Fabrication & Thermophysical Studies of Hexa Ferritesprr.hec.gov.pk/jspui/bitstream/123456789/1671/2/1503S.pdfMariam Ansari, Ms. Khush Bakhat and Ms Fatima-Tuz-Zahra for helping me

Fabrication & Thermophysical Studies of Hexa Ferrites

By

Ghulam Asghar

CIIT/SP05-PPH-003/ISB

PhD Thesis

In

Physics

COMSATS Institute of Information Technology

Islamabad-Pakistan

Spring 2011

COMSATS Institute of Information Technology


A Thesis Presented to

COMSATS Institute of Information Technology, Islamabad

In partial fulfillment

of the requirement for the degree of

PhD Physics

By

Ghulam Asghar


Spring, 2011


A Post Graduate Thesis submitted to the Department of Physics as partial

fulfillment of the requirement for the award of Degree of Ph. D. (Physics).

Name Registration No.

Ghulam Asghar CIIT/SP05-PPH-003/ISB

Supervisor

Dr. Muhammad Anis-ur-Rehman

Associate Professor, Department of Physics,

COMSATS Institute of Information Technology (CIIT),

Islamabad Campus.

June, 2011

Final Approval

This thesis titled


By

Ghulam Asghar

Registration No. CIIT/SP05-PPH-003/ISB

has been approved

for the COMSATS Institute of Information Technology, Islamabad

External Examiner:________________________________

Dr.

Supervisor: ___________________________________

Dr. M. Anis-ur-Rehman

Department of Physics/Islamabad

HoD:______________________________________

Dr. Ishaq Ahmed

HoD (Department of Physics/ Islamabad)

Dean, Faculty of Science: ______________________

Prof. Dr. Arshad Saleem Bhatti

Declaration

I Mr. Ghulam Asghar Reg. # CIIT/SP05-PPH-003/ISB, hereby declare that I have

produced the work presented in this thesis, during the scheduled period of study. I also

declare that I have not taken any material from any source except referred to wherever due

that amount of plagiarism is within acceptable range. If a violation of HEC rules on research

has occurred in this thesis, I shall be liable to punishable action under the plagiarism rules of

the HEC.

Date: _________________ Signature of the student:

___________________________

Ghulam Asghar

Reg. # CIIT/SP05-PPH-003/ISB

Certificate

It is certified that Mr. Ghulam Asghar Reg. # CIIT/SP05-PPH-003/ISB has carried

out all the work related to this thesis under my supervision at the Department of Physics,

COMSATS Institute of Information Technology, Islamabad and the work fulfills the

requirement for award of Ph. D degree.

Date: _________________

Supervisor:

_________________________

Dr. Muhammad Anis-ur-Rehman,

Associate Professor

Head of the Department:

_____________________________

Dr. Mahnaz Qadir Haseeb

Associate Professor

Department of Physics

Dedication

This dissertation is dedicated to my mother and father who prayed for me a lot.

Acknowledgements

All praise to Almighty Allah, the most gracious and merciful, whose blessings are

unlimited, and who blessed me with opportunity to pay my contribution in efforts to explore

some facts of his created striking and outstanding universe. Without the help and blessing of

my Allah, I was unable to complete my project. Countless prays for his holy prophet

Muhammad (P.B.U.H), who is forever, a light of guidance and wisdom for all humanity.

Special gratitude to my supervisor Dr. Muhammad Anis-ur-Rehman, for all kinds

of support he has provided me. I am sincerely obliged to him for providing the freedom with

respect to the research activities. It was his encouragement so that the project has been

completed. I have really no words to express my thoughts for him. God bless him all the

time in every walk of life.

I would like to pay my indebtedness to Prof. Dr. Arshad Saleem Bhatti, Dean of

Sciences, who remains always a source of courage for me. I am also grateful to Prof. Dr.

Sajid Qammar, Chairman of the Department of Physics for providing me research facilities

at CIIT, Islamabad and Dr. Ishaq Ahmad, for his continuous guidance.

I am very thankful to Dr. M. Ashraf Atta for his noble cooperation and to all my

teachers especially for their cooperation and healthy suggestions during study and research

work at COMSATS Institute of Information Technology Islamabad, Pakistan. Higher

Education Commission (HEC), Pakistan is highly acknowledged for providing financial

support through “Indigenous 5000 Scholarship Program”. This scholarship policy made it

possible for me to do this work. I also owe my profound thanks to Dr. Atta-ur-Rahman,

Ex. Chairman Higher Education Commission (HEC), Pakistan.

I would like to acknowledge Dr. M. Saif Ullah Awan, for providing facilities of some

of the characterization during my experimental work. My special thanks goes to my cousin

Ghulam Hasnain Tariq and Syed Nasir Khusro for their help in research work and to my other

lab fellows, Muhammad Akram, Ali Abdullah, Anwar-ul-Haq, M. Yasin Shami, Awais

Siddique Saleemi, Muhammad Mubeen, Muhammad Ali Malik, Ms. Zeb-un-Nisa, Ms.

Mariam Ansari, Ms. Khush Bakhat and Ms Fatima-Tuz-Zahra for helping me whenever I feel

stuck. I would also like to express my gratitude to Rafaqat Hussain, Niaz Ahmed Niaz,

Rizwan Ahmed Khan, Zahoor Ahmad, Muhammad Farooq Nasir and M. Hafeez.

I am deeply grateful to my family members, their love for me and providing me

encouragement at every step of life. I am really gratified to my daughter Areeba Naureen and

Inshraah Asghar who missed me very much whenever they wanted me with them; actually

their sacrifices are far above my words.

Ghulam Asghar


Abstract


Strontium hexaferrite nano material with nominal composition SrFe12O19 is prepared

by wet chemical methods. The effect of variation in synthesis parameters such as molar ratio

of cations (Fe/Sr), volume rate of addition of precipitating agent and the pH of the solution on

the phase purity and particle size is studied to optimize them for the synthesis by co-

precipitation method. The effect of molar ratio of cations (Fe/Sr) on phase purity is studied by

using X-ray diffraction (XRD) patterns. It is observed from indexed XRD patterns that molar

ratio of cations does not affect the phase purity of strontium hexaferrites as there is no

impurity peak present in any sample and all patterns are almost similar. The effect of volume

rate of addition of precipitating agent on phase purity and surface morphology are analyzed

by using XRD diffraction patterns and scanning electron micrographs (SEM). The indexed

XRD patterns show that the increase in the volume rate of addition of precipitating agent

improves the phase purity and SEM micrographs show that the size of the particles also

decrease with the increase in the volume rate of addition of precipitating agent. The effect of

pH variation on structural and electrical properties of strontium hexaferrite is analyzed by

using X-ray diffractometer, scanning electron microscope, temperature dependent dc

resistivity measurement system and precision component analyzer. Indexed XRD patterns

show that the secondary phases are decreased with the increase in pH of the solution and

single phase strontium hexaferrite is obtained for pH=13. The pH of the solution also imparts

a significant effect on structural morphology of prepared hexaferrite samples. The SEM

micrographs with varying pH samples clearly indicate that most of the particles are of

hexagonal shape. It can also be seen that the particle size and their distribution also decrease

with the increase in the pH of the solution. The dc resistivity is also increased by increasing

pH and this may be due to increase in the grain boundaries.

The composition SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) is prepared in order to

increase the coercivity of strontium hexaferrites. Results obtained indicate that Cr doping

causes the formation of secondary phases. It is also observed that for X ≤ 0.6, both dielectric

constant and coercivity is increased while saturation magnetization is decreased. The increase

in coercivity was due to variation in particle size and impurity phases which acted as pinning

centers. The decrease in saturation magnetization is because of the replacement of cation

(Fe3+) having high magnetic moment (5µB) on octahedral sites with cation (Cr3+) having

smaller magnetic moment (3µB). Another composition SrFe12-2xCrxZnxO19 with (X=0.0, 0.2,

0.4, 0.6, 0.8) is prepared with co-precipitation method in order to reduce the dielectric loss

tangent. The results show that Cr-Zn doping causes increase in the particle size and decrease

in dielectric loss tangent and make the strontium hexaferrite useful for high frequency

applications. The hysteresis loops of the Cr-Zn doped samples reveal that both coercivity and

saturation magnetization is decreased with increase in doping concentration. The same

composition SrFe12-2xCrxZnxO19 with x=0.0, 0.2, 0.4, 0.6, 0.8 is synthesized with WOWS sol-

gel method (WOWS stands for Without Water and Surfactants; a new simplified sol-gel

method developed in our lab). The structural and dielectric measurements results obtained

from the samples prepared with WOWS sol-gel method are better than the results obtained

from the same composition prepared with co-precipitation.

In some cases, the materials with high loss as well as high dielectric constant may be

desired in applications such as electromagnetic (EM) wave absorbing coatings. To achieve

these properties, reduction of oxygen from sintered SrFe12O19 is made. This treatment resulted

in the increase in the concentration of Fe2+ ions and free iron atoms and hence in the increase

in both dielectric constant and dielectric loss and making the material useful for microwave

absorption.

Table of Contents

Chapter 1 ............................................................................................................. 1

Introduction ......................................................................................................... 1

1. Introduction ..................................................................................................... 2

1.1 Nano science .............................................................................................. 2

1.2 Types of ferrites ............................................................................................. 3

1.2.1 Soft ferrites ............................................................................................. 3

1.2.2 Hard ferrites ............................................................................................ 4

1.3 Classification of hexaferrites ......................................................................... 4

1.4 M-type hexaferrites ....................................................................................... 5

1.5 Structural properties of strontium hexaferrites (SrM) ................................... 6

1.6 Magnetic properties of M-type strontium hexaferrites ................................. 8

1.6.1 Source of magnetism .............................................................................. 8

1.6.2 Classification of magnetic materials ....................................................... 9

1.6.3 Ferrimagnetism in ferrites ....................................................................... 9

1.6.4 Superexchange interaction .................................................................... 11

1.6.5 Magnetocrystalline anisotropy.............................................................. 12

1.6.6 Magnetic structure of M-type strontium hexaferrite (SrM).................. 12

1.6.7 Literature review about magnetic properties of SrM ............................ 14

1.7 The dc electrical properties of M-type strontium hexaferrites ................... 16

1.7.1 Temperature dependent dc electrical resistivity ................................... 16

1.8 Frequency dependent dielectric properties of strontium hexaferrites ......... 17

1.8.1 Electronic polarization .......................................................................... 18

1.8.2 Ionic polarization .................................................................................. 18

1.8.3 Orientation polarization ........................................................................ 19

1.8.4 Hyperelectronic polarization ................................................................ 19

1.8.5 Space charge polarization ..................................................................... 20

1.8.6 The dielectric constant .......................................................................... 21

1.8.7 The dielectric loss ................................................................................. 24

1.8.8 Literature review about frequency dependent electrical properties of

SrM ................................................................................................................ 25

1.8.9 Ferromagnetic resonance and Snoek’s Limit ....................................... 26

1.9 Mechanical properties of ferrites ................................................................. 31

1.10 Thermal transport properties ..................................................................... 32

1.11 Chemical stability ...................................................................................... 32

1.12 Applications of M-type hexaferrites ......................................................... 32

1.13 Motivation and objectives ......................................................................... 34

Chapter 2 ........................................................................................................... 38

Characterization Techniques ............................................................................. 38

2. Characterization Techniques ......................................................................... 39

2.1 X-ray diffraction .......................................................................................... 39

2.2 Scanning Electron Microscopy (SEM) ....................................................... 42

2.3 Frequency dependent ac measurements ...................................................... 44

2.4 Temperature dependent dc resistivity measurements .................................. 45

2.5 Magnetic hysteresis ..................................................................................... 48

Chapter 3 ........................................................................................................... 51

Synthesis techniques .......................................................................................... 51

3. Synthesis techniques ...................................................................................... 52

3.1 Solid state reaction method ...................................................................... 52

3.2 Wet chemical method .............................................................................. 52

3.2.1 Precursor methods ............................................................................. 53

3.2.2 Spray-drying ...................................................................................... 53

3.2.3 Freeze-drying method ....................................................................... 53

3.2.4 Microemulsion method ..................................................................... 53

3.2.5 Hydrothermal method ....................................................................... 54

3.2.6 Sol-gel method .................................................................................. 54

3.2.7 WOWS sol-gel method ..................................................................... 55

3.2.8 Co-precipitation method ................................................................... 55

Chapter 4 ........................................................................................................... 58

Optimization of synthesis parameters for phase purity ..................................... 58

4. Optimization of synthesis parameters for phase purity ................................. 59

4.1 Synthesis .................................................................................................. 59

4.2 Effect of variation in molar ratio (Fe/Sr) on structural properties of SrM

....................................................................................................................... 60

4.2.1 Conclusion ........................................................................................ 61

4.3 Volume rate of addition of precipitating agent on structural properties of

SrM ................................................................................................................ 62

4.3.1 Conclusion ........................................................................................ 64

4.4 Effect of variation in pH on structural and electrical properties of

strontium hexaferrites (SrM) ......................................................................... 64

4.4.1 Effect of pH on the structural properties of SrM .............................. 65

4.4.2 Effect of pH on dc electrical properties of SrM ................................ 69

4.4.3 Effect of pH on frequency dependent ac electrical measurements of

SrM ............................................................................................................. 71

4.4.4 Conclusion ........................................................................................ 75

Chapter 5 ........................................................................................................... 76

Structural, electrical and magnetic properties of Cr doped strontium

hexaferrites ........................................................................................................ 76

5. Structural, electrical and magnetic properties of Cr doped strontium

hexaferrites ........................................................................................................ 77

5.1 Structural properties of Cr doped SrM .................................................... 77

5.2 Frequency dependent ac electrical properties of SrM ............................. 78

5.2.1 The dielectric constant (ε') ................................................................ 80

5.2.2 The dielectric loss tangent (tanδ) ...................................................... 82

5.2.3 The dielectric loss factor () ........................................................... 83

5.2.4 The ac conductivity (ac) .................................................................. 84

5.3 Temperature dependent dc electrical properties of SrM ......................... 85

5.4 Magnetic properties of Cr doped SrM ..................................................... 86

5.5 Conclusion ............................................................................................... 89

Chapter 6 ........................................................................................................... 91

Structural, electrical and magnetic properties of Cr-Zn doped strontium

hexaferrites prepared by co-precipitation method .......................................... 91

6. Structural, electrical and magnetic properties of Cr-Zn doped strontium

hexaferrites prepared by co-precipitation method ............................................. 92

6.1 Structural properties of Cr-Zn doped SrM .............................................. 92

6.2 Frequency dependent ac electrical properties of Cr-Zn doped SrM ........ 95

6.2.1 The dielectric constant () ................................................................ 97

6.2.2 The dielectric loss tangent (tan) ...................................................... 99

6.2.3 The dielectric loss factor () ......................................................... 100

6.2.4 The ac conductivity (ac) ................................................................ 100

6.3 Temperature dependent dc electrical properties of Cr-Zn doped SrM .. 101

6.4 Magnetic properties of Cr-Zn doped SrM ............................................. 102

6.5 Conclusion ............................................................................................. 105

Chapter 7 ......................................................................................................... 106

Structural and electrical properties of Cr-Zn doped strontium hexaferrites

prepared by WOWS sol-gel method ............................................................... 106

7. Structural and electrical properties of Cr-Zn doped strontium hexaferrites

prepared by WOWS sol-gel method ............................................................... 107

7.1 Structural studies ................................................................................... 107

7.2 Dielectric properties ........................................................................... 109

7.2.1 The dielectric constant () .............................................................. 109

7.2.2 The dielectric loss tangent (tan) .................................................... 111

7.2.3 The ac conductivity (ac) ................................................................ 111

7.3 Conclusion ............................................................................................. 113

7.4 Comparison ............................................................................................ 114

7.4.1 Structural properties ........................................................................ 114

7.4.2 Dielectric properties ........................................................................ 115

7.4.3 Conclusion ...................................................................................... 117

Chapter 8 ......................................................................................................... 118

Oxygen reduced strontium hexaferrite for microwave absorbing coatings .... 118

8. Oxygen reduced strontium hexaferrite for microwave absorbing coatings 119

8.1 Reduction procedure .............................................................................. 119

8.2 Effect of oxygen reduction on structural properties of strontium

hexaferrites (SrM) ........................................................................................ 120

8.3 Frequency dependent ac electrical properties of oxygen reduced

strontium hexaferrites .................................................................................. 120

8.4 The dielectric loss tangent (tanδ) ........................................................... 120

8.6 Temperature dependent dc electrical properties of sintered SrFe12O19

before and after reduction ............................................................................ 124

8.7 Conclusion ............................................................................................. 124

9 Conclusions .................................................................................................. 125

9.1 Future work ............................................................................................... 127

10. References ................................................................................................. 128

LIST OF FIGURES

Fig 1.1: A typical hysteresis loop of soft and hard magnet. .......................................... 3

Fig 1.2: Chemical composition diagram of the ferrimagnetic ferrites [17]. .................. 4

Fig 1.3: Unit cell of the M-type hexaferrite. .................................................................. 7

Fig 1.4: Magnetic moment ordering in (a) diamagnetics, (b) paramagnetics, (c)

ferromagnetic, (d) antiferromagnetics and (e) ferrimagnetics ..................................... 10

Fig 1.5 Schematic diagram of polarization mechanisms and their frequency response

...................................................................................................................................... 20

Fig 1.6 Different polarization mechanisms in the absence and presence of an external

electric field [88]. ......................................................................................................... 21

Fig 1.7 The precession motion of magnetic moment around the applied magnetic

field. ............................................................................................................................. 27

Fig 2.1: Geometrical illustration of Bragg’s law [147]................................................ 39

Fig 2.2: PANalytical X'pert pro MPD X-ray diffractometer ....................................... 42

Fig 2.3: JEOL JSM-6700F scanning electron microscope (SEM) .............................. 43

Fig 2.4: (a) Two-terminal and (b) four-terminal resistance measurement

arrangements[148] ....................................................................................................... 46

Fig 2.5: Circuit diagram of the apparatus used for dc electrical resistivity

measurements ............................................................................................................... 47

Fig 2.6: The dc magnetometer (RIKEN DENSHI) ...................................................... 49

Fig 2.7: A typical hysteresis loop along with data obtained from the dc magnetometer

...................................................................................................................................... 50

Fig 3.1: Schematic diagram for the chemical co-precipitation method. ...................... 57

Fig 4.1: Indexed XRD patterns of SrFe12O19 for different molar ratios (MR=Fe/Sr) . 61

Fig 4.2: Indexed XRD patterns of the samples with different volume rate of addition

of the precipitating agent ............................................................................................. 62

Fig 4.3: SEM micrographs of the samples with different volume rate of addition of the

precipitating agent ........................................................................................................ 63

Fig 4.4: Indexed patterns of XRD of samples of SrFe12O19 for different values of pH

(- SrFe2O4, # -α-Fe2O3) .............................................................................................. 66

Fig 4.5: SEM micrographs of the sintered samples of SrFe12O19 for different values of

pH ................................................................................................................................. 68

Fig 4.6: Plot of ln of dc electrical resistivity of SrFe12O19 for different values of pH as

a function of temperature ............................................................................................. 70

Fig 4.7: Plot of drift mobility versus temperature of SrFe12O19 for different values of

pH as a function of temperature ................................................................................... 70

Fig 4.8: The plot of dielectric constant (έ) as a function of ln of frequency of

SrFe12O19 for different values of pH ............................................................................ 72

Fig 4.9: The plot of dielectric loss tangent (tanδ) as a function of ln of frequency of


Fig 4.10: The plot of dielectric loss factor (ε'') as a function of log of frequency of


Fig 5.1: Indexed XRD patterns of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .............. 78

Fig 5.2: Plot of dielectric constant (ε') as a function of ln of frequency for

SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 80

Fig 5.3: Plot of activation energy and dielectric constant at 3MHz versus Cr

concentration (X) ......................................................................................................... 81

Fig 5.4: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for

SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 83

Fig 5.5: Plot of dielectric loss factor () as a function of ln of frequency for

SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 84

Fig 5.6: Plot of ac conductivity (σac) as a function of ln of frequency for

SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ..................................................................... 85

Fig 5.7: Plot of lnρ as a function of 1/kBT for SrFe12-xCrxO19

(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit. ................................................... 86

Fig 5.8: Hysteresis loops of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ....................... 87

Fig 5.9: Plot of coercivity and saturation magnetization versus Cr concentration of

SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 88

Fig 6.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ...... 94

Fig 6.2: SEM micrographs of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............. 96

Fig 6.3: Plot of grain size (nm) as a function of Cr-Zn concentration (X) for

SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................. 97


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................. 98


SrFe12-xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................... 98


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ........................................................... 100


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................ 101

Fig 6.8: Plot of lnρ as a function of 1/kBT for SrFe12-2xCrxZnxO19

(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit. ................................................. 102

Fig 6.9: Hysteresis loops of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .............. 103

Fig 6.10: Plot of coercivity (Hc) and saturation magnetization (Ms) versus

Cr-Zn concentration (X) of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............... 104

Fig 7.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

prepared by WOWS sol-gel method. ......................................................................... 108


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method

.................................................................................................................................... 110


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method

.................................................................................................................................... 110

Fig 7.4: Plot of ac conductivity (σac)as a function of ln of frequency for

SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with sol-gel method. .......... 112

Fig 7.5: Plot of experimental and theoretically calculated dielectric constant () for

the sample sample X=0.6 (SrFe12-2xCrxZnxO19) as a function of frequency. ............. 112

Fig 8.1: Experimental setup for oxygen reduction ..................................................... 119

Fig 8.2: Indexed XRD patterns of sintered strontium hexaferrite samples before and

after reduction ............................................................................................................ 121

Fig 8.3: Dielectric constant () of sintered SrFe12O19 before and after reduction .... 122

Fig 8.4: Dielectric loss tangent (tanδ) of sintered SrFe12O19 before and after reduction

.................................................................................................................................... 122

Fig 8.5: Plot of temperature dependent dc electrical resistivity of sintered SrFe12O19

before and after reduction .......................................................................................... 123

LIST OF TABLES

Table 1.1: Ferrimagnetic oxides in BaO–MeO–Fe2O3 ternary phase [18] .................... 5

Table 1.2: Number of Fe3+ ions with their type, spin and geometry [40] .................... 13

Table 4.1: List of the chemicals used with their specification ..................................... 59

Table 4.2: Lattice constants (a & c), crystallite size (D114), cell volume (V), X-ray

density (ρx), bulk density (ρm), % α-Fe2O3 and particle size ....................................... 67

Table 4.3: Dielectric loss (tanδ), dielectric constant (έ), dc electrical resistivity (ρdc)

and drift mobility (μd) of SrFe12O19 for different values of pH ................................... 71

Table 5.1: Lattice parameters (a & c), crystallite size (D114), X-ray density (ρx), bulk

density (ρm), porosity, activation energy (ΔE), dc electrical resistivity (ρdc), dielectric

constant (), dielectric loss tangent (tanδ), ac conductivity( σac), coercivity (Hc) and

saturation magnetization (Ms) of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8). .............. 79

Table 6.1: Lattice parameters (a & c), average crystallite size (Dav), X-ray density

(ρx), bulk density (ρm), porosity, activation energy, dc electrical resistivity (ρdc),

dielectric constant (ε'), dielectric loss tangent (tanδ) and ac conductivity (σac) of the

prepared samples of SrFe12-2xCrxZnxO19 X 0.0, 0.2, 0.4, 0.6, 0.8. ............................ 93


density (ρm), % porosity, dielectric constant (ε'), dielectric loss tangent (tanδ) and ac

conductivity (σac) of the prepared samples of SrFe12-2xCrxZnxO19 X

0.0, 0.2, 0.4, 0.6, 0.8 by WOWS sol-gel method. ..................................................... 109

LIST OF ABBREVIATIONS

B Magnetic Induction

Dav Average crystallite size

∆E Activation energy

EMI Electromagnetic interface

έ dielectric constant

ε0 permittivity of free space

FE-SEM Field Effect Scanning Electron Microscopy

FMR Ferromagnetic resonance

H Magnetic field strength

Hc Coericivity

HA Magnetocrystalline anisotropy

M Magnetization

μd drift mobility

Ms Saturation magnetization

Mr Remanent magnetization

ρx X-ray density

ρm Bulk density

ρdc dc electrical resistivity

SrM M-type Strontium hexaferrite

SEM Scanning Electron Microscope

SMPS Switched-mode power supply

σac ac conductivity

tan δ dielectric loss tangent

TEM Transmission Electron Microscopy

WOWS Without Water and Surfactants, a new simplified sol-gel method

XRD X-Ray diffraction

Y Youngs modulus

YIG Y3Fe5O12 Yittrium iron oxide

Chapter 1 Introduction

1. Introduction

1.1 Nano science

The properties of a single atom of an element are different from the bulk material of the

same element. As the size of the particle decreases, the surface to volume ratio increases. The

properties of a material are greatly affected by this surface to volume ratio. A particle of size

50nm has only 5% of its atoms on its surface whereas a particle of 5nm has 50% of its atoms

on its surface.

Nano-science deals with the preparation and characterization of such materials with any

one dimension in the range of 1-100nm. At nanometer scale, the material exhibits properties

that are very much different from the bulk [1].

Nano-science has brought a revolution in the field of science. Nanotechnology is being

used in many fields of science such as solid state physics, chemistry, medical science,

biotechnology and materials engineering.

Nano structures have got much attention of researchers because of its wide range of

applications. A material comprised of nanometer-scale particles have high fraction of grain

boundaries and hence surface defects and large surface to volume ratio. These properties

determine the properties of a synthesized material and differentiate them from a bulk material

[2, 3].

In recent years, ferrites have got much attention of scientists due to their great scientific

and technological applications such as magnetic media, memory cores, high frequency

devices, catalysis and gas sensors [4-9]. In Italian language, iron is named as ferry. Ferrites

are iron containing complex oxides with different structures and possess interesting

electronic, magnetic, surface reactivity and optical properties in nano size [10]. As a

consequence of these numerous applications, new synthesis techniques have been developed

for the production of nano-ferrites [8-14].

The commercial preparation of various types of ferrites was not started until the

beginning of 20th century. The commercial ferrites could not make their room in the world

because their magnetic properties were inferior to those of the ferromagnetic alloys [4].

Synthesis of new ferrites with enhanced existing properties started in 1950’s due to rapid

expansion of their applications in the devices such as radio, television, carrier telephony,

computer circuitry and microwave devices.

1.2 Types of ferrites

The ferrites are usually classified into two types.

1.2.1 Soft ferrites

1.2.2 Hard ferrites

1.2.1 Soft ferrites

These are the magnetic materials having narrow hysteresis loops possessing moderate

saturation magnetization with very small coercivity (smaller than 1 kAm-1 [15]. Soft ferrites

are widely used in the cores of transformers and switched-mode power supply (SMPS) [16].

Spinel ferrites and garnets are one of the examples of soft ferrites.

Fig 1.1: A typical hysteresis loop of soft and hard magnet.

1.2.2 Hard ferrites

Hard ferrites are known as permanent magnets due to high coercivity (greater than 10

kAm-1). Hard ferrites are commonly used in electric motor and radios etc [15, 16].

Hexaferrites are one of the examples of hard ferrites.

The difference in the hysteresis loop of soft and hard ferrite is shown in the figure 1.1.

It is clear from the figure that soft magnetic materials have very narrow coercive field while

hard magnetic material have large coercive field.

1.3 Classification of hexaferrites

Depending on the chemical composition, hexaferrites are classified into following

six types [17]

(a) M-type hexaferrites, (b) W-type hexaferrites, (c) X-type hexaferrites, (d) Y-type

hexaferrites, (e) Z-type hexaferrites, (f) U-type hexaferrites.

The different compositions of the hexagonal compounds are shown in figure 1.2

as a part of ternary phase diagram for the BaO-MeO-Fe2O3 system.

Fig 1.2: Chemical composition diagram of the ferrimagnetic ferrites [17].

Where Me represents a divalent ion among the first transition elements. The stacking

orders of cubic and hexagonal basic units determine the type of composition such as X, W, U,

Z and Y-type hexaferrites. These types are given in table 1.1 [17].

Table 1.1: Ferrimagnetic oxides in BaO–MeO–Fe2O3 ternary phase [18]

Symbol Composition Crystallographic

build up

No. of molecules

/unit cell

c-axis

(Å)

M BaFe12O19 RSR*S* (MM*) 2M 23.2

X Ba2Me2Fe28O46 MM*S 3MeX 84.0

W Ba2Me2Fe16O27 MSM*S* 2MeW 32.8

U Ba2Me2Fe36O60 MM*Y* MeU 38.1

Z Ba2Me2Fe24O41 MYMY 2MeZ 52.3

Y Ba2Me2Fe12O22 3TS 3MeY 43.5

* Represents rotation at 180

1.4M‐typehexaferrites

M-type hexaferrites are a type of magnetic oxide with chemical formulae BaO.6Fe2O3

(BaM), SrO.6Fe2O3 (SrM) and PbO.6Fe2O3 (PbM). M-type hexaferrites possess higher

coercivity (400 kAm-1) than any other type of ferrite [19]. This family of ferrites have been a

subject of continuous interest for several decades due to the fact that these compounds have

been the work horse of the permanent magnet market [20] and for passive microwave

components, microwave absorber, magnetic recording media, electronic devices, medicine

and magneto-optical recording [21-24]. This material can easily be prepared into powder form

and converted into desired shape. The present work is on M-type strontium hexaferrites due to

its better magnetic properties, high Curie temperature, better chemical and thermal stability,

large uniaxial magnetocrystalline anisotropy, large electrical resistivity, high corrosion

resistance and easy handling because of its non-toxicity.

1.5Structuralpropertiesofstrontiumhexaferrites(SrM)

These M-type hexagonal magnetic oxides were initially developed by Went et al..

[25]. The synthesis of strontium hexaferrite was made for the first time by Adelsk¨old [26].

He also found that the crystal structure of strontium hexaferrite is similar to that of

magnetoplumbite. Later his determination was confirmed by the structural refinement of

strontium hexaferrites [27, 28]. The space group of this structure is P63/mmc. This structure

has hexagonal symmetry. Its ‘a’ axis is called minor axis and ‘c’ axis is considered as major

and preferred axis. This preferred axis positively contributes to M-type ferrites as permanent

magnetic material. The formula of magnetoplumbite structure is MFe12O19 where M is Ba, Sr,

or Pb.

The magnetoplumbite structure contains two formula units per unit cell. The unit cell

of magnetoplumbite structure contains ten layers of oxygen ions shown in figure 1.3. These

layers form spinel blocks S followed by R block containing Ba or Sr ion followed by S* and

R* blocks. S* and R* blocks are similar to S and R blocks but have a rotation over 1800 around

the c-axis. The layer in which Ba or Sr ion is present is hexagonally packed with two oxygen

layers at each side. The four oxygen layers between those containing the Ba or Sr ion are

cubically packed. This stacking of layers gives a rise to an overlap, of cubically and

hexagonally packed sections in the structure. The basal plane containing the barium ion is a

mirror plane of R block and consequently the block preceding and succeeding the R block

must be rotated over 1800 with respect to each other. This is also the reason that the

elementary cell of M-type structure contains 10 not the 5 oxygen layers. Five layers of oxygen

form one formula unit. Such two formula units results in one unit cell. The crystallographic

structure can be described as RSR*S* as given by Braun [29].

The unit cell of this structure contains 2 M ions (M= Ba+2, Sr+2 and Pb+2), 24 Fe3+ ions

and 38 O2- ions. The 24 iron (Fe) atoms are dispersed on five interstitial sites of the ten layers

mentioned above. Three octahedral (B) sites containing twelve ‘k’ sites, two ‘a’ sites and four

‘fiv’ sites, one tetrahedral (A) four ‘fvi’ sites and one bipyramidal (C) site contains two ‘b’ sites

[30].

Fig 1.3: Unit cell of the M-type hexaferrite.

The Fe3+ ions present at 2a sites are octahedrally corresponding with equal Fe–O

distances while the Fe3+ ions present at 4f2 and 12k sites are octahedrally corresponding with

different Fe–O distances ranging from 0.185 to 0.237 nm. The Fe3+ ions present at 4f1 are

tetrahedrally corresponding with oxygen and the Fe3+ ions present at 2b sites are coordinated

by five oxygen ions. The structure also contains short Fe–Fe distances. The Fe3+ ions present

at 4f2 sites have about 0.27 nm distances

between each other while the distance between Fe3+ ions at 12k sites is about 0.29–0.30 nm

[28].

1.6MagneticpropertiesofM‐typestrontiumhexaferrites

1.6.1Sourceofmagnetism

The magnetic moment, produced due to the movement of electrons in an atom, is

considered as fundamental source of magnetism. It can be related with the current loop.

Consider a current loop of area A and the current flowing through it is I then magnetic

moment ‘µ’ is given by

μ IA1.1

The unit of magnetic moment is A-m2.

Origin of magnetism is the motion of electron, spin motion and orbital motion. Both of

these motions are responsible for net magnetic moment on an electron and each electron

behaves like a tiny magnet. The nucleus of an atom also have magnetic moment but negligible

as compared to electron. These electrons respond to external magnetic field and produce

magnetization which is net magnetic dipole moment per unit volume of the material.

M 1.2

where ‘µ’ is the magnetic moment, ‘ν’ is volume and ‘M’ is magnetic dipole moment.

Therefore any atom with at least one electron should have some magnetic behavior. But

only those atoms or molecule show magnetic behavior which have unpaired electrons due to

Pauli’s exclusion principle.

The magnetic induction ‘B’ is given by

B B μ M1.3

where ‘Bext’ and ‘µ0’ are the strength of external magnetic field and permeability of free space

respectively. The magnetic field strength ‘H’ is given by

HBμ

1.4

Putting the value of ‘Bext’ in the above equation

B μ H M 1.5

The relationship between magnetization ‘M’ and the applied field ‘H’ is defined as

χMH1.6

where ‘χ’ is the magnetic susceptibility of the material.

1.6.2Classificationofmagneticmaterials

When a material is exposed to the field of an external magnet, its response determine

the magnetic properties of that material. Depending on the response to external magnetic field

(bulk magnetic susceptibility), materials are categorized into five groups and are named as

diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials.

Some of the materials with their response to external magnetic field are given in figure 1.4.

1.6.3Ferrimagnetisminferrites

According to Neel, the net magnetic moment of unpaired electrons (with spin up and

down) of the atoms present on the interstitial sites of ferrites is different. As a result, a net

magnetic moment appears due to the difference in number of magnetic ions or their magnetic

moments present on two type of interstitial sites [31].

Strontium hexaferrites (M-type ferrites) are ferrimagnetic in nature and its structure is

magnetoplumbite, the name after the naturally occurring of PbFe19O12. This family of ferrites

is named as hexaferrite due to its six fold symmetrical uniaxial crystallographic structure. In

many respects, hexaferrites are similar spinel ferrites in many respects due to the fact that they

Fig 1.4: Magnetic moment ordering in (a) diamagnetics, (b) paramagnetics, (c) ferromagnetic, (d) antiferromagnetics and (e) ferrimagnetics

possess the same ratio of octahedral to tetrahedral sites. The major difference is that M-type

ferrites include one extra site named as trigonal bipyramid site and one big site holding

divalent Pb, Ba, or Sr ions. Hexagonal ferrites are famous for their permanent magnet

applications. The trigonal bipyramid site containing Fe3+ ion is responsible for strong uniaxial

or planar anisotropy of M-type hexa ferrites. M-type hexaferrite structure is formed by

stacking of alternate building blocks beginning with a spinel block, named as S aligned with

the c axis along a <111> body diagonal. Due to this the ferromagnetic spin arrangements are

aligned with only one easy (or hard) magnetic direction. The spinel structure contains three

fold symmetry, in this direction. When S block is systematically rotated by 1800, six fold

symmetry is obtained. There are blocks which contain the layers having large cations, Ba2+

(radius 1.36Å), Sr2+ (1.16Å), Pb2+ (1.18Å) or combinations thereof. The layers in these blocks

also contain trigonal bipyramid sites. These blocks are named as R (or R*) [32].

1.6.4Superexchangeinteraction

When two magnetic dipoles having moment ~ 1µB separated by 1 Å have energy

~10-23 J at 1 K temperature. At high temperature (1000K) this interaction becomes very weak.

For long range magnetic order, exchange interaction phenomenon takes place. Number of

solids such as oxides has magnetic ground states. An exchange interaction between non-

adjacent magnetic ions mediated by a non-magnetic ion is called superexchange interaction. It

is a phenomenon in which two electrons from a double negative ion (such as oxygen) in a

solid go to different positive ions and couple with their spins, giving rise to a strong

antiferromagnetic coupling between the positive ions, which are too far apart to have a direct

exchange interaction is superexchange. In insulating materials such as metal oxides, the

distance between metal ions is large so antibonding state, according to Hund’s rule for

parallel spin alignment, due to mutual repulsion of electrons could not be established. So the

direct exchange between them does not contribute to the magnetic properties. Ferrites

contains cation ions at the interstitial sites of anion (oxygen) based crystal structure. Kramers

(1934) reported exchange mechanism between non neighboring cations ions mediated by

anions (oxygen) ions to discuss magnetic properties of ferrites. With some addition to

Kramers theory for indirect exchange, Neel postulated a new theory for antiferromagnetic

oxides and afterwards for ferrites. Mathematically, the theory of indirect exchange was

explained by Anderson in 1950. Anderson named this theory as superexchange. In this

mechanism of interaction, p orbital of oxygen filled in its ground state, exchange an electron

with the adjacent 3 orbitals of magnetic ions. To make a coupling with cation, electron of

oxygen ion should have a spin opposite to that of cation. This would result in leaving the

other spin of oxygen ion orbital free to couple with the unpaired spin of cation ion preferably

located opposite to the original cation. This is the cause of the stability of antiparallel

alignment of two cations adjacent to oxygen ion.

The ferrites contain two different lattice sites. In case of antiferromagnetic materials,

the moment of two different lattice sites are equal. In case of ferrites the moments are not

equal. This resulted in a net moment. This net moment is obtained from the difference in the

moments on two sites. This phenomenon is named as uncompensated antiferromagnetism or

ferrimagnetism [19, 33].

.

1.6.5Magnetocrystallineanisotropy

All magnetic materials (ferromagnetic or ferrimagnetic) possess one direction or more

than one direction in which their magnetic moments prefer to be oriented. This direction is

considered as easy axis for that material. The energy required to change the orientation of

magnetic moment from easy axis is called magnetocrystalline anisotropy energy. This

magnetocrystalline energy is expressed as

Ek = K1sin2ɵ + K2sin4 ɵ + …… , 1.7

where K1 and K2 are anisotropy constants and ɵ is the angle between magnetization and the

easy axis

The values of these anisotropy constants are strongly affected by the temperature

[34]. The c-axis is considered as easy axis for M-type hexaferrites. The cause of high

magnetocrystalline anisotropy in M-type hexaferrite could not be explained on the basis of

spin dipole-dipole alignment mechanism. It is due to the fact that all of the magnetic ions

(Fe3+) are S-state (L=0) and therefore the possibility of first-order spin-orbit or John-Teller

stabilizations do not exists. One possible reason might be the trigonal bipyramidal crystal

field which might stabilize the d5 electrons into a low-spin [17].

1.6.6MagneticstructureofM‐typestrontiumhexaferrite(SrM)

According to Gorter [35], M-type hexaferrites are ferrimagnetic in nature. The M-type

hexaferrites are of hexagonal structure. It contains 64 ions per unit cell on 11 different

symmetry sites [36]. The unit cell of this structure contains and 2 M ions (M= Sr2+, Pb2+ and

S2+), 24 Fe3+ ions and 38 O2-ions. The 24 Fe3+ ions are distributed over five distinct sites i.e.

12k, 2a, 4f2, 4f1 and 2b. Out of these five, three sites 2a, 4f2 and 12k are octahedral, 4f1 is

tetrahedral and 2b is trigonal bipyramid site which is surrounded by five oxygen atoms [37].

The oxygen ions are present at 4e, 4f, 6h and 12k sites form a closed pack lattice [38]. The M

ions are present at 2d sites [39]. The site occupancy, the number and spin of the 12 Fe3+ ions

is given in table 1.2. The table shows that there are 6 Fe3+ ions at 12k site with spin up, 1 ion

at each 4f2 and 4f1 site with spin down and 1 ion in 2a and 2b site each having spin up. So

there are 8 Fe3+ ions with spin up and 4 with spin down. So the net moment obtained is of 4

Fe3+ present in formula units. One Fe3+ ion has the magnetic moment of 5μB and so the net

magnetic moment of one formula unit is 20 μB.

Table 1.2: Number of Fe3+ ions with their type, spin and geometry [40]

Site type 12K 2a 4f1 4f2 2b

Geometry Octahedral Octahedral Tetrahedral Octahedral Trigonal bipyramidal

Fe3+ ions 6 1 2 2 1

Spin Up Up Down Down Up

1.6.7LiteraturereviewaboutmagneticpropertiesofSrM

M-type hexaferrites are famous for their application as permanent magnets. These

materials cover 90wt% of the annual production of permanent magnets. These magnets are

manufactured about 300 and 500 tons per year in Europe [41]. This family of ferrites are of

also much importance due to their application for high density perpendicular recording media

[42]. Recently IBM has reported a magnetic tape composed of barium hexaferrite

nanoparticles possessing 15 times higher recording density than the commercially available

tapes in market [43].

The magnetic properties of a material depend upon number of factors such as the

chemical composition, microstructural parameters, impurities, and temperature etc. Thus the

scientists paid much attention to the synthesis of strontium hexaferrite having fine particle

size with narrowed distribution and minimum agglomeration[44]. The magnetic properties of

ferrites are greatly influenced by the nature of cation and their distribution in the structure.

One formula unit of strontium hexaferrite contain 12 iron atoms which are present on five

distinct sites: three parallel (12k, 2a, and 2b) and two antiparallel (4f1 and 4f2) [30, 45]. The

site occupancy, the number and spin of the 12 Fe3+ ions is given in table 1.2. The table shows

that there are 6 Fe3+ ions at 12k site with spin up, 1 ion at each 4f2 and 4f1 site with spin down

and 1 ion in 2a and 2b site each having spin up. So there are 8 Fe3+ ions with spin up and 4

with spin down. The magnetic moment of 4 Fe3+ ions with spin up and 4 Fe3+ ions with spin

down cancel each other’s effect and only the net moment of 4 Fe3+ ions with spin up is left in

one formula units. As the magnetic moment of one Fe3+ ion is 5μB so the total moment of 4

Fe3+ ions with spin up is 20 μB per formula unit. The saturation magnetization of strontium

hexaferrite depends upon the cation distribution on these different interstitial sites.

The magnetic properties of strontium hexaferrite can be tailored by replacing ferric

ion (Fe3+) with the ion of other element having same or different valency. If Fe3+ ion with spin

down is replaced by such an ion whose unpaired electrons are less than that of Fe3+ ion then

the number of unpaired electrons with the spin up would be increases. As a result net

magnetic moment will increase because the net magnetic moment of one formula unit of

strontium hexaferrite arises because of the difference of the spin up and spin down electrons.

This increase in net magnetic moment will result in the increase in saturation magnetization.

Similarly if Fe3+ ion with spin up is replaced by such an ion whose unpaired electrons are less

than that of Fe3+ ion then the number of unpaired electrons with the spin up would be

decreased. As a result net magnetic moment will decrease and hence saturation magnetization

will decrease. Except very few cases, saturation magnetization decreases with the replacement

of Fe3+ ion with any other cation on the interstitial sites.

The coercivity mainly depends upon magneto crystalline anisotropy and

microstructural parameters such as grain size, stresses in the crystal lattice, porosity,

vacancies, spin canting and impurities. The high magneto crystalline anisotropy results in

high coercivity. Strontium hexaferrite (M-type hexaferrite) is considered as hard magnet due

to their high magneto crystalline anisotropy. The decrease in grain size (single domain) and

increase in porosity and impurities increases the coercivity of the material and vice versa. The

intrinsic coercivity of strontium hexaferrite is also very high (about 7 kOe). The coercivity of

this material has been has been increased as high as 13kOe and decreased to few hundred

oestered according to its requirement for different applications [46].

Strontium hexaferrites possess better magnetic properties than barium hexaferrites

because of smaller ionic radius of strontium than barium. Strontium hexaferrites have also an

advantage over barium ferrite due to problems in the synthesis of barium ferrite in better form

[47].

A lot of research has been made to investigate and improve the magnetic properties of

strontium hexaferrites due to their large number of application in the market based on their

magnetic properties. Different researchers used different synthesis techniques, [44, 48-54]

with different annealing temperatures to investigate and tailor the magnetic properties of

strontium hexaferrites for different applications. Numerous magnetic studies has been carried

out with the substitution of Fe3+ or Sr2+ ions with elements such as Sm [55], Mn, Co, Zr [56],

Al [57], Gd [58], Si-Ca [59], Ba [60], La-Co [61], Al-Cr [62], Co–Nd [63], Zr–Ni [64], Al–

Ga [65], Zn, Ti, Ir [66], Zn-Nb [67], Zn, Co, Ti [68], La, Sm, Nd [69]. Some researchers

added different oxides SiO2, CaO [70] or used core (SrFe12O19) - shell (SiO2) model [71],

Fe/Sr ratio [72], some prepared composites by using different oxides such as SiO2, Fe2O3,

Bi2O3, H3BO3 and SrCO3 [73, 74] and studied the magnetic properties for different

applications.

1.7 The dc electrical properties of M-type strontium hexaferrites

Most of the ferrites are considered as high resistive materials. The resistivity of these

materials depends upon the microstructural parameters, dopants and their site occupancy,

annealing temperature and time. The electrical conduction in strontium hexaferrites is

discussed by using Verwey model [75]. The unit cell of strontium hexaferrites contains 24 Fe

atoms distributed on three different interstitial sites named as octahedral, tetrahedral and

trigonal bipyramedal sites explained above. It is the number of ferrous ions, formed due to the

reduction of oxygen during annealing, present on the octahedral sites that play a dominant

role in the process of conduction as well as dielectric polarization [76]. In an ideal crystal

lattice of strontium hexaferrite, only ferric (Fe3+) ions are present on interstitial sites. But

during heat treatment, a number of ferric (Fe3+) ions are transformed to ferrous (Fe2+) ions due

to small reduction and gives a rise to oxygen vacancies. According to Verwey model, the

electrical conduction in ferrites is mainly due to hopping of electrons between ions of the

same element present in more than one valence state, distributed randomly over

crystallographic equivalent lattice sites [77]. The distance between cation ions present on

interstitial sites of ferrites is different. It is smaller for the cations present on octahedral B

sites than those present on octahedral B and tetrahedral A sites. The probability of electron

hopping between tetrahedral A and octahedral B sites much smaller as compared to the

hopping between (B)–(B) sites. As only Fe3+ ions are present on tetrahedral A site so electron

hopping between (A)–(A) sites is not possible. If any Fe3+ ion is transformed to Fe2+ ion

during processing then it preferentially occupy (B) sites only. In ferrites, the conduction is due

to the hopping of electrons between the ions of same element having different oxidation state

(Fe2+ to Fe3+) present octahedral B sites [78, 79].

1.7.1Temperaturedependentdcelectricalresistivity

The dc electrical resistivity of these ferrites could be increased by decreasing the

concentration of Fe2+ ions by any method. The resistivity of nano materials increases with the

decrease in grain size. The decrease in grain size results in the increase in grain boundaries

which acts as poorly conducting medium. Hence the dc resistivity could be controlled by

varying the grain size of the synthesized material. The impurities present in the crystal

structure produce the stress in the lattice which usually results in the increase in resistivity.

The moisture present in the material provides conducting path to charge carriers and hence

decreases the resistivity.

The resistivity of these materials lies in the range of the semiconductors (several

ohms to several mega ohms) at different temperatures. Also their resistivity decreases with the

increase in temperature. On the basis of these properties, ferrites are considered as

semiconductors. The energy required by charge carrier to overcome the barriers while moving

from one point to another is called activation energy.

1.8Frequencydependentdielectricpropertiesofstrontiumhexaferrites

Two equal and opposite charges separated by a very small distance as compared with

the distance to an observer form an electric dipole. An electric dipole µ is given by

μ Qd 1.8

where ‘Q’ and ‘d’ is the charge and the distance between the two charges respectively.

The direction electric dipole moment µ is taken from the negative to the positive charge and

unit is C-m. Another unit of electric dipole moment is debye which is equal to 3.33 x10-30 C-

m.

When a high resistive (dielectric) material is exposed to an applied electric field, the

displacement of the charge inside the material takes place without transferring to electrodes

and give a rise to the formation of electric dipoles and as a whole the material is said to be

polarized and this process is known as polarization. The ability of the material to respond to

the field is called its polarizability (α).

There are five different types of polarization mechanisms named as electronic

polarization, ionic polarization, orientation polarization, interfacial polarization (Maxwell-

Wagner) and hyper electronic polarization. At lower frequencies (< 106 Hz), all polarization

mechanisms prevail. The total polarizability α due to all of these polarization mechanisms

may be expressed as α = αe + αi + αo+ αs.

1.8.1Electronicpolarization

When an atom is placed in an electric field, a deformation in the spherical charge

distribution of negative charge around the nucleus takes place. This leads to the electronic

polarizability αe which is the measure of the ease with which the charge centers may be

dislocated. The dipole moment produced due to this polarization is independent of frequency

and is directly related to the strength of the applied electric field. This polarization mechanism

is present in all materials.

The electronic polarizability αe can be calculated by making a supposition that the

atom is of perfect spherical shape. The electronic polarizability αe for one atom can be

calculated by

α 4πε R 1.9

where 0 is permittivity of free space and ‘R’ is the radius of atom [80].

1.8.2Ionicpolarization

In ionic materials, in addition to deformation of electronic cloud around the nucleus, a

displacement in the cations and anions also takes place. This displacement of ions gives a rise

to Ionic polarizability ‘αi’. It is more pronounced in weakly bonded molecules and prevails in

the frequency range (1012 - 1013 Hz).

Ionic polarizability ‘αi’ can be found by using the equation [80]

α qy. d

1.10

where ‘q’ is the charge; ‘y’ is Young’s Modulus and ‘d0’ is the separation between ions.

1.8.3Orientationpolarization

There are materials such as polar liquids, gases or polymers whose molecules possess

dipole moment even when no external field is present. The moments of these materials have

random directions. When these materials are placed in an external electrical field, their dipole

moments become align and leads to the dipolar polarizability αd.

Debye proposed a model to calculate orientation polarizability αd for the polar

materials with small dipole moment and field strength. Debye relation for orientation

polarizability αd is given by [81]

α μ3kT

1.11

where ‘µ’, ‘k’ and ‘T’ are the dipole moment, Boltzmann constant, temperature respectively.

The polarization produced due to the orientation of polar molecules is greater than

electronic polarization.

1.8.4Hyperelectronicpolarization

Hyperelectronic polarization occurs in some long polymeric molecules Pohl et al. [82,

83]. This polarization appears at low frequencies and is because of the pliant interaction of

charge pairs of excitons, located on long, polarizable polymers. The movement of charge

pairs to long range in large molecules will cause large polarization. The hyperelectronic

polarization is much greater to that of the electronic polarization in the band of frequency

ranging from several kHz to several MHz.

1.8.5Spacechargepolarization

Space charge polarization is also known as interfacial polarization. When the external

field is applied, the charge carriers move through the material and stop at grain boundaries,

cracks and defects. This movement causes large scale distortion inside the material and leads

to space charge polarizability αs. This interfacial polarization prevails up to (103 Hz). The

interfacial polarization is responsible for high dielectric response in many materials such as

polymers and ceramics [84]. The contribution of this polarization mechanism to the total

polarization is large at low frequency region (Hz to kHz). The high dielectric constant at

lower frequencies is mainly because of interfacial polarization [85]. The degree of the

polarization depends on the grain size and its morphology, number of grain boundaries and

defects as well as the difference in the conductivity between crystalline and amorphous

region [84, 86, 87]. Space charge polarization mechanism is very complicated and no

satisfactory models are available to calculate the interface polarizability yet. However

Maxwell-Wagner model is usually used to discuss polarization mechanism in polycrystalline

materials.

Fig 1.5 Schematic diagram of polarization mechanisms and their frequency response

Fig 1.6 Different polarization mechanisms in the absence and presence of an external electric field [88].

1.8.6Thedielectricconstant

Dielectric constant is the response of a material to an applied electric field. When a

highly resistive (dielectric) material is placed in a static electric field, it gets polarized

instantaneously and the dielectric constant is considered as real number. In a polarized

dielectric material, negative charge gather towards positive electrode and positive charge

gather towards negative electrode providing external electrical field. This displacement of

charge in a dielectric material gives a rise to electric dipoles. These electric dipoles, produced

due to the displacement of charge, create their own electric field which is opposite to the

applied field [89].

When the material is placed in an alternating electric field, the dielectric constant

varies with the variation in frequency. At lower frequencies, space charge polarization

mechanism is dominant as compared to other mechanisms. At relatively higher frequencies

(MHz), space charge polarization mechanism vanishes and dipole polarization mechanism

becomes dominant. In this way, at ultra high frequencies, the dielectric constant due to only

electronic polarization prevails.

When a dielectric material is subjected to an alternating field, the orientation of the

dipoles changes as the field reverses its direction. At lower frequencies of the applied field,

the orientation of dipoles can easily follow the applied field. At relatively higher frequencies,

the orientation of dipole starts lagging the applied field due to inertial effects and spatially

oriented defects and the dielectric constant becomes a complex quantity.

The complex dielectric constant is given by

ε∗ ε jε 1.12

where ‘*’, ‘’and ‘’ are complex, real part and imaginary part of dielectric constant

respectively.

The space charge polarization results in the high dielectric constant. Maxwell-Wagner

two layer model [90] is used to discuss the dielectric constant obtained because of space

charge polarization. According to this model, interfacial polarization takes place because of

two layers of the dielectric material. One thick layer acts as resistive medium while second

thin layer act as insulating medium. Using Maxwell-Wagner two layer model, one can derive

complex dielectric constant which is given by,

ε∗ εε ε1 jωτ

jσωε

1.13

where ‘’, ‘s’, ‘dc’, ‘’ and ‘0’ are dielectric constant for electronic polarization, dielectric

constant at dc field, conductivity, relaxation time and dielectric constant of free space

respectively and =2f.

The real part of dielectric constant obtained from above equation is given by

ε εε ε1 ω τ

1.14

For hexaferrites, the space charge polarization is discussed in term of charge hopping

mechanism. The hopping polarizability α depends upon the width and height of the potential

barrier between two sites and is given by [80],

αq r3kT

P A → B P B → A 1.15

where ‘q’ is the charge, ‘r’ is the distance between A and B sites, ‘k’ is the Boltzmann

constant, ‘T’ is the temperature, and P A → B P B → A is the average product of two

hopping probabilities.

1.8.7Thedielectricloss

The dielectric loss is the amount of energy dissipated in a material due to electrical

conduction, dielectric relaxation, dielectric resonance and loss from non-linear processes [30].

The dielectric loss in a material also takes place due to delay between the electric field and the

electric displacement vectors [91]. There are two types of dielectric losses namely intrinsic

and extrinsic losses. Intrinsic dielectric losses depend on the crystal structure, frequency of

applied field, temperature and the interaction of the phonon with the ac electric field [92, 93].

Extrinsic losses depends upon crystal imperfections such as impurities, microstructure

defects, dislocations, vacancies, porosity, grain boundaries, microcracks, random crystallite

orientation, dopant atoms etc. The extrinsic losses could be reduced by controlling the above

parameters during synthesis and other different processing. The crystals with different

symmetry groups and defects cause different losses at particular frequency and temperature

[92].

Energy losses in a dielectric material are mainly because of two factors.

1. Conduction of free carriers

2. Relaxation effects

The dielectric loss due to conduction of free carriers is given by

tanδ4πσωε

1.16

where =2f, is dielectric constant and is conductivity.

If a graph of log (tanδ) versus log ω is drawn then it would be a straight line. If dielectric loss

factor ε is proportional to 1/, the conduction is frequency independent and is considered as

dc energy loss.

Different impurities and defects present in dielectric material show relaxation effects.

The dielectric loss due to these relaxation effects could be found by using the equation

tanδ 1.17

These losses (due to relaxation effects) are different from that of conduction losses. The plot

of relaxation loss against frequency is not a straight line but it shows a maximum peak at

particular frequency.

The total loss could be obtained by adding the loss due to the conduction of free

charge carriers and due to relaxation effects and is given by

tanδ4πσωε

ε ε ωτ1 ω τ

1.18

At lower frequencies (kHz), the contribution of the first term on right hand side becomes

predominant and the contribution of second term is negligible.

In ferrites the dielectric losses are discussed using Koop’s phenomenological theory on the

basis of Maxwell-Wagner two layer model. According to Koop’s, the grain in a bulk material

acts as a resistor and grain boundary acts as thin insulating layer [94].

1.8.8LiteraturereviewaboutfrequencydependentelectricalpropertiesofSrM

When a highly resistive material is placed in a circuit for electrical isolation, then that

material is named as insulator and when the same material is placed in an electrical field then

that material is named as dielectric. When a high resistive (dielectric) material is placed in an

electric field, the displacement of the charge inside the material takes place without

transferring to electrodes and as a whole the material is said to be polarized and this process is

known as polarization. At lower frequencies (< 106 Hz), all polarization mechanisms such as

electronic polarization (atomic polarization), orientation polarization (dipole polarization),

ionic polarization, interfacial polarization (Maxwell-Wagner) prevail.

Ferrites are considered as highly resistive materials and are being widely used in the

devices operating at higher frequencies. In microwave devices, both intrinsic and extrinsic

losses are of significant importance. Intrinsic losses mainly arise due to the fundamental

interactions in ferromagnetic materials with in the magnetic system. The extrinsic losses are

developed because of the crystal imperfections (depends on the synthesis technique and

conditions), porosity, grain boundaries, surface roughness, polycystallinity (random local

anisotropy), slow and fast relaxing impurities (rare earth slow relaxers, Fe3+, Fe2+ hopping,

etc.).

In microwave devices, conduction, dielectric and magnetic losses are also of much

importance. These losses determine the performance of the device. The off resonance losses

in ferrite based microwave devices such as circulators and micro strip tunable filters plays

very important role in their performance [95].

The high-frequency ferrites (spinels, garnets and hexaferrites) are magnetically

anisotropic and gyromagnetic in nature. These characteristics originate from the precessional

motion of the magnetic moments. For a particular direction of biasing magnetic field, the

precessional motion of moments gives a sense of rotation in one direction. When the biasing

field is reversed, sense of rotation is also reversed. The frequency of the precessional motion

is proportional to the strength of biasing field. The strength of this biasing field depends on

the magnetocrystalline anisotropy field, demagnetizing field and applied magnetic field in

ferrites. When the sense of rotation of precessional motion of magnetic moments and applied

field (circularly polarized EM wave) is same then the interaction between them would be very

strong. Reversing the direction of EM wave (applied field) reverses the sense of rotation. The

strong interaction would be observed only in one direction of propagation of EM wave. Such

a direction dependent interaction of EM wave in ferrites enabled them to be used for

circulators, isolators and other non-reciprocal devices. The interaction in EM wave and

ferrite can be tailored by using variable applied field. This characteristic of ferrites allows

them to be used as phase shifters, filters and other tunable devices. The above interaction

becomes strongest at ferromagnetic resonance (FMR). At FMR, a strong absorption of

incident microwave energy (attenuation of a wave) takes place.

1.8.9FerromagneticresonanceandSnoek’sLimit

The ferromagnetic resonance is produced due to the precessional motion of a

ferromagnetic material when exposed to an external magnetic field. The external magnetic

field produces a turning effect in magnetization. This turning effect of

Fig 1.7 The precession motion of magnetic moment around the applied magnetic field.

external magnetic field results in the precessional motion of the magnetic moment. The

frequency of precessional motion is dependent on the orientation of the material and applied

magnetic field strength shown in figure 1.8.

This precession motion can be described by the Landau-Lifshitz-Gilbert equations

γM H∝

1.19

where ‘ϒ’ is the gyromagnetic factor of 28GHz/T, Heff is the effective DC field which

includes anisotropic field, demagnetization field and external applied field. ‘α’ is the damping

factor.

When a small ac field along with effective field is applied along the magnetic

moment direction to find ferromagnetic resonance (FMR), the mean magnetic moment M will

precess very close to Heff. In non-dissipated condition, the above equation becomes

∂M∂t

γM H 1.20

In the absence of external field, magnetization rotation dominates. This resonance is only

because of the internal anisotropic field given by:

ω γH γ2KM

1.21

where ‘K1’ is the crystalline anisotropy of material

According to Snok’s law, the product of permeability and ferromagnetic resonance

frequency (FMR) is a constant and is proportional to the saturation magnetization and is given

by:

ω μ 12γ3

4πM 1.22

As the saturation magnetization of a material has particular value so the permeability of that

material is also limited to specific value. This Snoek’s limit shows that the saturation

magnetization permeability and permeability of a material are directly related to each other

[96].

Ferromagnetic resonance frequency of ferrites is strongly affected by their

magnetocrystalline anisotropy field (HA). As the magnetocrystalline anisotropy field (HA) of

spinel ferrites is very small so their ferromagnetic resonance (FMR) frequency falls near or

below 1GHz. This limits the operating frequency of devices based upon spinel ferrites to C, S

and X-bands. The ferrites used in these devices are biased by permanent magnets. The field of

these permanent magnets is used to saturate the ferrite and to shift FMR frequency to higher

frequency thus increases the limit of operating frequency of devices. Spinel devices becomes

untenable above X-band frequency.

The garnets, another type of ferrites, possess excellent structural and chemical

stability. The garnet (Y3Fe5O12 (YIG)) is being widely used in the devices operating at

microwave frequency due to its low ferromagnetic resonance (FMR) line width, 0.6Oe [97],

which results in very small microwave loss. YIG is biased by using permanent magnets. The

operating frequency of YIG based devices is below 1–2GHz.

The magnetic damping due to ferromagnetic resonance (FMR) determines the total

loss in microwave devices. The line width of ferromagnetic resonance determines the

magnetic loss. The line width of single crystal is much narrow (low magnetic loss) than that

of polycrystalline materials (high magnetic loss). The ferromagnetic resonance line width of

M-type hexaferrites is very large. This characteristic of ferrite is used to develop various types

of microwave absorbing devices [98]. The operating frequency of ferrites is determined by

their ferromagnetic resonance (FMR) frequency.

At higher operating frequencies, the ferrites having large magnetocrystalline

anisotropy (M-type hexa ferrites) are used. M-type strontium hexaferrites generates very low

residual losses at high frequency application (GHz) than spinel (soft) ferrites due to their high

magneto crystalline anisotropy [99]. The magnetocrystalline anisotropy (HA) of M-type

hexaferrites is approximately 1000 times greater than that of spinel ferrites [17, 100, 101].

The zero field FMR frequency of these materials is about 36GHz. These materials also

required external magnetic field to saturate. The strength of the biasing field required to shift

ferromagnetic resonance (FMR) frequency to high frequency is substantially lower due to

high magnetocrystalline anisotropy.

M-type hexaferrite based devices could be operated at frequencies Ka-band which is far

below their resonance frequency.

Ferrite based devices can operate up to Ka-band (for below resonance operation). The

properties of M-type hexaferrites are greatly influenced by the distribution of cations present

at different interstitial sites of their crystal structure. The magnetocrystalline anisotropy (HA)

of these ferrites could be varied by substituting Fe, present on interstitial sites, with different

cations. The HA could be increased by doping Al, Cr and Ga [102]. This leads to increases

the operating frequency of device application upto and including U, E, and W bands [103,

104]. Conclusively the operating frequency of M-type hexaferrites and their substituted

compositions could be tailored from 1 to 100GHz. The large magnetocrystalline anisotropy

field (HA) also results in high remnant magnetization. Ferrite based microwave devices

require permanent magnet to bias the magnetic material to be operated at particular frequency.

The use of these permanent magnets hinders in the reduction of size and weight of devices.

The M-type hexaferrites are used as self-biased materials due to their large remnant

magnetization. So these materials are also useful for the reduction in size and weight of

devices.

The performance of ferrite based devices strongly depends on the ferromagnetic

resonance frequency (FMR) line width [105]. Hexaferrites are considered as strong candidate

for the devices operating at high frequencies because of their large magnetocrystalline

anisotropy field. This field could be used to bias these ferrite based materials.

The pollution produced by electromagnetic waves can be controlled by developing

such materials which could absorb these waves. Conducting materials could be used to shield

the electromagnetic waves but the reflection produced by these materials pollutes the

atmosphere. Keeping this problem in view, scientists paid considerable attention to develop

such materials which could absorb electromagnetic energy. Depending on the working

frequency (low or high), a variety of absorber materials are synthesized to suppress

electromagnetic waves [106-109]. Metallic magnetic material could not be used at relatively

higher frequencies due to their high eddy current losses. For high frequency electromagnetic

wave absorption, magneto-dielectric materials were developed. The development of

magnetic–dielectric absorbers received considerable attention due to their non zero complex

permittivity (r = - j) and permeability (µr = µ- jµ). The thickness of electromagnetic

wave absorbing material decreases by (µ)1/2 times [110]. Ferrite (spinel or hexagonal)

materials are used as magnetic fillers to prepare composites for different frequency band

width and thickness [23, 111].

Strontium hexaferrites (M-type ferrites) are highly resistive (108 -cm) with good

magnetic properties. These are low density materials and find a number of applications in

market due their economical production and high microwave magnetic losses, dc resistivity

and chemical stability [112-114]. Different researchers have reported their work on

hexaferrites for microwave applications [115-119].

A low dielectric loss is necessary for high performance devices as it may result in

higher efficiency and lower noise [120] which is especially important for high frequency

applications (e.g. in MHz). So if the dielectric losses of strontium hexaferrites are reduced to

very low value by controlling its microstructural properties and Fe2+ ions concentration then

the material would be more useful for high frequency applications.

M-type strontium hexaferrites also have lot of potential to find its frequency

dependent application due to their tunable electrical properties but their electrical properties

has not been reported too much in the literature. A very small work has been reported by

different scientists on the electrical properties of strontium hexaferrites. The scientists

working on strontium hexaferrites used different synthesis methods [56, 76] and dopants such

as Mn, Co, Ti [121, 122], SiO2 [123], Si-Ca [59], Ba [60], Pb [124], Ba-Ce-Ni [125], Al-Cr

[62], Zr–Ni [64], Al–Ga [65], Zr-Cu [126], Ca [127] and investigated their dielectric

properties in different frequency regions (MHz and GHz). But still there is a plenty of room to

improve their frequency dependent electrical properties by varying synthesis conditions and

doping elements with different concentration.

1.9 Mechanical properties of ferrites

The mechanical strength of a material is related to porosity. Strontium hexaferrites

have large porosity (low density) due to which their mechanical strength is low however their

compressive strength is high [33]. The mechanical properties such as hardness, tensile and

flexural strength of strontium hexaferrites have not been studied so far to much extent.

1.10 Thermal transport properties

This property though becomes very important when this material is used in heat generating

devices. Thermal properties of these materials could not get too much attention of the

researchers. The thermal conductivity, coefficient of linear thermal expansion and specific

heat are of great importance. Strontium hexaferrites are ceramic materials and have poor

thermal conductivity. Haberey et al. [128] reported that the linear thermal coefficient of

strontium hexaferrite is its axis dependent. It is 14.0× 106 K-1 when the sample is placed

parallel to c-axis and 10.0×106 K-1 when placed perpendicular to the c-axis. Specific heat

capacity, thermal diffusivity and thermal conductivity mainly depend upon microstructural

properties, temperature and chemical composition. According to the measurements of Hussain

and Maqsood [129] heat capacity per unit volume of strontium hexaferrite is 2.73 MJ m-3 K-1,

thermal diffusivity is 1.132 mm2 S-1 and thermal conductivity is 2.69 Wm-1 K-1. When the

temperature of strontium hexaferrite is raised, its heat capacity per unit volume decreases

while thermal diffusivity and thermal conductivity increases.

1.11Chemicalstability

Along with the above characteristics, these materials also show good chemical

stability. M-type hexaferrites are found to be stable in week acids such as citric acid, acetic

acid (CH3COOH) and phenol solutions. These are also stable in alkalis such as NH3, NaOH,

NaCl, KOH and other chemicals. But M-type hexaferrites are soluble in strong acids such as

H3PO4, H2SO4, HCl, HNO3 and HF.

1.12ApplicationsofM‐typehexaferrites

The demand of a material in market depends upon a number of factors such as its

physical properties and production cost. Before 1930s, AlNiCo alloys were used as hard

magnets. These materials were very brittle and their production methods were limited [130].

Hexaferrites, which are also considered as hard ferrites, were discovered around 1950s [131].

Very soon this material became work horse of permanent magnet market due to their low cost

production. As permanent magnets are hard and brittle so their machining is difficult. To

overcome this problem, bonded magnets are used. The most common bonded magnets are M-

type hexaferrites. These materials are widely used in different moving-coil instruments. These

instruments works on the principle that when a current carrying coil is placed in an external

magnetic field (permanent magnet field), a turning effect is produced. These permanent

magnets are also used in equipments used in research apparatus, industrial machinery and

consumer products. They are also used in various biomedical apparatus, generators, industrial

motors, power tools, audio/video equipment, printers, copiers, personal computers,

automobiles and household appliances [132].

The strontium hexaferrites (M-type hexaferrites) also find their applications in the

magnetic recording media-tapes and disks because of its large saturation magnetization.

Presently, most of the magnetic memories consist of islands which are magnetized into digital

bits. The beauty of this technology is that it provides more dense recording at relatively low

cost.

From the viewpoint of microwave devices operating at high frequency, ferrites,

because of their high dc electrical resistivity (low eddy current losses) and excellent magnetic

property, are widely used as dielectric material as compared to the metallic magnetic

materials. The working frequency of soft ferrites (spinel ferrite (MeFe2O4)) is limited to 1GHz

with wide bandwidth absorption. M-type hexaferrites are more useful for high frequency

(several GHz) absorption application due to their high magneto crystalline anisotropy which

results to very low residual losses in comparison with soft ferrite [99, 133].

At frequencies, above 1 GHz, electrical energy cannot be transmitted through wires.

It is transmitted in the form of electromagnetic waves through space or contained in wave-

guides. These electromagnetic waves (microwaves) are controlled by magnetic components.

These magnetic components possess excellent dielectric properties at microwave frequencies.

These materials transmit electromagnetic waves with small losses. These components are

called phase shifters, circulators and Faraday rotators. These components are widely used in

space communication, in controlling satellites, aircrafts and radars [33].

In different systems such as satellite communication system, wireless communication

system, radar and precise guidance system, microwaves of high frequency (several gigahertz)

are used. Electromagnetic interface (EMI) in gigahertz range is a big issue and miniaturization

of devices is also of great interest [134]. The development of microwave absorbers to reduce

back scattering has got much attention of scientists [135]. These materials are of great

importance for microwave absorber coatings on military aircrafts for radar jamming. M-type

hexaferrites are famous for their large magneto crystalline anisotropy. The working frequency

(GHz) of these materials can be varied by tailoring anisotropy field [136, 137].

1.13Motivationandobjectives

M-type hexaferrites (MFe12O19 where M=Ba, Pb or Sr) find many technologically

demanding and challenging applications because of their high dc electrical resistivity, high

Curie temperature, better chemical and thermal stability, high corrosion resistance and large

uniaxial magnetocrystalline anisotropy. M-type hexaferrites are being widely used in

automobile industry, dc electric motors, data storing devices, magneto-optic recording media,

door catchers, loud speakers, plasteferrite, injection-molded pieces, microwave devices and a

lot more [106, 138-140]. In the present work, strontium hexaferrite is selected due to its better

electrical and magnetic properties, high Curie temperature than other M-tpe hexaferrite

family, easy to handle because of its non-toxicity.

The properties of this ferrite are largely dependent on the processing routes used for

its fabrication [18, 42, 141]. A number of synthesis methods have been used to prepare

hexaferrites. Some methods are complex and expensive while some of them are resulted in

the formation of secondary phases and required very high annealing temperature for longer

time to reduce the impurity phases. Hussain et al. [124] prepared M-type hexaferrite by

standard ceramic method and annealed the samples at 900C for 10 h and even then

secondary phases were not removed. Kikuchi et al. [61] prepare M-type hexaferrite with

polymerizable complex method. He annealed his samples at temperature above 900C for 24

h to get single phase hexaferrite. Narang et al. [60] prepared M-type hexaferrites by

conventional ceramic method and annealed the sample at 1250C for 20 h but impurity phase

was not removed. Ghasemi et al. [56] prepared M-type hexaferrites by sol–gel method. The

viscous residue was heated at 200C for 20 h to get dried gel. This dried gel was annealed at

1000 C for 1 h. The XRD indicates the presence of small amount of impurity phase. Jacobo

et al. [63] prepared hexaferrites by using self combustion technique and annealed the powder

at 1100C for 2h but could not get a rid of impurity phases. Iqbal et al. [64] synthesized M-

type strontium hexaferrites by co-precipitation technique. The samples were annealed at

925C for 1h and got single phase strontium hexaferrite powder. Charalampos et al. [142]

annealed barium hexaferrites, prepared by co-precipitation method, at 920C for 2h. This

literature survey has motivated us to use co-precipitation and sol-gel method for the synthesis

of strontium hexaferrites because these are very simple methods and require low annealing

temperature for shorter time for phase purity. These wet chemical methods also provide

greater chemical homogeneity, greater reactivity, high purity and fine particle size with

narrow distribution [33].

The phase purity and particle size of hexaferrites, prepared by co-precipitation

method, are strongly dependent on different synthesis parameters such as volume rate at

which the precipitating agent is added and the pH of the solution. Different synthesis

parameters and post synthesis treatments [63, 127, 142-144] have been used by researchers to

control impure phases. In this thesis work, we analyzed that how the synthesis parameters

(molar ratio of cations, volume rate at which the precipitating agent is added and the pH of the

solution) affect the phase purity and particle size of strontium hexaferrite material. For this

purpose, molar ratio of cations (Fe/Sr) is varied from 12 to 08 with step size of 1, volume rate

at which the precipitating agent is added (NaOH(aq)) is varied from 30ml/min to 2000ml/min

and pH is varied from 13 to 08 with step size of 1.

Strontium hexaferrites (M-type hexaferrites) are known to be the work horse of

permanent magnet market due to their high magnetocrystalline anisotropy. A lot of work has

been done to tailor the magnetic properties of these materials for different applications such as

permanent magnetic for dc motors, data storing devices and bio-medical applications.

The electrical properties of strontium hexaferrite materials are also of much

importance but have not been reported too much in the literature. The operating frequency

limit of ferrite based devices is strongly affected by their magnetocrystalline anisotropy field

(HA). The magnetocrystalline anisotropy (HA) of M-type hexaferrites is approximately 1000

times greater than that of spinel ferrites [17, 101, 131]. The magnetocrystalline anisotropy

(HA) and hence operating frequency limit of M-type hexaferrites based microwave devices

such as circulators, isolators and other non-reciprocal devices, phase shifters, microwave

absorbing devices could be tailored by using different dopants.

Özgür et al.[102] reported that Cr causes the increase in magnetocrystalline

anisotropy of M-type hexaferrites. Keeping this observation in view, the composition SrFe12-

xCrxO19 with X 0.0, 0.2, 0.4, 0.6, 0.8 is prepared by simple co-precipitation method and

the structural, electrical and magnetic properties these Cr varying sample are studied. This

composition has been prepared by co-precipitation method and its dielectric properties have

not been reported yet.

A low dielectric loss is necessary for high performance devices as it may result in

higher efficiency and lower noise [120] which is especially important for high frequency

applications. So if the dielectric loss of strontium hexaferrites is reduced to very low value by

controlling its microstructural properties and Fe2+ ions concentration then the material would

be more useful for high frequency applications.

Angeles et al. [145] reported that Zn2+ ions preferentially occupy tetrahedral A and

may also occupy bipyramidal C sites where it replaces Fe3+. For charge neutrality, the Fe2+

ions present on octahedral B site [127, 146] are converted into Fe3+ ions. This results in the

decrease in the Fe2+ ions concentration responsible for dielectric loss tangent. Tetrahedral site

occupancy of Zn2+ leads the ferrites to increase their saturation magnetization.

To increase both coercivity and saturation magnetization (permeability), Cr and Zn

doped strontium hexaferrite having composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 is synthesized by co-precipitation and WOWS sol-gel

method (WOWS stands for Without Water and Surfactants). The effect of this composition on

structural, electrical and magnetic properties has been studied. Strontium hexaferrites with

this composition and with the above synthesis techniques has been studied for the first time to

best of our knowledge.

Strontium hexaferrites are being widely used as microwave absorbing coatings for

radar jamming purpose. For such type of coatings, the material should be non toxic and

possess high chemical and thermal stability [112-114]. The material should have high

dielectric constant and high permeability to reduce the thickness of coating layer and high

microwave magnetic loss.

It is reported in literature that when oxygen is reduced from crystalline strontium

hexaferrite at elevated temperatures (800-900)C, then some of the Fe3+ ions present on

interstitial sites becomes free iron atoms which causes the increase in saturation

magnetization. It was expected that it would also result in the increase in carrier concentration

and hence dielectric constant.

Keeping this in view, single phase strontium hexaferrites is hydro-nitroginated at

850C for one hour and its effect on dielectric properties is studied. The study of dielectric

properties of oxygen reduced strontium hexaferrites has not been reported so far to best of our

knowledge.

Chapter2CharacterizationTechniques

2.CharacterizationTechniques

2.1X‐raydiffraction

To study the structural properties of a material, X-ray diffraction technique is

commonly used. When a beam of fast moving electrons, travelling in evacuated tube, strikes

on the surface of a material (target), X-rays are produced. These X-rays are considered as

characteristics of that material. Most of the X-ray diffractometer contain Cu as target material.

The X-rays, generated from Cu, strikes on the surface of the material under testing.

If ‘d’ is the distance between the planes (considered as the characteristics of a

material) and ‘’ is the wave length of X-rays then according to Bragg’s law.

= 2dsinɵ 2.1

where ‘ɵ’ is the angle between the plane of the crystal and incident X-ray beam.

Fig 2.1: Geometrical illustration of Bragg’s law [147]

For constructive interference this path difference should be the integral multiple of

the wavelength so the above equation becomes

n= 2dhkl sinɵ 2.2

where ‘hkl’ are Miller indices

Using the data of the peaks, obtained from XRD patterns of polycrystalline hexagonal

structure, different structural parameters such as lattice constant (a & c), crystallite size (D),

cell volume (V) and X-ray density (ρx) can be calculated by using the following formulae.

The lattice parameters (a & c) of hexagonal crystal structure can be calculated by using the

formula

1 43

2.3

where ‘hkl’ are Miller indices and ‘d’ is the interplaner spacing.

The crystallite size ‘D’ is calculated by using Full Width at Half Maximum (FWHM),

obtained from the diffraction peaks in Scherrer’s formula given by

D0.89Cosɵ

2.4

where ‘’ is wave length, ‘’ is FWHM and ‘ɵ’ is the Bragg angle

The volume of unit cell can be calculated from formula

V √32a c2.5

X-ray density (theoretical density) is calculated by the formula

ρ 2.6

where ‘n’ is number of formula units per unit cell, ‘Mm’ is the molar mass, ‘V’ is the volume

of unit cell and ‘NA’ is the Avogadro’s number.

Porosity is calculated by the formula

P 1ρρ

2.7

In the present work, variation in structural parameters and phase formation of the

synthesized material was examined by X-ray diffraction machine (PANalytical X'pert pro

PMD) with Cu-Kα as the radiation source. The machine was operated at 40kV and 30mA.

An image of PANalytical X'pert pro MPD X-ray diffractometer is shown in figure 2.2.

2.2ScanningElectronMicroscopy(SEM)

X-ray diffraction provides information about the internal structure of crystal such as

interplaner spacing, lattice constant and hence provides the information about the type of

crystal structure. But XRD does not provide the image of the particles of a material. To get

direct image of particle, electron microscopy is used. The principle of electron microscope is

similar to that of optical microscope and the only difference is of wave length and hence

resolution. The wave length of electron beam used in electron microscope is much smaller

(about 5 orders of magnitude) than that of visible light. This short wave length results in high

resolution and provides much clear scans of short structures.

Fig 2.2: PANalytical X'pert pro MPD X-ray diffractometer

In scanning electron microscope, the secondary and back scattered electrons emitted

from the surface of the material under testing are received by a detector and amplified. The

variation in emitted and back scattered electrons is converted in to SEM image. This provides

highly resolved details of small structures. This technique helps a lot in understanding the

minute variation in the surface morphology of a material.

When the electron beam strikes the surface of a material under testing, the surface gets

charged and its electric field defocuses the electron beam. To avoid this problem, the surface

of the sample should be conducting or made conducting by a very thin coating of a

conducting material. This coating layer is grounded to ground any charge accumulated on

surface.

In the present work, the surface morphology and grain size of the prepared material

was studied by the field emission scanning electron microscope (FE-SEM) JEOL JSM-6700F.

An image of JEOL JSM-6700F scanning electron microscope (SEM) in our department is

shown in figure 2.3.

Fig 2.3: JEOL JSM-6700F scanning electron microscope (SEM)

2.3Frequencydependentacmeasurements

Frequency dependent electrical properties are measured by precision component

analyzer (6400B). This system provides both two probe and four probe methods to take

different measurements in the frequency range 20Hz to 3MHz. The ac measurement derive

level of this instrument varies from 1mV to 10V rms. This instrument measures real and the

imaginary parts of an impedance vector. Then it converts them into the required parameters

automatically. Impedance is the measure of the total opposition to the flow of an alternating

current at particular frequency offered by the circuit or device. It is an important parameter to

characterize an electronic component or instrument. The real part of impedance vector is

named as resistance R and imaginary part is named as reactance X. In terms of rectangular

coordinates, the impedance vector is expressed as Z=R+jX. The reciprocal of the impedance

(1/Z) gives admittance (Y). The expression for admittance becomes Y=G+jB where G

represents conductance and B susceptance. The SI unit of impedance is ohm (Ω) and

admittance is siemen (S).

The quality factor Q determines the purity of reactance. This term is more often used

for inductors. For capacitors, the term dissipation factor (tanδ) is commonly used. The

dissipation factor (tanδ) is usually expressed in terms of tanδ and is defined as the ratio of the

energy dissipated by the component to the energy stored in a component.

The values of capacitance (C) obtained from precision component analyzer at

different frequencies are used to calculate dielectric constant () using the formula:

εCdAε

2.8

where ‘d’, ‘A’ and ‘0’ are the thickness, area of pellet and is the permittivity of free space

respectively.

Dissipation factor (D) obtained from precision component analyzer at different

frequencies is taken as dielectric loss tangent (tanδ).

Dielectric loss factor (ε´´) is calculated by using the equation

ε ε tanδ2.9

Frequency dependent ac conductivity (σac) is calculated by using the equation

σ ε ε tanδ2.10

2.4Temperaturedependentdcresistivitymeasurements

During literature survey it was observed that the electrical properties of M-type hexaferrites

yet could not get too much attention of scientists although their electrical properties are of

much importance. These materials have lot of applications in market (especially in high

frequency devices) because of their large dc resistivity and good dielectric properties.

The dc electrical resistivity is usually measured by two methods.

1. Two probe method

2. Four probe method

Usually two probe method is used for the materials having electrical resistivity much higher

than the resistivity of contact probs. The figure 2.4 (b) shows that the resistivity of the probes

also adds up in the resistivity of the material but as their resistivity is much smaller than the

resistivity of the material so can easily be ignored.

Secondly two probe method is better to use for resistivity measurements at high temperatures

(400 C). On the other hand, four probe method, shown in figure 2.4 (b), is used where the

material have very low electrical resistivity. In this method, the resistivity of contacts is not

included in the resistivity of the material under consideration. Usually silver

paste is used for connections which becomes unstable at high temperatures.

Fig 2.4: (a) Two-terminal and (b) four-terminal resistance measurement arrangements[148]

Fig 2.5: Circuit diagram of the apparatus used for dc electrical resistivity measurements

Strontium hexaferrites are considered as very high resistive material so two probe

method is used for their temperature dependent resistivity measurements. The circuit diagram

of dc resistivity measurement system is shown in figure 2.4. The sample is sandwiched

between two electrodes. The sample along with electrodes is placed in a tube furnace. The

furnace is connected with ac power supply and its temperature is controlled by a power

regulator. The electrodes are connected with dc source. The pressure contacts are made

between electrodes and pellet. A volt meter and a sensitive ammeter are connected in parallel

and series with the circuit respectively. The variation in current due to change in temperature,

with step size of 5C, is noted. The values thus obtained are used to calculate temperature

dependent dc resistivity and activation energy of the samples.

The dc resistivity ‘ρ’ is calculated by using the formula

ρ RAL2.11

where ‘R’ is the resistance, ‘L’ is thickness and ‘A’ is the area of cross-section of pellet.

The activation energy is calculated the Arrhenius relationship given by

ρ ρ exp∆Ek T

2.12

where ‘ρ’, ‘ρ0’, ‘ΔE’, ‘kB’ and ‘T’ are resistivity at temperature T, resistivity extrapolated to

1/T = 0, constant, activation energy, Boltzmann’s constant and temperature respectively

[123].

The drift mobility (µd) is calculated by using equation

μ1neρ

2.13

where ‘e’ is charge of electron, ‘ρ’ is resistivity and n is the concentration of charge carriers given by the equation

nN D P

M2.14

where ‘NA’ is the Avogadro’s number, ‘DB’ is the bulk density, ‘PFe’ is

the number of iron atoms and ‘M’ is the molecular weight of the chemical formula.

2.5Magnetichysteresis

The dc magnetometer (RIKEN DENSHI) was used to obtain hysteresis loop and

hence different magnetic parameters. The electromagnet can generate the field up to 3 tesla.

The North and South Pole faces have the diameter (ф = 50 mm) and can be separated up to

100 mm. The dc magnetometer provides both MH-loop and BH-loop in a single step with

controlled sweep rates. The image of the dc magnetometer (RIKEN DENSHI) is shown in

figure 2.6.

The magnetic parameters such as coercivity Hc is taken directly from loop table while

Ms is calculated by using formula:

M 2.15

The Ms thus obtained is divided by density of sample in order to get its values in emu/g

Fig 2.6: The dc magnetometer (RIKEN DENSHI)

Fig 2.7: A typical hysteresis loop along with data obtained from the dc magnetometer

Chapter 3

Synthesis techniques

3. Synthesis techniques

The properties of a material are greatly dependent upon microstructure of nano

particles. The microstructural stress, grain size, , vacancies, phase purity, impurities, porosity,

surface defects etc. are largely affected by the synthesis and post synthesis technique

followed. These parameters could be controlled to large extent by choosing appropriate

synthesis technique along with suitable preparation conditions. Scientists have used different

synthesis techniques to prepare strontium hexaferrites.

There are two different routes to synthesis a material.

1. Solid state reaction method

2. Wet chemical methods

3.1 Solid state reaction method

In solid state reaction method, individual oxides are mixed in stoichiometric

quantities and are ground well to obtain homogeneous mixture. The grinding is made by

different methods such as ball milling, mortar and pestle etc. The ground powder is subjected

to annealing usually at elevated temperatures for longer time. This results in large particle size

with wide particle size distribution. Appearance of impurity phases also remain a big

issue[59, 149, 150].

3.2 Wet chemical method

Wet chemical method is also considered as non conventional method which includes

1. Precursor method

2. Spray-drying method

3. Freeze-drying method

4. Microemulsion method

5. Hydrothermal method

6. Sol-gel method

7. WOWS sol-gel method

8. Co-precipitation method

3.2.1 Precursor methods

The precursor method provides a precise stoichiometry for the synthesis of ferrites. In

this method the precursors, containing the reactants in the required stiochiometry, are

decomposed upon heating and ferrites are formed.

3.2.2 Spray-drying

Precipitates of a material could be produced by the evaporation of solvent from

concentrated solution of cations. In order to keep the particle size small, the concentrated

solution is converted into fine droplets using high pressure and solvent is removed by a

stream of hot gas. The particles obtained are converted into a compact powder and annealed

to get required phase of material.

3.2.3 Freeze-drying method

This method also involves the conversion of concentrated solution of cations into fine

droplets but these droplets are rapidly freezed by passing them through a bath at very low

temperature such as liquid nitrogen or ice-acetone. The particles obtained are dried by

sublimation of the ice in vacuum.

3.2.4 Microemulsion method

Microemulsion is the dispersion of two immiscible liquids particles stabilized by a

surface active coating [151]. It is the mixture of oil, water and surfactant. The oil is mixture of

olefins and hydrocarbons. In this technique, the surfactants formed an interface between oil

and water. The surfactant forms hydrophobic tails dissolved in oil and the hydrophilic head

dissolved in the aqueous phase. The nature of surfactant plays very important role in the

synthesis of nano materials. Water oil microemulsion method has been used by the scientists

in synthesizing metals, halides and oxides [152-156]. Different surfactants have been used by

the scientists for the synthesis of M-type hexaferrites [151, 157-160]. Microemulsion is high

cost technique [161]. Microemulsion technique involves the probability of absorption of

surfactants at the surface of nano particles which acts as impurity [162]. This method yields

very small material and faces reproducibility problems [163, 164].

3.2.5 Hydrothermal method

It is the technique used to crystallize a material by using aqueous solution at high

temperature and pressure. Hydrothermal synthesis takes place in a closed container in which

water is used as solvent. When this container is heated, a very high pressure is produced in the

container and the reaction which takes place at such elevated pressure is known as

hydrothermal reaction. When the temperature of a liquid increases from its critical point, the

liquid is said to supercritical and the fluid behaves both like liquid as well as gas. The surface

tension of that liquid becomes negligible and dissolve the compound easily. The critical

temperature and pressure of water is 374 C and 218 atm respectively. Different compounds

such as SrFe12O19 have been synthesized by this method [53, 165-168]. But the reaction rate is

slow and in order to increase the reaction rate ultrasonic, electric field or microwaves are used

which increases their cost. The hydrothermal system in which ultrasonics are used for heat

treatment is called ultrasonic-hydrothermal. Similarly the hydrothermal system using

microwaves and electric fields for heating are called microwave-hydrothermal and

electrochemical-hydrothermal respectively[169-171].

3.2.6 Sol-gel method

This method is also known as chemical solution deposition method. This method is

widely used for the synthesis of nano materials starting from chemical solutions (sol) called

precursors. Most of the precursors are metal chlorides and alkoxides which refer to various

forms of hydrolysis and polycondensation reactions. The sol is converted into a solid called

gel. This gel contains some solvent which is removed by a drying process. The rate of

removal of solvent determines porosity and microstructure of final product. The gel obtained

is converted into required material after heat treatment. This heat treatment improves the

crystallinity and densification of the material. In this method pH, temperature and

composition of the solution determine the particle size [157]. The gel formed by this method

takes long time to dry. It also requires high annealing temperature for long time to get phase

purity.

3.2.7 WOWS sol-gel method

The WOWS sol-gel method has number of advantages over other sol-gel methods.

This method does not require surfactants or templates [172].

In WOWS sol-gel method, stoichiometric amounts of different precursors are

dissolved in ethylene glycol. The molar ratio between metal salts to ethylene glycol is kept at

1:14 to dissolve the metal salts homogeneously. The pH of solution was 1. The solution is

stirred for 30 min at room temperature in order to prepare homogeneous solution. The

temperature of this solution is raised to 100oC with continuous stirring. By doing so, a thick

gel is formed. The gel so obtained is heated to 300oC. At 300oC, the gel dried and was burned

slowly and transformed into fine powder.

The proposed reaction for all the nitrates with ethylene glycol may be like following

3.2.8 Co-precipitation method

In co-precipitation reactions, the solution of the salts, required for final compound,

are prepared. Usually water soluble salts are used but the salts which are not soluble in water,

are dissolved in acids. The solutions of all the precursors are mixed and a precipitating agent

(basic solution such as solution of NaOH, KOH, NH4OH etc) is added. With the addition of

precipitating agent, nucleation (precipitates) sites are developed and growth of grains and

their agglomeration starts to become more thermodynamically stable particles [173]. The

number of nucleation sites, their growth and agglomeration depends upon pH, temperature,

stirring speed and time of solution [174]. Faster the nucleation process, the smaller would be

the particle size with narrow size distribution [163]. In co-precipitation reaction, chlorides,

OFe

CH2

CH2

OH

OH

CH2

CH2

OFe

OH

+ Fe(NO3).9H2O

CH2

CH2

OFe

nitrates or acetates are formed which are removed during washing process and hydroxides are

removed during heat treatment. The synthesis of our work is based on co-precipitation method

because it has number of advantages such as it provides [33]

1. Greater homogeneity

2. Greater reactivity

3. High purity - no grinding

4. Fine particle size with narrow distribution

5. Elimination of calcinations

This method is also famous for its low cost and provides reproducible results. Co-

precipitation provides low porosity, greater homogeneity of small grain sizes and cation

distribution in them. Because of these excellent properties, we used co-precipitation

method for the synthesis of our material (M-type strontium hexaferrites). WOWS sol-gel is

also used to compare the results obtained with co-precipitation method. The steps followed

for the synthesis of strontium hexaferrites by co-precipitation method are shown in figure 3.1.

Fig 3.1: Schematic diagram for the chemical co-precipitation method.

Chapter 4

Optimization of synthesis parameters for phase purity

4. Optimization of synthesis parameters for phase purity

Strontium hexaferrite is synthesized by chemical co-precipitation technique. Different

synthesis parameters such as molar ratio (Fe/Sr), volume rate at which the precipitating agent

is added and pH of solution is optimized. The effect of variation in molar ratio (Fe/Sr) on the

phase purity of strontium hexaferrite is observed from X-ray diffraction patterns. Different

volume rate of addition of precipitating agent are used and their effect on phase purity and

structural morphology is studied by using X-ray diffraction patterns and scanning electron

micrographs (SEM images). The pH of the solution plays a major role in the formation of a

precipitates. The pH of the solution was varied and its effect on phase purity, structural

morphology and electrical properties is studied

4.1 Synthesis

In the present work, the chemicals used for the synthesis of different sample series

were of analytical grade and their details are given below in table 4.1.

Table 4.1: List of the chemicals used with their specification

S. No. Compound Formula Molar mass % Purity Company

01 Strontium nitrate Sr(NO3)2 211.63 g 99.0 Merck

02 Ferric nitrate nine

hydrate Fe(NO3)3.9H2O 404.00 g 98.0 Merck

03 Chromium nitrate

six hydrate Cr(NO3)3.6H2O 400.21 g 98.0 Uni-Chem

04 Zinc nitrate nine

hydrate Zn(NO3)3.9H2O 297.40 g 98.0

Sigma-

Aldrich

05 Sodium hydro-

oxide NaOH 40.00 g 99.0

Sigma-

Aldrich

For the compositions of strontium hexaferrite synthesized by co-precipitation

technique [143], all the salts (Fe(NO3)3.9H2O & Sr(NO3)2) were dissolved in de-ionized water

with required molarities. The solutions of the precursors were mixed and heated on hot plate

with violent stirring. When the temperature of the solution was reached to 70C, calculated

amount of 2M NaOH solution added abruptly and maintained the pH of the solution at

130.03. This final solution was stirred for one hour to get fine particle size with greater

homogeneity. The precipitates thus obtained were washed well with de-ionized water. The

washed precipitates were dried at 105C in an electric oven. The dried precipitates were

crushed in to fine powder and then annealed at 925C in a programmed furnace. The

temperature was increased at the rate of 5C/min. The annealed powder was converted in

pellets by using uniaxial press and then sintered at 910C in furnace for 20min. These

sintered pellets were used for different characterizations. All the composition of strontium

hexaferrites with different dopants such Cr and Cr-Zn were synthesized under the similar

conditions mentioned above.

4.2 Effect of variation in molar ratio (Fe/Sr) on structural properties of SrM

The chemical formula of M-type strontium hexaferrite is SrFe12O19. According to this

formula, the molar ratio between Fe and Sr is 12. But researchers used different molar ratio in

order to get single phase material [143, 175]. In order to investigate the effect of molar ratio

on phase purity, the molar ratio is varied from 12 to 08 with step size of one and its effect on

phase purity is studied by X-ray diffraction analysis. Indexed XRD patterns of molar ratio

varying samples are shown in figure 4.1.

Fig 4.1: Indexed XRD patterns of SrFe12O19 for different molar ratios (MR=Fe/Sr)

It can be seen from the figure that all XRD patterns are almost similar and no

impurity peak is present. It shows that variation in molar ratio (Fe/Sr) in the range 12-08 does

not affect phase purity of strontium hexaferrite. We used theoretical molar ration (Fe/Sr) =12

for the synthesis of all compositions.

4.2.1 Conclusion

A systematic study is made to know that how molar ratio (Fe/Sr) affects the phase

purity of hexaferrites. Different samples with varying molar ratio in the range 12-08 has been

prepared and characterized. This was required as one can find only a scattered study in the

already reported like Chen et al. [[143]] used molar ratio (Fe/Sr) greater than 12. Ataie et al.

[176] prepared strontium hexaferrites by co-precipitation method and used molar ratio (Fe/Sr)

8 to get phase pure hexaferrite material. Iqbal et al. [[65]] synthesized strontium hexaferrites

by chemical co-precipitation method and used 11 molar ratio (Fe/Sr) and got single phase

material. The effect of variation in molar ratio (Fe/Sr) on phase purity is studied by using

XRD patterns. The indexed XRD patterns indicate that almost all the patterns are similar and

no impurity peak is detected. Thus variation in molar ratio in the range 12-08 does not affect

the phase purity of strontium hexaferrites.

4.3 Volume rate of addition of precipitating agent on structural

properties of SrM

During optimization of different synthesis parameters, it is observed that volume rate

at which the precipitating agent is added, also greatly affect the formation of phase pure

material. Number of samples was prepared by varying only volume rate at which the

precipitating agent is added and its effect on phase purity and structural morphology was

observed by using X-ray diffraction patterns (XRD) and scanning electron microscope

(SEM). Indexed XRD patterns of the samples with different volume rate at which the

precipitating agent is added, are shown in figure 4.2.

The XRD patterns clearly indicate that as the volume rate at which the precipitating

agent is added, decreases, the impure phase increases. It is concluded from the above results

that abrupt addition of precipitating agent results in more pure phases as compared to drop by

drop addition.

The effect of volume rate of addition of the precipitating agent on structural morphology was observed from SEM micrographs shown in figure 4.3. The figure shows the variation in particle size with the change in volume rate at which the precipitating is added.

30 40 50 60 70

160ml/min

2000ml/min

30ml/min

*

**

*

*

(4 4

0)

(0 1

4)

(0 2

12

)

(1 1

12

)(0

0 1

4)

(0 3

2)

(1 2

4)

(0 1

11

)

(0 1

10

)

(0 2

5)

(0 2

3)

(0 1

8)

(1 1

4)

(0 1

7)

(1 1

2)

(0 0

8)

Inte

nsity

(a.

u.)

2

*

(Deg.)

- Fe2O3

A

B

C

Fig 4.2: Indexed XRD patterns of the samples with different volume rate of addition

of the precipitating agent

Fig 4.3: SEM micrographs of the samples with different volume rate of addition of the precipitating agent

Clearly the particle size obtained is smaller in case of abrupt addition of precipitating agent as

compared to drop by drop addition. It is concluded from these results that abrupt addition of

precipitating agent is useful to obtain single phase strontium hexaferrite with small particle

size. So abrupt addition of precipitating agent is used in the synthesis of all composition

presented in this work.

4.3.1 Conclusion

During optimization of different synthesis parameters, it was observed that volume

rate of addition of precipitating agent also imparts a major effect on phase purity of strontium

hexaferrites. The effect of volume rate of addition of precipitating agent on phase has been

studied by using XRD and SEM micrographs. The indexed XRD patterns indicate that the

increase in volume rate of addition of precipitating agent improves the phase purity. The SEM

micrographs show that the particle size decreases with the increase in volume rate of addition

of precipitating agent. This may be due to the reason that the high volume rate of addition of

precipitating agent produces more nucleation sites than that with slow addition. As the

number of nucleation sites increases, the agglomeration of particles decreases. So high

volume rate of addition of precipitating agent is good to get phase pure and small particle

sized strontium hexaferrite material. This parameter has not been studied so far to the best of

our knowledge and its addition to the literature would be very useful for the scientists

working on strontium hexaferrites.

4.4 Effect of variation in pH on structural and electrical properties of strontium hexaferrites (SrM)

In co-precipitation technique, pH of the solution is the key factor which imparts its

large influence on the particles homogeneity, shape, size and their distribution and hence on

the electrical properties of the synthesized material. Keeping these effects in view, pH varying

samples (from 13 to 8 with step size of one) of strontium hexaferrites are synthesized in order

to investigate its effect on phase purity, particle size with its distribution and on electrical

properties.

4.4.1 Effect of pH on the structural properties of SrM

The structural properties of strontium hexaferrite samples, with different pH are

investigated from XRD patterns and SEM micrographs. The indexed XRD patterns of pH

varying samples are given in figure 4.4. Indexed XRD patterns show that as pH is decreased

from 13, extra phase of α-Fe2O3 started appearing and increases with the further decrease in

pH. Parameters such as crystallite size (D), lattice parameters a (Å) and c (Å), cell volume (V)

and X-ray density (ρx) are calculated from indexed XRD patterns and are given in table 4.2.

Structural morphology is studied from SEM micrographs and is given in figure 4.3. The

micrographs of pH varying samples of strontium hexa-ferrite shows that the particle size

increases with the decrease in pH and most of the particles have hexagonal structure with

diameter in the range of 0.4–3.5µm.

The particle size distribution also decreases on increasing the pH of the solution and

becomes very narrow for pH=13. This behavior is different as reported by Hessien et al.

[144]. The particle size distribution of pH varying samples is given in table 4.2. As the pH

increased, the rate of nucleation sites formation increased which resulted in decrease in

agglomeration.

Fig 4.4: Indexed patterns of XRD of samples of SrFe12O19 for different values of pH

(- SrFe2O4, # -α-Fe2O3)

Table 4.2: Lattice constants (a & c), crystallite size (D114), cell volume (V), X-ray density (ρx), bulk density (ρm), % α-Fe2O3 and particle size

pH=13 pH=12 pH=11 pH=10 pH=09 pH=08

Lattice Constant a (Å)

c (Å)

5.82(5)

23.12(4)

5.88(2)

23(6)

5.86(3)

22.84(9)

5.87(2)

23.03(4)

5.89(2)

23.04(2)

5.86(3)

23.06(4)

Crystallite size (D114 )

(nm)

52 52 70 59 69 59

Volume V (Å3) 677(13) 687(7) 690(10) 686(7) 693(5) 686(10)

X-ray Density ρx (g/cm3) 5.21 5.13 5.11 5.14 5.09 5.14

Bulk Density ρm (g/cm3) 3.27±0.02 3.18±0.03 3.39±0.02 3.53±0.02 3.29±0.03 3.63±0.04

% age of α-Fe2O3 0 5 10 16 20 24

Particle Size (µm) 0.6-0.9 0.5-1.2 0.5-1.5 0.5-2.0 0.8-3.0 0.4-3.5

Fig 4.5: SEM micrographs of the sintered samples of SrFe12O19 for different values of pH

4.4.2 Effect of pH on dc electrical properties of SrM

Temperature dependent dc electrical resistivity measurements

The variation in electrical current was measured due to the variation in temperature at

constant voltage by using two probe method shown in figure 2.4 (a). The data thus obtained

was used to calculate dc electrical resistivity of samples with varying pH. Figure 4.6 clearly

indicates that the dc resistivity decreases with the increase in temperature so M-type ferrites

behave like semiconductors [17]. This is due to the fact that the kinetic energy of the electrons

increases with the rise in temperature. The rate of hopping of electrons from one octahedral

site to the other increases and hence the resistivity decreases. Moreover the decrease in

resistivity with the decrease in pH is because of variation in particle size. As the particle size

increases, the number of grain boundaries decreases which acts as resistive medium and hence

resistivity decreases. The activation energy obtained from the slope of linear fit of the plot of

resistivity measurements also decreases with the increase in pH. It is mainly due to the

decrease in grain boundaries.

Effect of temperature on drift mobility of charge carriers is shown in figure 4.7. The

graph of drift mobility verses temperature shows that it increases with the rise in temperature.

This is because the electrons start moving easily from one interstitial site to another due to the

increase in temperature. The values of activation energy and drift mobility at 603K are given

in the table 4.3.

15 20 25 30 35 408

10

12

14

16

18

20

22

24

pH=08pH=09pH=10pH=11pH=12pH=13Linear fit

ln

(

-cm

)

1/kBT (eV)-1

Fig 4.6: Plot of ln of dc electrical resistivity of SrFe12O19 for different values of pH as a function of temperature

350 400 450 500 550 600 650 700 750

0.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

pH=08

d

V-S

c

m-2

Temperature (K)

0

1x104

2x104

3x104

4x104

5x104

d

V-S

c

m-2

pH=09pH=10pH=11pH=12pH=13

Fig 4.7: Plot of drift mobility versus temperature of SrFe12O19 for different values of pH as a function of temperature

Table 4.3: Dielectric loss (tanδ), dielectric constant (έ), dc electrical resistivity (ρdc) and drift mobility (μd) of SrFe12O19 for different values of pH

.

pH=13 pH=12 pH=11 pH=10 pH=09 pH=08

tanδ (1kHz) 1.64(2) 2.05(3) 2.36(03) 2.70(2) 3.65(2) 4.00(4)

tanδ (3MHz) 0.061(1) 0.063(1) 0.082(1) 0.142(1) 0.305(1) 0.341(1)

ε´ (1kHz) 211.0(3) 158.0(2) 146.0(2) 128.0(2) 117.0(2) 111.0(2)

ε´ (3MHz) 30.0(1) 28.0(1) 26.0(1) 23.0(1) 20.8(1) 14.6(1)

ρdc (Ω-cm) at

603K 1.66x107 1.20x107 5.90 x106 1.12 x107 8.28x105 4.96x104

4.4.3 Effect of pH on frequency dependent ac electrical measurements of SrM

Dielectric constant

The dielectric properties of ferrites could be explained by Maxwell–Wagner two layer

model in section 1.8.6. The first layer is composed of large number of grains that acts as

conducting layer at higher frequencies and the other layer consists of grain boundaries that act

as highly resistive medium at lower frequencies. The polarization and conduction mechanism

in ferrites are similar i.e. by electron exchange between ferrous (Fe2+) and ferric (Fe3+) ions

[126].

The charge polarization takes place by electron hoping between ferrous (Fe2+) and

ferric (Fe3+) ions. As the applied field frequency increases, it becomes more difficult for the

electron to hope from ferrous (Fe2+) and ferric (Fe3+) ions with the alternating frequency, the

net displacement of charge in one direction decreases and hence dielectric constant decreases.

It is observed that at relatively lower frequencies dielectric constant is high. It might be

because of moisture, voids, dislocations, density and impurities [177]. The Figure 4.8 shows

that the dielectric constant (ε') depends upon the pH value of the samples. It decreases with

the decrease in pH and this effect is prominent at low frequencies. This is because of the grain

size and

6 8 10 12 140

40

80

120

160

200

240

280

pH=08pH=09pH=10pH=11pH=12pH=13

Die

lect

ric C

onst

ant

(')

ln f

Fig 4.8: The plot of dielectric constant (έ) as a function of ln of frequency of SrFe12O19 for different values of pH

secondary phase (α-Fe2O3) which increases with the decrease in pH. The concentration of Fe2+

ions decreases due to which the polarization decreases and hence dielectric constant decreases

[178]. This could also be explained by Koops [179] model according to which the dielectric

constant at low frequency is because of grain boundaries. These grain boundaries acts as high

resistive medium and thus contribute to high dielectric constant. As the decrease in pH results

in increase in the grain size and decrease in grain boundaries and hence decrease in dielectric

constant [180].

Dielectric loss tangent (tanδ)

The figure 4.9 shows that the dielectric loss (tan δ) for all samples is greater at low

frequencies and decreases rapidly with the increase in frequency. At lower frequencies, high

dielectric loss may be because of impurities, crystal defects and moisture. The decrease in

dielectric loss with increasing frequency is because of charge polarization. As the applied

field frequency increases, the polarization lags the alternating field. The net polarization

decreases and hence dielectric loss decreases. The figure 4.9 also shows that at lower

frequencies, dielectric losses increases with the decrease in pH and becomes nearly uniform at

higher frequencies. This is because of conduction losses due to the electron hoping between

Fe2+ and Fe3+ ions [177]. Figure 4.5 indicates that resistivity of the samples decreases with the

decrease in pH and hence conductivity increases. As the decrease in pH results increase in the

grain size and decrease in grain boundaries. Thus conductivity increases and conduction

losses increases. The values of dielectric constant (ε') and dielectric loss tangent (tanδ) of

samples of SrFe12O19 for different values of pH is given in table 4.3.

6 8 10 12 14

0

1

2

3

4

5

6

7


Die

lect

ric L

oss

Tan

gent

(ta

n)

ln f

Fig 4.9: The plot of dielectric loss tangent (tanδ) as a function of ln of frequency of SrFe12O19 for different values of pH

The dielectric loss factor (ε'') is calculated by using equation 2.12 [181]. The plot of

dielectric loss factor (ε'') as a function of frequency for all samples is shown in the figure

4.10. The trend is similar to dielectric loss (tan δ) as shown in the figure 4.8. At lower

frequencies, high dielectric loss factor (ε'') may be because of impurities, crystal defects and

moisture. Hudson et al. has shown that the dielectric losses in ferrites are mainly because of

conduction mechanism due to space charge polarization. As the frequency of the applied

electric field increases, the charges could not follow the alternating field. As a result the

polarization decreases and hence

6 8 10 12 14

0

200

400

600

800

1000


Die

lect

ric

Loss

Fac

tor

('')

ln f

Fig 4.10: The plot of dielectric loss factor (ε'') as a function of log of frequency of SrFe12O19 for different values of pH

dielectric loss decreases. The figure 4.10 also shows that at lower frequencies, dielectric

losses factor (ε'') increases with the decrease in pH and becomes nearly uniform at higher

frequencies. This is because of the similar reason as explained above in dielectric loss (tan δ).

These results are consistent with dc resistivity measurements shown in figure 4.6. As the

decrease in pH results in increase in the grain size and decrease in grain boundaries. Thus

conductivity increases and conduction losses increases and hence dielectric loss factor (ε'')

increases.

4.4.4 Conclusion

pH of the solution plays a key role in controlling microstructural parameters and

phase purity of a material synthesized by co-precipitation method. In order to study the effect

of pH on structural and electrical properties of strontium hexaferrites, pH of the solution is

varied from 13 to 08. Indexed XRD patterns indicate that as pH increases the impurity phase

of α-Fe2O3 decreases and a single phase strontium hexaferrite is obtained for the sample

prepared by keeping pH=13. SEM micrographs show that the increase in pH results in the

decrease in particle size as well as its distribution. This may be due to the high pH of the

solution results in the increase in the number of nucleation sites and hence decreases the

possibility of agglomeration. The temperature dependent dc electrical resistivity

measurements indicate that dc resistivity increases with the increase in pH of the solution.

The increase in resistivity is due to the decrease in particle size which results in the increase

in the grain boundaries acting as highly resistive medium. The frequency dependent dielectric

loss decreases with the increase in pH of the samples. This behavior is also attributed to the

decrease in grain size. As the grain size decreases, the resistive area (grain boundaries)

increases and hence dielectric loss decreases. The decreasing trend of dielectric loss is

consistent with the variation in dc resistivity of samples prepared by varying pH. Both

structural and electrical measurements show that high pH of the solution is useful for the

synthesis of strontium hexaferrites.

Chapter 5

Structural, electrical and magnetic properties of Cr doped strontium hexaferrites

5. Structural, electrical and magnetic properties of Cr doped strontium hexaferrites

In ferrites, the conduction is because of the electron hopping from Fe2+ and Fe3+ ion

present on octahedral B sites [78, 79]. The presence of this Fe2+ also contributes a lot to the

dielectric properties of ferrites. It is reported in the literature that Cr3+ ions preferentially

occupy octahedral B sites [182] where it replaces Fe3+ ions. It impedes the motion of charge

carriers and results in tne increase in coercivity. The aim of the present work is to study the

effect of Cr substitution on the interstitial sites of strontium hexaferrite on frequency

dependent dielectric properties such as dielectric constant, dielectric loss tangent and

frequency dependent ac conductivity which has not been reported. In this work, effect of Cr

substitution on structural, electrical (temperature and frequency dependent) and magnetic

properties of strontium hexaferrite has been discussed.

5.1 Structural properties of Cr doped SrM

The crystallographic structure and phase formation of the composition SrFe12-

xCrxO19 with X 0.0, 0.2, 0.4, 0.6, 0.8 sintered at 940C is studied by using powder X-ray

diffraction data. All the peaks of X-ray diffraction (XRD) patterns shown in figure 5.1 are

identified by using ICDD patterns with reference code 01-080-1197. The indexed XRD

patterns show that all the compositions have major phase of strontium hexaferrite material.

The secondary phase of α-Fe2O3 showed an increasing trend with the increase in Cr content.

All the XRD patterns confirmed the successful substitution of Cr cations on the interstitial

sites of strontium hexaferrite lattice as there was no separate peak of these substituted cations

detected except for X=0.8. In this composition, a peak of Cr2O3 was appeared indicating that

the composition become over saturated with Cr and Cr formed its oxide instead of replacing

Fe3+ ions on interstitial sites of strontium hexaferrites. Parameters calculated from indexed

XRD patterns are given in table 5.1.

25 30 35 40 45 50 55 60 65 70

(1 1

2)

(0 0

8)

2 (Deg.)

Inte

nsity

(a.

u.)

0.4

0.6

0.8 (2 0

14

)

(2 1

8)

Cr2O3

#

(2 0

2)

*

(1 0

4)

(0 2

13)

(0 2

12

)

(1 1

12)

(0 0

14)

(0 3

2)

(0 2

7)

(0 1

10

)

(0 2

5)

(0 2

3)

(0 1

8)

(1 1

4)

(0 1

7)

*

-Fe2O3#

0.0

0.2

Fig 5.1: Indexed XRD patterns of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

( α-Fe2O3, # Cr2O3)

5.2 Frequency dependent ac electrical properties of SrM

For solids, Maxwell-Wagner two layer model is being commonly used to discuss

their dielectric properties. According to Maxwell-Wagner two layer model, the grain in a bulk

material acts as a resistor and grain boundary acts as thin insulating layer discussed earlier.

The dielectric properties are measured at room temperature in the frequency range (10kHz-

3MHz) using precision component analyzer.


density (ρm), porosity, activation energy (ΔE), dc electrical resistivity (ρdc), dielectric

constant (), dielectric loss tangent (tanδ), ac conductivity( σac), coercivity (Hc) and

saturation magnetization (Ms) of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8).

0.8 0.6 0.4 0.2 0.0 SrFe12-xCrxO19

5.878(2)

23.019(3)

3.92

5.888(5)

23.027(5)

3.91

5.889(2)

23.043(3)

3.91

5.888(2)

23.039(3)

3.91

5.884(2)

23.040(4)

3.92

Lattice Constant

a(Ȧ)

c(Ȧ)

c/a

58 39 61 81 44 Crystallite Size (D114) (1nm)

5.12 5.11 5.10 5.10 5.10 X-ray Density ρx (g/cm3)

3.02(2) 2.92(3) 3.00(2) 2.72(1) 2.88(2) Bulk Density ρm (g/cm3)

41 43 41 47 43 % Porosity

0.453(8) 0.444(8) 0.923(6) 0.391(4) 0.94(1) Activation Energy (eV)

1.92×106 4.23×105 2.23×108 4.11×105 1.78×108 DC resistivity ρdc (Ω-cm) at 200oC

59.0(1) 120.0(2) 37.0(1) 94.0(2) 23.0(1) Dielectric constant (ε') at 3MHz

0.240(1) 0.300(1) 0.210(1) 0.370(1) 0.320(1) Dielectric loss (tanδ)

at 3MHz

2.34×10-3 6.06×10-3 1.29×10-3 5.9×10-3 1.29×10-3 ac conductivity ac (S-m-1) at 3MHz

5.85 6.32 6.07 5.96 5.37 Coercivity Hc (kOe)

15.82 21.31 36.09 35.64 47.55 Saturation Magnetization Ms

(emu/g)

In ferrites, as described earlier, the polarization mechanism is similar to their

conduction mechanism i.e. because of the electron hopping from Fe2+ and Fe3+ ion present on

octahedral B sites. The presence of this Fe2+ contributes a lot to the dielectric properties of

ferrites. It is reported in the literature that Cr3+ ions preferentially occupy octahedral B sites

[182] where it replaces Fe3+ ions and increases the concentration of Fe2+ ions by transforming

Fe3+-Fe2+ for higher Cr content.

9 10 11 12 13 14 150

50

100

150

200

250

300

350

400

450 SrFe12-x

CrxO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Co

nst

an

t (

')

ln f


SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

5.2.1 The dielectric constant (ε')

The graph shown in figure 5.2 reveals that the dielectric constant ε' for all

compositions decreases sharply at lower frequencies and then becomes fairly constant for

higher frequencies. The dielectric polarization mechanism in ferrites is similar to their

conduction mechanism. The electron hoping on octahedral sites easily follow an external field

with low frequency and hence more the displacement of charge could take place inside the

grains and pile up at poorly conducting grain boundaries and hence causes large dielectric

constant. At high frequency, the hoping of the charge could not follow the applied field and

only local charge polarization could take place and hence net dielectric constant decreases.

0.0 0.2 0.4 0.6 0.80.0

0.5

1.0

1.5A

ctiv

atio

n e

ne

rgy

(eV

) Activation Energry Dielectric Constant at 3MHz

Cr Concentration

0

20

40

60

80

100

120

140

160

180

200

Die

lect

ric

con

sta

nt ( )

Fig 5.3: Plot of activation energy and dielectric constant at 3MHz versus Cr

concentration (X)

The figure 5.3 indicates that ε' have a wave like trend with the increase in doping content.

This could be explained on the basis of random distribution of Cr3+ ions on octahedral (2a and

12k) sites. As the polarization in ferrites is mainly due to hopping of electrons between ions

of the same element present in more than one valence state and the hopping probability

depends upon the separation between the ions involved and the activation energy [183]. The

distance between two iron atoms present on 12k sites is smaller than on any other site due to

small bond angle [184] so the presence of Fe2+ ions on 12k plays major role on the electrical

properties of strontium hexa-ferrites. It can be seen that dielectric constant increases for

X=0.2. It is possible that Cr3+ (0.755 Ȧ) ions may substitute Fe2+ (0.77 Ȧ) ions present on

octahedral (2a) site due to their similar ionic radii and for charge neutrality, Fe3+ ions present

on octahedral (12k) site transform into Fe2+ ions. As a result, concentration of Fe2+ ions on

12k sites increases and consequently polarization increases and hence dielectric constant

increases. It may also be due to the decrease in activation energy given in table 5.1. For

X=0.4, the decrease in dielectric constant may be due to the reason that Cr3+ ions may replace

Fe3+ present on 12k sites. The presence of Cr3+ ions on 12k sites impedes the motion of charge

carriers from Fe2+ to Fe3+ ions [185, 186] and thus increases the activation energy and

decreases the dielectric constant. For X=0.6, the increase in dielectric constant may be

attributed to the increase in Fe2+ ions concentration as higher Cr content causes the

transformation of Fe3+ - Fe2+ ions [187]. This may also be due to decrease in the activation

energy. For X=0.8, the decrease in dielectric constant, along with random distribution of Cr3+

ions, may also be due to the increase in activation energy.

5.2.2 The dielectric loss tangent (tanδ)

The general trend of the dielectric loss tangent (tan) shown in figure 5.4 could be explained

in terms of conduction losses which are the main source of dielectric losses. The major

source of conduction losses is the concentration of Fe2+ on octahedral sites. A relation

between conduction and dielectric losses was discussed by Iwauchi et al. [188]. As charge

polarization is similar to conduction mechanism in ferrites. In lower frequency region, the

dispersion in dielectric loss tangent is more than at high frequency region. It is due to the fact

that lower the frequency, more the time available for the displacement of the charge inside the

grains, more would be the conduction and higher would be the dielectric losses. As the

frequency increases, the electron hopping could not follow the applied frequency, the

conduction inside the grain decreases and hence dielectric loss tangent decreases. The figure 4

also reveals that the dielectric loss tangent has similar trend to that of dielectric constant with

the increase in doping concentration. This is mainly due to the variation in Fe2+ ion

concentration on 12k sites due to the substitution of Cr3+ on octahedral sites (explained

earlier) and activation energy. The compositions, where Fe2+ ion are more on 12k sites and

low activation energy, have higher dielectric losses and vice versa. Initially the dielectric loss

tangent increases due to the increase in Fe2+ ion concentration on 12k site, decrease in the

activation energy and high porosity (high moisture). Similarly it decreases due increase in

Cr3+ concentration on 12k site which impedes the motion of charge carriers, increase in the

activation energy and due to decrease in porosity (low moisture). The again increase in

dielectric loss tangent (X=0.6) may be attributed to the increase in Fe2+ ions concentration

discussed above. It decreases for X=0.8 due to increase in activation energy. The values of

dielectric loss tangent obtained from this composition of strontium hexaferrite are much

smaller than reported in [24, 65] so making this composition more suitable for high frequency

applications.

9 10 11 12 13 14 150

2

4

SrFe12-x

CrxO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Lo

ss T

an

ge

nt

(ta

n

ln f


SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

5.2.3 The dielectric loss factor ()

The dielectric loss factor () [76] is calculated by using formula given in equation

2.12. The plot of the dielectric loss factor () shown in figure 5.5 indicates that it decreases

initially with the increase in frequency and then becomes fairly constant at higher frequencies.

It is because the hoping of electron could not follow the applied frequency and consequently

the dielectric loss factor () decreases. The variation in dielectric loss factor () with the

increase in Cr doping is due to the random distribution of Cr3+ ion on octahedral (2a and 12k)

sites, change in activation energy and Fe2+ ions concentration, explained in detail in the

previous section.

9 10 11 12 13 14 150

400

800

1200

1600

2000SrFe

12-xCr

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Lo

ss F

act

or

'')

ln f


SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

5.2.4 The ac conductivity (ac)

The ac conductivity (ac) is calculated by using formula given in equation 2.13.

Figure 5.6 reveals that ac remains approximately constant at lower frequencies and then starts

increasing at higher frequencies. The hoping frequency of electron is small at lower

frequencies and it increases with the increase in frequency. This is attributed to the fact that

the required energy correlated with forward-backward hoping is only a fraction of the energy

necessary to activate long range diffusive conduction [189]. The variation in ac conductivity

with doping concentration is due to the variation in activation energy, Fe2+ ion concentration

and site occupancy of Cr ions.

9 10 11 12 13 14 150.0000

0.0008

0.0016

0.0024

0.0032

0.0040

0.0048

0.0056 SrFe12-x

CrxO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

ac (

S-m

-1)

ln f


SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

5.3 Temperature dependent dc electrical properties of SrM

The temperature dependent dc electrical resistivity was measured in the range of

373K-623K. Plot of ln of dc electrical resistivity as a function of 1/kBT is shown in figure

5.7. The dc electrical resistivity decreases with the increase in the temperature. This is due to

the fact that the kinetic energy of the electrons increases with the rise in temperature. The rate

of hopping of electrons from one octahedral site to the other increases, the drift mobility of

charge carriers increases and hence the resistivity decreases. The dc electrical resistivity of

ferrites mainly depends upon porosity, particle size and cations distribution on octahedral

sites. Higher porosity and smaller particle size cause the increase in dc electrical resistivity.

The Fe2+ ion concentration on octahedral site contributes a lot to the dc electrical resistivity.

Higher the concentration more would be the charge carriers available for conduction and

smaller would be the resistivity. The plot also indicates that the resistivity first decreases and

then increases with the increase in Cr concentration. This is due to variation in Fe2+

concentration on octahedral 12k site. The initial decrease (X=0.2) may also be due to the

increase in crystallite size which results in the decrease in grain boundaries (highly resistive

medium) and decrease (X=0.4) due to decrease in crystallite size and increase in activation

energy. For X=0.6, the dc resistivity decreases although the crystallite size decreases. This is

attributed to the increase in Fe2+ ion concentration and decrease in activation energy. The

resistivity again increases (X=0.8) due to the increase in activation energy and random

distribution of Cr3+ ion. The variation in dc electrical resistivity with increase in Cr content is

consistent with the variation in activation energy and dielectric measurements. Activation

energy was calculated from the slope of the linear plots of lnρ versus reciprocal of the

temperature shown in figure 5.7.

18 20 22 24 26 28 30 3210

12

14

16

18

20

22 X=0.0 X=0.2 X=0.4 X=0.6 X=0.8 Linear Fit

ln

cm

)

1/kBT (eV)-1

Fig 5.7: Plot of lnρ as a function of 1/kBT for SrFe12-xCrxO19

(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit.

5.4 Magnetic properties of Cr doped SrM

The magnetic parameters such as coercivity (Hc), saturation magnetization (Ms) and

remanence (Mr) calculated from hysteresis loops of the composition SrFe12-xCrxO19 with X

0.0, 0.2, 0.4, 0.6, 0.8 are shown in figure 5.8. The saturation magnetization (Ms) decreases

with the increase in Cr concentration. It is due to the substitution of Cr3+ cations on different

interstitial sites. In M-type

Fig 5.8: Hysteresis loops of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

hexaferrite, 24 Fe3+ ions are distributed on five different interstitial sites. There are three

octahedral (2a, 12k and 4f2), one tetrahedral (4f1), and one trigonal bipyramid (2b) site. The

sites (2a, 12k and 2b) are parallel and (4f1 and 4f2) are antiparallel. The iron atoms present on

these sites are coupled by super exchange interactions through the O2- ions, form the

ferrimagnetic structure[190, 191]. The M-type hexa-ferrite (magnetoplumbite structure)

contains two formula units per unit cell. 12 Fe3+ ions are arranged with eight spins in the up

direction and four in the down direction, giving a net moment of 4 Fe3+ ions per formula unit

times 5µB per ion, which gives a total of 20µB per formula unit [33].

0.0 0.2 0.4 0.6 0.8

5.4

5.6

5.8

6.0

6.2

6.4

Ms

(em

u/g

)

Hc (k

Oe

)

Cr concentration

Hc

16

20

24

28

32

36

40

44

48

-O- Ms

Fig 5.9: Plot of coercivity and saturation magnetization versus Cr concentration of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

According to ferrimagnetic theory [192], magnetism in ferrite originates from the net

magnetic moment of ions with spin up and spin down in interstitial sites. Figure 5.9 indicates

that saturation magnetization decreases with the increase in doping content. The saturation

magnetization mainly depends upon the distribution of Cr3+ ions on interstitial sites and

magnetic dilution with the substitution of Fe3+ ions by the lower magnetic moment ions. As

Cr3+ ions with magnetic moment (3 µB) preferentially occupy 12k and 2a sites with spin up so

net magnetic moment decreases and hence saturation magnetization decreases [182]. This

decrease may also be due to the appearance of non magnetic phase α-Fe2O3. Figure 5.9 also

indicates that coercivity (Hc) first increases for X0.6 and then decreases. The increase in

coercivity is attributed to the continuous increase in non magnetic phase α-Fe2O3 which acts

as pinning centers for domain wall motion. This increase could also be explained by using the

equation [193] given below:

H2KM

5.1

where ‘HA’ is the magnetocrystalline anisotropy field, ‘K1’ is constant and ‘Ms’ is saturation

magnetization.

According to equation 5.1, the decrease in saturation magnetization (Ms) results in an increase

in the magnetocrystalline anisotropy and hence in intrinsic coercivity.

Different factors such as Cr concentration, grain size and porosity contribute to the variation

in coercivity but different concentration, different factors are dominant. A sharp increase in

coercivity for X=0.2 is due to increase in the porosity and non magnetic phase α-Fe2O3. It

increases to very small value for X=0.4 due to decrease in crystallite size and increase in α-

Fe2O3 and increases sharply for X=0.6 due to sharp increases in porosity and decrease in

crystallite size. For X=0.8, the coercivity decreases due to decrease in porosity.

5.5 Conclusion

Cr doped strontium hexaferrites with composition SrFe12-xCrxO19 (X=0.0, 0.2, 0.4,

0.6, 0.8) has been prepared with co-precipitation method. The indexed XRD patterns indicate

that Cr doping causes the formation of secondary phases. For X=0.8, a peak of Cr2O3 is

appeared indicating the solubility limit of Cr in strontium hexaferrites. Hysteresis loops of the

samples with different Cr concentration indicate an increasing trend in coercivity. The

increase in coercivity is mainly due to increase in impurity phases. These impurity phases acts

as pinning centers and resist the domain wall motion.

Abbas et al. [194] has synthesized Cr doped strontium hexaferrites with composition

SrFe12-xCrxO19 (X=0.0, 0.1, 0.3, 0.5) by solid state reaction method. The sintering temperature

used by them is greater than 1200 ºC for 2h while the similar composition synthesized by us

with co-precipitation method is sintered at much lower temperature (940 ºC) for one hour.

The coercivity obtained by the above authors for X=0.5 is around 4.3 kOe which is much

smaller than that obtained in our case (>6 kOe) for similar composition. So Cr doped

strontium hexaferrite synthesized by co-precipitation method is better than that of solid state

reaction method with respect to both energy (sintering temperature) and magnetic properties.

As the increase in the coercivity results in the increase in the limit of the operating frequency

of the devices so Cr doped strontium hexaferrite synthesized by co-precipitation method is

more useful for the devices operating at high frequency.

Chapter 6

Structural, electrical and magnetic properties of Cr-Zn doped strontium hexaferrites prepared by

co-precipitation method

6. Structural, electrical and magnetic properties of Cr-Zn doped strontium hexaferrites prepared by co-precipitation method

In most of the electronic devices operating at high frequency, materials with very low

dielectric loss are required. One of the major sources of dielectric loss is the concentration of

charge carriers. In ferrites, the charge carriers are provided by Fe2+ ions. It is reported that

when Zn2+ being a divalent ion is doped in M-type hexaferrites, it preferentially occupy

tetrahedral (A) site where it replaces Fe3+ ions. For the charge neutrality, Fe2+ ions present on

octahedral (B) are changed to Fe3+ ions and hence the concentration of Fe2+ ions decreases.

Keeping this thing in view, Zn2+ was doped to decrease the concentration of Fe2+ ions in the

structure and hence to decrease the dielectric losses. It is reported that Cr (after certain

concentration) impedes the motion of charge carriers and previous study indicates that it

increases the coercivity and hence increases the operating limit of high frequency devices. So

keeping these observed results in view, Cr was also doped to reduce dielectric losses and

making the material useful for high frequency applications.

In the present work, Cr and Zn ions are substituted on the interstitial sites of strontium

hexaferrite and their effect on structural, electrical and magnetic properties of strontium

hexaferrites has been studied.

6.1 Structural properties of Cr-Zn doped SrM

The phase formation studies of the composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 are made by using powder X-ray diffraction data. All the

peaks of X-ray diffraction (XRD) patterns shown in figure 6.1 are identified by using ICSD

patterns with reference code 01-080-1197. All the XRD patterns confirmed the successful

substitution of Cr and Zn cations on the interstitial sites of strontium hexaferrite lattice as

there was no separate peak of these substituted cations detected. The impurity phase of α-

Fe2O3 is vanished for higher concentration of zinc. It might be due to the lower activation

energy of zinc ferrites. The parameters calculated from XRD patterns are given in table 6.1.

Table 6.1: Lattice parameters (a & c), average crystallite size (Dav), X-ray density

(ρx), bulk density (ρm), porosity, activation energy, dc electrical resistivity (ρdc), dielectric

constant (ε'), dielectric loss tangent (tanδ) and ac conductivity (σac) of the prepared samples of

SrFe12-2xCrxZnxO19 X 0.0, 0.2, 0.4, 0.6, 0.8 .

.

0.8 0.6 0.4 0.2 0.0 SrFe12-2xCrxZnxO19

5.883(9)

23.036(9)

3.91

5.884(2)

23.037(9)

3.91

5.884(3)

23.028(8)

3.91

5.882(8)

23.028(2)

3.91

5.884(2)

23.040(4)

3.92

Lattice Constant a(Å)

c(Å)

c/a

32 42 30 48 44 Crystallite Size (D114) (nm)

5.11 5.11 5.11 5.11 5.10 X-ray Density ρx (g/cm3)

2.49(2) 2.51(1) 2.58(2) 2.65(3) 2.88(2) Bulk Density ρm (g/cm3)

51 50 49 48 43 % Porosity

0.64(1) 0.67(1) 0.76(1) 0.84(1) 0.94(1) Activation Energy (eV)

3.65×106 5.5×106 1.75×107 3.08×107 1.78×108 DC resistivity ρdc (Ω-cm) at 200oC

8.6(1) 11.1(1) 16.5(1) 18.9(1) 23.0(1) Dielectric constant (ε') at 3MHz

0.020(1) 0.080(1) 0.150(1) 0.200(1) 0.320(1) Dielectric loss (tanδ)

at 3MHz

2.44×10-5 1.48×10-4 4.14×10-4 6.32×10-4 1.25×10-3 ac conductivity ac (S-m-1) at 3MHz

Structural morphology was studied by using scanning electron microscopy. The SEM

micrographs figure 6.2 showed that most of the particles are of hexagonal shape and their size

has increasing trend with the increase in Cr-Zn concentration. The average particle size for all

compositions was estimated by using diagonal method and its variation with Cr-Zn

concentration is given in the figure 6.3.

25 30 35 40 45 50 55 60 65 70

Inte

nsity

(a.

u.)

X=0.0

2 (Deg.)

X=0.2

X=0.4

X=0.6

X=0.8

(1 0

4)

*

(0 3

2)

(0 0

14)

(1 1

12)

(0 2

12)

(0 3

8)

(0 2

7)

(0 1

10)

(0 2

5)

(0 2

4)

(0 2

3)

(0 2

1)(1

1 4

)(0

1 7

)(1

12)(0

0 8

)

*

-Fe2O

3

Fig 6.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

( α-Fe2O3)

6.2 Frequency dependent ac electrical properties of Cr-Zn doped SrM

For solids, Maxwell-Wagner two layer model is being commonly used to discuss

their dielectric properties. According to Maxwell-Wagner two layer model, the grain in a bulk

material acts as a resistor and grain boundary acts as thin insulating layer discussed earlier.

The dielectric properties are measured at room temperature in the frequency range (10kHz-

3MHz) using precision component analyzer.

Fig 6.2: SEM micrographs of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

0.0 0.2 0.4 0.6 0.8

200

300

400

500

Gra

in S

ize

(n

m)

Cr-Zn concentration (X)

Fig 6.3: Plot of grain size (nm) as a function of Cr-Zn concentration (X) for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

In ferrites, as described earlier, the polarization mechanism is similar to their

conduction mechanism i.e. because of the electron hopping from Fe2+ and Fe3+ ion present on

octahedral B sites. The presence of this Fe2+ also contributes a lot to the dielectric properties

of ferrites. It is reported in the literature that Cr3+ ions preferentially occupy octahedral B sites

[182] where it replaces Fe3+ ions and increases the concentration of Fe2+ ions by transforming

Fe3+-Fe2+ for higher Cr content. Zn2+ ions preferentially occupy tetrahedral A and may also

occupy bipyramidal C sites [145] where it replaces Fe3+. For charge neutrality, the Fe2+ ions

present on octahedral B site [127, 146] are converted into Fe3+ ions.

6.2.1 The dielectric constant ()

The graph shown in figure 6.4 reveals that the dielectric constant ε' for all

compositions decreases sharply at lower frequencies and then becomes fairly constant for

higher frequencies. The dielectric polarization mechanism in ferrites is similar to their

conduction mechanism. At low frequency, the electron could easily follow the applied field

and the time available for electrons to hope from one site to other sites is more. Hence more

the

9 10 11 12 13 14 150

20

40

60

80

100 SrFe12-2x

CrxZn

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Co

nst

an

t

')

ln f


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

9 10 11 12 13 14 15

0.0

0.4

0.8

1.2

1.6

2.0SrFe

12-2xCr

xZn

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Lo

ss T

an

ge

nt

(ta

n

ln f


SrFe12-xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

displacement of charge could take place inside the grain and hence causes large dielectric

constant. At high frequency, the hoping of the charge could not follow the applied field and

only local charge polarization could take place and hence net dielectric constant decreases.

The graph also indicates that ε' has a decreasing trend with the increase in doping contents.

This is due to the fact that in ferrites, the space charge polarization directly depends upon Fe2+

ion concentration in a grain. As Zn2+ ions have strong preference to occupy tetrahedral (A)

sites, so concentration of Fe2+ ions on tetrahedral site decreases as explained earlier. So

electric polarization decreases and consequently ε' decreases. This may also be due to the

reason that the Cr ions do not participate in the conduction process but impedes the

transformation of Fe2+ - Fe3+ ions [185, 186].

6.2.2 The dielectric loss tangent (tan)

The general trend of the dielectric loss tangent (tan) shown in figure 6.5 could be

explained in terms of conduction losses which are the main source of dielectric losses. A

relation between conduction and dielectric losses was discussed by Iwauchi et al. [188].

According to Iwauchi, charge polarization in ferrites is similar to their electrical conduction

mechanism. In lower frequency region, the dielectric loss tangent is more than at high

frequency region. It is due to the fact that lower the frequency, more the time available for the

displacement of the charge inside the grains which acts as a conductive medium, more would

be the conduction and higher would be the dielectric losses. As the frequency increases, the

electron hopping could not follow the applied frequency, the conduction inside the grain

decreases and hence dielectric loss tangent decreases. The figure 6.5 also shows that the

dielectric loss tangent also decreases with the increase in doping concentration. This may be

due to the decrease in Fe2+ ion concentration, which is responsible for conduction losses,

because of increase in Zn2+ content as explained earlier. The values of dielectric loss tangent

obtained from this composition of strontium hexa-ferrite are much smaller than reported in

[24, 65] so making this substitution (X=0.2 to 0.8) more suitable for high frequency

applications.

6.2.3 The dielectric loss factor ()

The dielectric loss factor () is calculated by using equation 2.12. The plot of the

dielectric loss factor () shown in figure 6.6 indicates that it decreases initially with the

increase in frequency and then becomes fairly constant at higher frequencies. It is because the

hoping of electron could not follow the applied frequency and consequently the dielectric loss

factor () decreases. The decrease in dielectric loss factor () with the increase in Cr-Zn

doping is due to the decrease in Fe2+ ion concentration.

9 10 11 12 13 14 15

0

40

80

120

160

200 SrFe12-2x

CrxZn

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Lo

ss F

act

or

'')

ln f


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)


The ac conductivity (ac) is calculated by using the equation 2.13. Figure 6.7 shows

that ac increases up to the relaxation frequency and then decreases and becomes constant for

a range of frequencies and then starts increasing at higher frequencies. The hoping frequency

of electron is small at lower frequencies and it increases with increase in frequency. This is

attributed to the fact that the required energy correlated with forward-backward hoping is only

a fraction of the energy necessary to activate long range diffusive conduction [189].

.

9 10 11 12 13 14 15

0.0000

0.0004

0.0008

0.0012

0.0016SrFe

12-2xCr

xZn

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

ac (

S-m

-1)

ln f


SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

6.3 Temperature dependent dc electrical properties of Cr-Zn doped

SrM

The temperature dependent dc electrical resistivity was measured in the range of

373K-623K. Plot of ln of dc electrical resistivity as a function of 1/kBT is shown in figure

6.8. The dc electrical resistivity decreases with the increase in the temperature due to increase

in drift mobility of charge carriers. The plot also indicates that resistivity has a decreasing

trend with the increase in Cr-Zn concentration. This mainly because of increase in the grain

size. As the grain size increases, grain boundaries (which acts as highly resistive medium)

decrease and consequently resistivity decreases. Activation energy was calculated from the

slope of the linear plots of lnρ versus reciprocal of the temperature using Arrhenius relation

shown in figure 6.8.

18 20 22 24 26 28 30 32

10

12

14

16

18

20

22

24 SrFe12-2x

CrxZn

xO

19

ln d

c (

-cm

)

1/kBT (eV)-1

X=0.0 X=0.2 X=0.4 X=0.6 X=0.8 Linear Fit

Fig 6.8: Plot of lnρ as a function of 1/kBT for SrFe12-2xCrxZnxO19

(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit.

The values of the activation energy as given in table 1, also decreases with the increase in

doping content and that is due to increase in the grain size.

6.4 Magnetic properties of Cr-Zn doped SrM

The magnetic parameters such as coercivity (Hc), saturation magnetization (Ms) and

remanence (Mr) are calculated from hysteresis loops of the composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 are shown in figure 6.9.

Fig 6.9: Hysteresis loops of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

The saturation magnetization (Ms) also decreases with the increase in Cr-Zn concentration. It

is due to the substitution of Cr-Zn cations on different interstitial sites already discussed.

0.0 0.2 0.4 0.6 0.8

4.4

4.6

4.8

5.0

5.2

5.4

Ms

(em

u/g

)

Hc (k

Oe

)

Cr-Zn concentration (X)

28

32

36

40

44

48

Fig 6.10: Plot of coercivity (Hc) and saturation magnetization (Ms) versus

Cr-Zn concentration (X) of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)

The Figure 6.10 indicates that saturation magnetization decreases with the increase in doping

content. It may be due to following reasons. According to ferrimagnetic theory [192],

magnetism in ferrite originates from the net magnetic moment of ions with spin up and spin

down in interstitial sites. The variation in saturation magnetization mainly depends upon the

distribution of Cr3+ ions on interstitial sites and then magnetic dilution with the substitution of

Fe3+ ions by the lower magnetic moment ions. It is reported that Cr3+ ions with magnetic

moment (3 µB) preferentially occupy 12k and 2a sites with spin up. This results in the

decrease in the total magnetic moment of cations with spin up. As the net magnetic moment

of one formula unit is due to the difference of the magnetic moments of cations with spin up

and spin down so net moment decreases and hence saturation magnetization decreases [195].

On the other hand, Zn2+ with zero magnetic moment occupy tetrahedral (A) site with spin

down and bipyramidal (C) site with spin up [145]. If Zn ions only occupy tetrahedral site then

saturation magnetization should increase but the results show that it decreases. It is possible

only if the Zn ions also occupy the sites with spin up. The third factor which also contributes

to decrease in saturation magnetization is the formation of non magnetic phase (α-Fe2O3). The

XRD patterns shown in figure 6.1 indicates that the compositions (X=0.2 and 0.4) have non

magnetic phase (α-Fe2O3). This non magnetic phase also contribute to the sharp decrease in

saturation magnetization of these compositions. A small increase in saturation magnetization

for X=0.6 is due to the absence of this phase. Figure 6.7 also shows that coercivity decreases

with the increase in Cr-Zn content. This decrease in coercivity (Hc) of the samples (X≤0.6) is

attributed to the increase in particle size shown in figure 6.3. The increase in the grain size

and hence decrease in the grain boundaries results in decrease in opposition to the domain-

wall motion and consequently in the coercivity. For X=0.8, it increases due to decrease in

particle size.

6.5 Conclusion

The indexed XRD patterns of the composition SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4,

0.6, 0.8) sintered at 940C show the successful substitution of Cr and Zn on interstitial sites of

strontium hexaferrite lattice. The particle size obtained from SEM micrographs indicates an

increasing trend with the increase in doping concentration. The dielectric constant and

dielectric loss tangent measured in the frequency range 10kHz-3MHz shows a decreasing

trend with the increase in Cr-Zn concentration. This is mainly due to the fact that Zn doping

reduces the concentration of Fe2+ ions in ferrites. The dielectric losses obtained with this

doping are lowest. So this doping in strontium hexaferrites is more useful for the devices

operating at high frequency. The temperature dependent dc resistivity measurements show a

small decreasing trend with the increase in doping content. This behavior is attributed to the

increase in particle size. As the particle size increases, grain boundary decreases and hence

resistivity decreases. The hysteresis loops of Cr-Zn doped samples indicate that both

saturation magnetization and coercivity decreases with the increase in doping concentration.

The decrease in saturation magnetization is due to the replacement of Fe3+ with Cr3+ having

less magnetic moment than that of Fe3+ ion. The decrease in coercivity is due to the increase

in particle size. Iqbal et al. [126] prepared strontium hexaferrites with the composition

SrZrxCuxFe12-2xO19 (X = 0.0–0.8) with co-precipitation method. The dielectric loss tangent

obtained is in the range 0.98-1.55 respectively. Ashiq et al. [62] synthesized SrAlxCrxFe12-

2xO19, (X=0.0–0.6) via co-precipitation route. The dielectric loss tangent obtained is in the

range 1.26-0.95 respectively. In the present work, the dielectric loss tangent obtained is in the

range 0.32-0.02 which is much lower than the already reported. So it is expected that Cr-Zn

substitution in strontium hexaferrites would also be more suitable for high frequency

application due to low loss.

Chapter 7

Structural and electrical properties of Cr-Zn doped strontium hexaferrites prepared by WOWS sol-gel method

7. Structural and electrical properties of Cr-Zn doped strontium hexaferrites prepared by WOWS sol-gel method

Cr-Zn doped strontium hexaferrite with composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 is synthesized by simplified sol-gel method. This new

method is developed in our lab and is named as WOWS (With Out Water and Surfactants)

sol-gel method [196]. M-type strontium hexaferrite is prepared for the first time with this

method.

7.1 Structural studies

The phase formation studies of the composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 , prepared by WOWS sol-gel method, are made by using

powder X-ray diffraction data. All the peaks of X-ray diffraction (XRD) patterns shown in

figure 7.1 are identified by using ICDD patterns with reference code 01-080-1197. The

indexed XRD patterns show that all the compositions have single phase of strontium

hexaferrite material. This confirmed the successful substitution of Cr and Zn cations on the

interstitial sites of strontium hexaferrite lattice as there was no separate peak of these

substituted cations detected. The parameters calculated from XRD patterns are given in table

7.1. Both lattice parameters a & c are increased with the increase in Cr-Zn concentration. This

change is attributed to Zn2+ whose ionic radius is greater than that of Fe3+ ion [197]. The

replacement of large cation on the interstitial sites of strontium hexaferrites causes the crystal

structure to expand and in other words lattice parameters increases and X-ray density

decreases.

25 30 35 40 45 50 55 60 65 70

Inte

nsi

ty (

a.u.

)

X=0.0

2 (Deg.)

X=0.2

X=0.4(0

2 1

2)

(1 1

12)

(0 0

14)

(0 1

10)

(0 2

5)

(0 2

3)

(0 2

1)(1

1 4

)

(0 1

7)

(1 1

2)

(0 0

8)

X=0.6

X=0.8

Fig 7.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method.

Table 7.1: Lattice parameters (a & c), crystallite size (D114), X-ray density (ρx), bulk density (ρm), % porosity, dielectric constant (ε'), dielectric loss tangent (tanδ) and ac conductivity (σac) of the prepared samples of SrFe12-2xCrxZnxO19 . , . , . , . , . by WOWS sol-gel method.

SrFe12-2xCrxZnxO19 X=0.0 X=0.2 X=0.4 X=0.6 X=0.8 Lattice Constant a(Å) c(Å) c/a

5.868(2) 23.008(1) 3.92

5.879(3) 23.009(2) 3.91

5.886(2) 23.014(4) 3.91

5.892(2) 23.032(3) 3.91

5.891(2) 23.059(6) 3.91

Crystallite Size D1 1 4 (nm)

114 106 139 138 104

X-ray Density ρx (g/cm3)

5.14 5.12 5.11 5.10 5.09

Bulk Density ρm (g/cm3)

2.82(2) 2.13(1) 2.16(2) 2.28(2) 2.32(3)

% Porosity 45 58 58 55 54 Dielectric constant (ε') at 3MHz

5.53(1) 5.35(1) 4.92(1) 4.72(1) 4.58(1)

Dielectric loss (tanδ) at 3MHz

0.037(1) 0.038(1) 0.035(1) 0.028(1) 0.023(1)

7.2 Dielectric properties

7.2.1 The dielectric constant ()

The figure 7.2 indicates that ε' have a decreasing trend with the increase in doping contents.

This is due to the similar reason as described in section 6.2 and 6.2.1. The values of dielectric

constant are much smaller than those obtained from similar composition synthesized by co-

precipitation method. This is because of better crystallinity.

9 10 11 12 13 14 154.4

4.8

5.2

5.6

6.0

6.4

6.8

7.2

7.6

8.0

X=0.0 X=0.2 X=0.4 X=0.6 X=0.8

ln f

Die

lect

ric C

on

sta

nt '

)

SrFe12-2x

CrxZn

xO

19

Fig 7.2: Plot of dielectric constant (ε') as a function of ln of frequency for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method

9 10 11 12 13 14 150.0

0.1

0.2

0.3

0.4

0.5

Die

lect

ric L

oss

Tan

gen

t (ta

n

ln f

X=0.0 X=0.2 X=0.4 X=0.6 X=0.8

SrFe12-2x

CrxZn

xO

19

Fig 7.3: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method

7.2.2 The dielectric loss tangent (tan)

Figure 7.3 shows that the dielectric loss tangent also decreases with the increase in

doping concentration. This may be due to the decrease in Fe2+ ion concentration, which is

responsible for conduction losses, because of increase in Zn2+ content as explained earlier.

This may also be due to the increase in lattice parameters. The values of dielectric loss

tangent obtained from this composition of strontium hexaferrite are much smaller than

reported in [24, 65]. So it is expected that this substitution (X=0.2 to 0.8) would also be more

suitable for the devices operating at high frequencies than already reported composition.


The ac conductivity (ac) is calculated by using the equation 2.13. Figure 7.4 show

that ac is fairly constant for a range of low frequencies and then starts increasing at higher

frequencies. The hoping frequency of electron is small at lower frequencies and it increases

with the increase in frequency. This is attributed to the fact that the required energy correlated

with forward-backward hoping is only a fraction of the energy necessary to activate long

range diffusive conduction [189]. The decrease in ac with the increase in Cr-Zn concentration

is due to decrease in Fe2+ ion concentration discussed earlier in this chapter.

9 10 11 12 13 14 15

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

2.5x10-5

3.0x10-5

3.5x10-5

X=0.0X=0.2X=0.4X=0.6X=0.8

SrFe12-2x

CrxZn

xO

19

ac (

S-m

-1)

ln f

Fig 7.4: Plot of ac conductivity (σac)as a function of ln of frequency for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with sol-gel method.

2 4 6 8 10 12 14 16

0

10

20

30

40

50

60

70 Experimental Theoretical

Die

lect

ric

Co

nst

an

t

')

ln f

Fig 7.5: Plot of experimental and theoretically calculated dielectric constant () for the sample X=0.6 (SrFe12-2xCrxZnxO19) as a function of frequency.

The dielectric properties (dielectric constant and dielectric loss tangent) of strontium

hexaferrites follow Maxwell-Wagner two layer model, already discussed in section 1.8.6.

Figure 7.5 shows the plot of theoretically calculated values by using the equation 1.14 and

experimentally observed measurements of dielectric constant of sample X=0.6 prepared by

sol-gel method. The obtained results match well with the theoretical results. A small

difference in both curves may be due to the fact that grain boundaries do not act as perfectly

insulating regions. A leakage current takes place at grain boundaries. The leakage current is

more at lower frequencies and less at higher frequencies.

7.3 Conclusion

The indexed XRD patterns of the composition SrFe12-2xCrxZnxO19 with X=0.0, 0.2,

0.4, 0.6, 0.8 prepared with co-precipitation method show that there are some impurity phases

present in some samples and these are successfully eliminated by WOWS sol-gel method.

This may be due to different synthesis temperatures (70ºC for co-precipitation and 200ºC for

sol-gel) and methods. This indicates that WOWS sol-gel method provide much better control

on impurity phases than that prepared by co-precipitation method. Lattice parameters

calculated from XRD data show a random trend for the samples prepared by co-precipitation

method while a regular trend is observed in the samples prepared by WOWS sol-gel method

for the same composition. Lattice constant depends on composition and cations distribution

and these are dependent on synthesis method so WOWS sol-gel method provide much better

control on cations distribution than that of co-precipitation method. The dielectric constant

and dielectric loss obtained from the samples prepared by WOWS sol-gel method is much

smaller than that of prepared by co-precipitation method.

Ashiq et al. [62] prepared strontium hexaferrites with the composition SrAlxCrxFe12-

2xO19 (X=0.0–0.6) with co-precipitation method. The dielectric loss tangent, measured at

1MHz, is in the range 1.26-0.95 respectively. Hussain et al. [124] synthesized strontium

hexaferrites with the composition Sr0.5Pb2+0.5Fe12−xPbx

3+O19 (X=0.0-1.0) with solid state

reaction method. The dielectric loss tangent, measured at 1MHz, is in the range 0.55-0.06

respectively. Iqbal et al. [125] prepared strontium hexaferrites with the composition Sr0.5Ba0.5-

xCexFe12-Y NiYO19 (X=0.0-0.1 and Y=0.0-1.0) with co-precipitation method. The dielectric

loss tangent obtained is in the range 1.26-0.95 respectively. Iqbal et al. [64] synthesized

strontium hexaferrites with the composition SrZrxNixFe12-2xO19 (X=0.0–0.8) with co-

precipitation technique. The values of dielectric loss tangent varies from 0.35-0.2 measured at

1MHz. Hussain et al. [123] prepared strontium hexaferrites with the composition

SrFe12O19+SiO2 (0-0.2 wt%) by solid state reaction method. The dielectric loss tangent,

measured at 1MHz, is in the range 1.0-0.21 respectively. Iqbal et al. [65] synthesized

SrAlxGaxFe12–2xO19 (X=0.0–0.8) via co-precipitation route. The dielectric loss tangent

obtained is in the range 0.98-0.26 respectively.

In the present work, the values of the dielectric loss tangent, measured at 1MHz, is in

the range 0.04-0.02 respectively which is much smaller than already reported for different

compositions of strontium hexaferrites. So it is expected that Cr-Zn doped strontium

hexaferrites synthesized by WOWS sol-gel method would also be more useful for the devices

operating at higher frequencies due to very low dielectric loss than any of the already reported

compositions of strontium hexaferrites.

7.4 Comparison

In this section, a comparison between structural and dielectric properties of the composition SrFe12-2xCrxZnxO19 with X 0.0, 0.2, 0.4, 0.6, 0.8 prepared by co-precipitation and sol-gel methods is presented.

7.4.1 Structural properties

The structural properties of the samples prepared by co-precipitation and sol-gel

method are compared by using indexed XRD patterns.

Figures 6.1 and 7.1 show that the crystallinity of the samples prepared with sol-gel

method is much better. In case of samples prepared with co-precipitation, there are some

impure phases (α-Fe2O3) present while in case of sol-gel method; no impurity peak in any

sample is present. Figure 7.1 indicates that the variation in both lattice parameters (a & c) is

composition dependent for samples prepared with sol-gel while this dependence is not

observed in case of co-precipitation. As lattice constant is composition dependent so WOWS

sol-gel method provide much better control on cation distribution and stresses in lattice which

results in phase purity of strontium hexaferrites.

25 30 35 40 45 50 55 60 65 70

Inte

nsity

(a.

u.)

X=0.0

2 (Deg.)

X=0.2

X=0.4

(0 2

12

)

(1 1

12

)(0

0 1

4)

(0 1

10

)

(0 2

5)

(0 2

3)

(0 2

1)(1

1 4

)

(0 1

7)

(1 1

2)

(0 0

8)

X=0.6

X=0.8

7.4.2 Dielectric properties

The dielectric properties (dielectric constant and dielectric loss) of the samples

prepared with co-precipitation and sol-gel method are compared by using the data obtained

from precision component analyzer.

A comparison of dielectric constant for the composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 prepared by co-precipitation and sol-gel method is given in

figures 6.2 and 7.2. The plot shows that the dielectric constant of the composition synthesized

by sol-gel is much lower than that of the samples prepared by co-precipitation method.

25 30 35 40 45 50 55 60 65 70

Inte

nsi

ty (

a.u.

)

X=0.0

2 (Deg.)

X=0.2

X=0.4

X=0.6

X=0.8(1

0 4

)

*

(0 3

2)

(0 0

14)

(1 1

12)

(0 2

12)

(0 3

8)

(0 2

7)

(0 1

10)

(0 2

5)

(0 2

4)

(0 2

3)

(0 2

1)(1

1 4

)

(0 1

7)

(1 1

2)(0 0

8)

*

-Fe2O

3

Fig 6.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 with (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by co-precipitation

method. ( α-Fe2O3)

Fig 7.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 with (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method.

9 10 11 12 13 14 154.4

4.8

5.2

5.6

6.0

6.4

6.8

7.2

7.6

8.0

X=0.0 X=0.2 X=0.4 X=0.6 X=0.8

ln f

Die

lect

ric

Co

nst

ant

')

SrFe12-2x

CrxZn

xO

19

A comparison of dielectric loss tangent for the composition SrFe12-2xCrxZnxO19

with X 0.0, 0.2, 0.4, 0.6, 0.8 prepared with co-precipitation and sol-gel method is given in

figures 6.3 and 7.3. The plot shows that the dielectric loss tangent of the composition

synthesized by sol-gel is much smaller than that of the samples prepared by co-precipitation

method.

9 10 11 12 13 14 150.0

0.1

0.2

0.3

0.4

0.5

Die

lect

ric

Loss

Tan

gent

(ta

n

ln f

X=0.0 X=0.2 X=0.4 X=0.6 X=0.8

SrFe12-2x

CrxZn

xO

19

(WOWS sol-gel)

Figure 6.2: The dielectric constant for the composition SrFe12-2xCrxZnxO19 with

(X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by co-precipitation.

Figure 7.2: The dielectric constant for the composition SrFe12-2xCrxZnxO19 with (X=0.0,

0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method

9 10 11 12 13 14 150

20

40

60

80

100 SrFe12-2x

CrxZn

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric

Con

sta

nt '

)

ln f

9 10 11 12 13 14 15

0.0

0.4

0.8

1.2

1.6

2.0SrFe

12-2xCr

xZn

xO

19

X=0.0X=0.2X=0.4X=0.6X=0.8

Die

lect

ric L

oss

Tan

gen

t (ta

n

ln f

(Co-precipitation)

Fig 6.3: The dielectric loss tangent for the composition SrFe12-2xCrxZnxO19 with

(X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by co-precipitation.

Fig 7.3: The dielectric loss tangent for the composition SrFe12-2xCrxZnxO19 with

(X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method.

7.4.3 Conclusion

The indexed XRD patterns of the composition SrFe12-2xCrxZnxO19 with X=0.0, 0.2,

0.4, 0.6, 0.8 prepared with co-precipitation method show that there are some impurity phases

present in some samples but are not present in the same composition prepared by WOWS sol-

gel method. This indicates that WOWS sol-gel method provide much better control on

impurity phases than that of co-precipitation method. Lattice parameters calculated from XRD

data show a random trend for the samples prepared by co-precipitation method while a regular

trend is observed in the samples prepared by WOWS sol-gel method for the same

composition. As lattice constant is composition dependent so WOWS sol-gel method provide

much better control on cations distribution than that with co-precipitation method. The

dielectric constant and dielectric loss obtained from the samples prepared by WOWS sol-gel

method is much lower than that of prepared by co-precipitation method. As these properties

are dependent on cations distribution of microstructures so WOWS sol-gel synthesis method

is better than that of co-precipitation.

Chapter 8

Oxygen reduced strontium hexaferrite for microwave absorbing coatings

8. Oxygen reduced strontium hexaferrite for microwave absorbing coatings

In ferrites, the major source of charge carriers is Fe2+ ion. By changing the grain

boundaries and surface defects of the grains and the concentration of Fe2+ ions inside the

grain, the electrical properties of the material could be tailored according to the desired

application. When sintered strontium hexaferrite is exposed to hydrogen at high temperature,

oxygen from the surface as well as from the bulk is removed. It results in the formation of

free iron atoms as well as Fe2+ ions which could result in the increase in dielectric properties

such as dielectric constant and dielectric loss [198].

In the present work, oxygen is intentionally reduced to increase the concentration of

Fe2+ ions and surface defects in order to increase the dielectric constant and making the

material useful for microwave absorber coatings.

8.1 Reduction procedure

Fig 8.1: Experimental setup for oxygen reduction

Annealed powder of strontium hexaferrite was placed in tube furnace. The

temperature of the tube furnace was raised to 800C at the rate of 5C/min. Then hydrogen

was passed through the tube with the flow rate of 50scc/min for one hour and then the furnace

was turned off. Experimental setup for oxygen reduction is shown in figure 8.1. The effect of

this oxygen reduction on structural and electrical properties of annealed strontium hexaferrite

was investigated by using X-ray powder diffractometer (XRD) and precision component

analyzer respectively.

8.2 Effect of oxygen reduction on structural properties of strontium

hexaferrites (SrM)

Effect of oxygen reduction on structural properties of strontium hexaferrites was

studied by using XRD. The indexed XRD patterns of annealed strontium hexaferrite powder

before and after oxygen reduction is shown in figure 8.2. It can clearly be seen from XRD

patterns that after oxygen reduction the major peaks, having miller indices (107) and (114), of

M-type strontium hexaferrites are vanished and other phases of strontium ferrites are formed.

It indicates that oxygen reduction disturbs the structure of M-type hexaferrites.

8.3 Frequency dependent ac electrical properties of oxygen reduced strontium hexaferrites

Frequency dependent electrical properties are mainly characterized by two parameters

named as dielectric constant and dielectric loss tangent. The dielectric constant and dielectric

loss tangent of reduced samples are shown in figures 8.3 and 8.4 respectively.

Figure 8.3 indicates that dielectric constant of reduced strontium hexaferrite sample is

very high as compared to non reduced sample. This is due to the reason that reduction of

oxygen causes the transformation of Fe3+ ions to Fe2+ ions or free iron atoms. This results in

the increase in the concentration of available charge carriers and hence increases the space

charge polarization due to which dielectric constant has increased to large extent.

8.4 The dielectric loss tangent (tanδ)

The plot of dielectric loss tangent (tanδ) of reduced and non reduced strontium

hexaferrite sample is shown in figure 8.4. The figure shows that dielectric loss tangent

Fig 8.2: Indexed XRD patterns of sintered strontium hexaferrite samples before and after reduction

9 10 11 12 13 14 15

0

1000

2000

3000

4000

5000

6000 After ReductionBefore Reduction

Die

lect

ric

Co

nst

an

t

')

ln f

Fig 8.3: Dielectric constant () of sintered SrFe12O19 before and after reduction

of reduced sample is much higher than that of non reduced sample. This is due to the increase

in carrier concentration because of the reduction of oxygen. As the carrier concentration

increases, the conduction losses increases and dielectric losses increases. A comparison of the

dielectric constant and dielectric loss tangent of reduced and non reduced sample is given in

table 8.1.

9 10 11 12 13 14 15

0

20

40

60

80

100

120

140

160After ReductionBefore Reduction

Die

lect

ric

Lo

ss T

an

ge

nt

(ta

n

ln f

Fig 8.4: Dielectric loss tangent (tanδ) of sintered SrFe12O19 before and after reduction

Table 8.1: Dielectric constant () and dielectric loss tangent (tanδ) of reduced and non reduced sample

Table 8.1 shows that % age increase in dielectric constant is much bigger than that of

loss tangent. The results show that the oxygen reduced sample is very suitable for microwave

absorbing coatings.

24 26 28 30 32 34 364

6

8

10

12

14

16

18

20

22

24

1/kBT (eV-1)

After reduction Befire reduction

ln

dc (

-cm

)

Fig 8.5: Plot of temperature dependent dc electrical resistivity of sintered SrFe12O19 before and after reduction

Composition SrFe12O19

(Before reduction)

SrFe12O19

(After Reduction)

Dielectric const. ε' at 3 MHz 23 1530

Loss tangent (tanδ) at 3 MHz 0.32 2.76

8.6 Temperature dependent dc electrical properties of sintered SrFe12O19 before and after reduction

Figure 8.5 show that dc electrical resistivity is dropped to very low values after

oxygen reductions. This is because the reduction of oxygen results in the increase in

concentration of Fe2+ ions and charge carriers.

8.7 Conclusion

Reduction of oxygen from sintered SrFe12O19 has been made in order to increase the

concentration of charge carriers. The effect of oxygen reduction on the structure of SrFe12O19

is analyzed by using X-ray diffractions patterns. Indexed XRD patterns show that the

characteristic peaks (1 0 7) and (1 1 4) are vanished. This shows that the reduction of oxygen

results in the formation of new phases. The temperature dependent dc electrical resistivity is

decreased due to increase in carrier concentration. The frequency dependent dielectric

constant and dielectric loss tangent are increased to large extent. The dielectric results show

that oxygen reduction makes the strontium hexaferrites useful for microwave absorption.

Further investigations are required to know the detailed phenomenon behind this behavior and

also for reflection losses.

9 Conclusions

In the present work, single phase strontium hexaferrites (SrFe12O19) are synthesized

by two different methods. These methods used are co-precipitation and sol-gel methods. In

co-precipitation technique, the properties of a synthesized material are strongly affected by

synthesis conditions. A systematic study of the major synthesis parameters which impart a

significant contribution on the properties of hexaferrites has been made. These major

synthesis parameters include molar ratio of cations (Fe/Sr), volume rate of addition of

precipitating agent and the pH of the solution. A number of samples has been prepared by

varying any one of the synthesis parameters such as molar ratio of cations (Fe/Sr), volume

rate of addition of precipitating agent and the pH of the solution and keeping all other

parameters same. After analyzing the results obtained, these parameters are optimized for

phase purity and particle size. To study the effect of molar ratio (Fe/Sr) on phase purity, the

samples were prepared with molar ratio MR = 08, 09, 10, 11, 12. The indexed XRD patterns

of molar ratio (Fe/Sr) varying samples show that almost all patterns are similar and contain no

impurity peak. It is concluded that molar ratio (MR) within the range (08 to 12) does not

affect the phase purity of strontium hexaferrites. To study the effect of volume rate of addition

of precipitating agent on phase purity, the samples are prepared with very slow (drop by

drop), intermediate and very fast addition of solution of the precipitating agent. X-ray

diffraction patterns are used to analyze the phase formation. It is observed that variation in

volume rate of addition of precipitating agent also plays very important role in the phase

purity of strontium hexaferrites. The results show that the increase in the volume rate of

addition of precipitating agent improves the phase purity. The effect of this parameter on

surface morphology is analyzed by using scanning electron micrographs (SEM). The SEM

micrographs indicate that the particle size decreases with the increase in volume rare of

addition of precipitating agent. This may be due to the reason that high volume rate of

addition of precipitating agent increases the number of nucleation sites. As the number of

nucleation sites increases, the possibility of agglomeration decreases. To analyze the effect of

one of the most important synthesis parameter i.e. pH of the solution, the samples with

different pH (08 to 13) are prepared. The obtained results of the samples prepared with

different pH of the solution indicate that pH of the solution imparts significant effects on

phase and microstructural properties of strontium hexaferrites. High pH of the solution

improves the phase purity and decreases the particle size. This decrease in particle size may

be due to the reason that high pH of the solution increases the number of nucleation sites. As

the number of nucleation sites increases, the possibility of agglomeration decreases. It is

observed that the dc electrical resistivity is increased in the samples which are prepared with

the increase in the pH of the solution. This is due to the increase in grain boundaries.

Ferrites are being commonly used as dielectric materials in the devices operating at

high frequency. To improve coercivity which results in the increase in the operating

frequency of ferrites, Cr is doped in strontium hexaferrite and the composition SrFe12-xCrxO19

with X=0.0, 0.2, 0.4, 0.6, 0.8 is prepared by co-precipitation method and its dielectric studies

are reported. The X-ray diffraction patterns indicate that Cr doping results in the formation of

secondary phase of α-Fe2O3. A peak of Cr2O3 is also appeared for X=0.8 indicating the

solubility limit of Cr in strontium hexaferrites. It is also observed that for X0.6, both

dielectric constant and coercivity are increased while saturation magnetization is decreased.

The increase in coercivity is mainly due to the impurity phase acting as pinning center while

the decrease in saturation magnetization is due to the replacement of Fe3+ ions (5µB) with Cr3+

ions, having less magnetic moment (3µB), on octahedral sites.

As Cr causes the increase in coercivity and it is reported that Zn causes the decrease

in Fe2+ ions concentration and dielectric losses. So keeping these observation in view, both Cr

and Zn are doped simultaneously in strontium hexaferrites and the resulting composition

SrFe12-2xCrxZnxO19 with X=0.0, 0.2, 0.4, 0.6, 0.8 is prepared with co-precipitation method.

The XRD patterns show that in two samples, impurity peak of α-Fe2O3 is appeared. SEM

micrographs indicate that the particle size shows an increasing trend with the increase in

doping concentration. As the particle size is increased, density is decreased and hence

porosity is increased. It is also observed that Cr-Zn doping causes the decrease in the

dielectric constant and dielectric loss tangent due to decrease in Fe2+ ions concentration

because of Zn doping. Frequency dependent ac conductivity is also decreased with the

increase in Cr-Zn concentration. This is due to the decrease in the carrier concentration (Fe2+

ions) because of the Zn doping. Temperature dependent dc electrical resistivity measurements

show that there is a small drop in dc resistivity. This is attributed to the fact that Cr-Zn doping

resulted in the increase in particle size. The increase in particle size caused the decrease in the

grain boundaries acting as highly resistive medium and as a consequence resistivity is

decreased. The hysteresis loops of the Cr-Zn doped samples reveal that both coercivity and

saturation magnetization are decreased with increase in doping concentration. The decrease in

coercivity is due to the increase in particle size.

As the properties of these materials are sensitive to synthesis methods and conditions

so the composition SrFe12-2xCrxZnxO19 with x=0.0, 0.2, 0.4, 0.6, 0.8 is also prepared with

WOWS sol-gel method (a much simplified method developed in our lab) in order to compare

the properties of this composition prepared by these two methods. The structural results

obtained from XRD patterns indicate that the variation in both the lattice parameters (a & c) is

composition dependent for samples prepared with sol-gel while this dependence is not

observed in case of the samples prepared with co-precipitation method. As lattice constant is

composition dependent so WOWS sol-gel method provides much better control on cation

distribution, stresses in lattice and hence phase purity of strontium hexaferrites. The frequency

dependent dielectric measurements are much better than that of the samples prepared with co-

precipitation method. The dielectric loss of the sample (X=0.0) synthesized by this method is

about 90% lower than that of prepared with co-precipitation method. It is due to the better

microstructural properties such as particle size and its distribution provided by WOWS sol-

gel method.

In some cases, high loss may be desired in applications such as heating and EM wave

absorption. To increase the dielectric losses, reduction of oxygen from sintered SrFe12O19 is

made. The indexed XRD pattern indicate that the characteristic peaks (1 0 7) and (1 1 4) of

strontium hexaferrites are missing. This shows that due to the reduction of oxygen, some

chemical bonds of SrFe12O19 are broken up and new phases are formed. The reduction of

oxygen resulted in the increase in the concentration of Fe2+ ions and free iron atoms. The

temperature dependent dc electrical resistivity is sharply decreased due increase in carrier

concentration. The frequency dependent dielectric constant and dielectric loss are increased to

large extent due increase in carrier concentration and hence making the material useful for

microwave absorption.

9.1 Future work

The dielectric properties of the samples prepared by WOWS sol-gel method should

be studied in higher GHz range. The difference in dielectric properties of the samples

prepared by co-precipitation and WOWS sol-gel method should be studied in terms of site

occupation of dopants in crystal lattice by using EXAFS. The synthesized material should be

sintered in oxygen environment in order to reduce the Fe2+ ion concentration and hence to

reduce the dielectric losses. The elements with low atomic number should also be tried. The

effect of oxygen reduction on dielectric properties of strontium hexaferrites should also be

studied systematically.

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Fabrication & Thermophysical Studies of Hexa Ferritesprr.hec.gov.pk/jspui/bitstream/123456789/1671/2/1503S.pdfMariam Ansari, Ms. Khush Bakhat and Ms Fatima-Tuz-Zahra for helping me