1

Chapter I

Introduction

2

Chapter 1

INTRODUCTION

INTRODUCTION:

During the past decade and a half the production and use of Nano materials

has established a foot hold. The term nanotechnology is employed to describe the

creation and exploitation of materials with structural features in between those of

atoms and bulk materials, with at least one dimension in the nanometer range (1nm =

10-9

m).The Scientist and Technocrats have well realized that the use of Nano sized

materials have not only helped in the production of compact and smaller machines

and equipment’s, but has also relaxed the strain on the fast depleting of the limited

resources. It has been further recognized that the ultrafine particles have properties

which are different form their counterpart.

Magnetic nanoparticles are of great interest in recent years due to their

extensive use in the technological and chemical applications. Among these, spinal

ferrites have attracted considerable attention due to their useful electrical and

magnetic properties and applications in several important technological fields.

About for half of the century ferrites have been established as new category of

magnetic materials. Research and development continue to take place in many new

theories, synthesis methodologies; characterization and analysis techniques are

currently under development in the field of ferrites to be used in ever widening range

of applications. Generally the term ferrite is referred to all magnetic oxides containing

iron as a major component. They have a general chemical formula MFe2O4 [M = any

divalent cation (Zn, Cu, Ni, Co, Mg, Fe etc.) [Ramesh and Spaldin 2007].Ferrites are

considered as advance materials for their crucial role as pace setters and the role they

found in pushing the development of civilization at a great pace. [Santos, Costa et al.

2009]Spinal ferrites are considered as important catalysts for a number of industrial

processes such as in ammonia synthesis, Fisher-Tropsch, dehydrogenation of

butylene [Li, Wang et al. 2014; Rennard and Kehl 1971] and decomposition of

alcohols and H2O2[Lahiri and Sengupta 1991].

Nanocomposites include multiphase solid materials wherein one of the phases

has a dimension of less than 100 nm. The mechanical, electrical, optical,

3

electrochemical, and catalytic properties of the Nano composite will differ markedly

from that of the component materials. Other kinds of Nano particulates may result in

enhanced optical properties, dielectric properties or mechanical properties such as

stiffness and strength.

In the recent years, cumulative consideration has been paid in the area of

Nanocomposites magnet [Asti, Solzi et al. 2004; Erokhin, Berkov et al. 2012]as it

delivers an integrated system comprising of components whose properties are

harmonizing to each other [Roy and Kumar 2013]. One such dynamic field of

research is the exchange spring magnet [Uzdin, Vega et al. 2012; Kneller and Hawig

1991; Shield, Zhou et al. 2006; Zhou, Skomski et al. 2005; Suess, Schrefl et al.

2005], where high saturation magnetization of the soft and the high magnetic

anisotropy of the hard magnetic phases are exchange coupled in the Nano metric

scale.

One of the fascinating properties of ferrites is the possibility to prepare

different compositions and thereby alter the magnetic properties. One of the

challenges is to improve the magnetic properties of soft ferrites such as saturation

magnetization, magnetic hysteresis, demagnetizing force and anisotropic energy.

Researchers are trying to produce hard and soft ferrites by using simple methods. In

view this, many studies have focused on new systems, such as

CoFe2O4/ZnFe2O4[Masala, Hoffman et al. 2006], earth-iron-boron [Maeda, Sugimoto

et al. 2004] and Fe/Z-type ferrite [Liu, Itoh et al. 2006].The results suggest that

coupling exchange exists between the nanoparticles and the interaction significantly

influences magnetization and coercivity of the composite powders. [Masala, Hoffman

et al. 2006], they reported that exchange interaction between hard and soft magnetic

phases improve the microwave absorption and magnetic properties of Nano

composites.

Recently, due to development of electronic technology, the trends of

miniaturization and excellent electromagnetic properties are the utmost requirements

of materials to be used for different purpose and these have been and are being

fulfilled by the materials called Composites [Grössinger et al. 2008; Goldman,

Gardner, Moss et al. 1966]. For few years extensive research has been carried out on

Multiferroic (MF) composite materials [Ma, Hu et al. 2011; Ramesh and Spaldin

4

2007] which have been under the focus of researchers due to their potential

applications in electronics technology (as magnetic–electric sensors in radio-

electronics, optoelectronics, microwave electronics and transducers). In MF materials,

magnetic and electric orders coexist simultaneously and the coupling between spin

and charge degrees of freedom gives rise to a wide range of magneto electric

phenomenon [Eerenstein, Mathur et al. 2006; Fitchorov, Chen et al. 2011]. The

control of polarization by applying magnetic fields or the magnetization by applying

electric fields, which is known as the magneto electric (ME) effect, appears in the

materials when the electric polarization and magnetic orders are coupled to each other

[Verma and Negi 2010; Verma and Kotnala 2011].

The ME effect can also be given as direct ME effect which is characterized as

magnetic-field-induced polarization and electric-field-induced magnetization, respectively

[Chu, Martin et al. 2008] .The different types of single-phase Multiferroic such as BiFeO3

[Chu, Martin et al. 2008], TbMn2O5 [Hur et al. 2004], BaTiO3-CoFe2O4 [Agarwal et al.

2012], 0.62Pb (Mg1/3Nb2/3)O3-0.38PbTiO3, Ni47.4Mn32.1Ga20.5/PZT [Wang et al. 2010] etc.

are investigated in literature. Mostly these MF systems are extensively studied and they

are the focus of current research because of the advancement in every field. To overcome

the scarcity of single-phase Multiferroic, one approach is to enhance the specific

characteristics by doping or the other is the development of new Multiferroic materials

such as ferroelectric- ferromagnetic. However the composite of ferrite such as NiFe2O4,

NiZnFe2O4 and CoFe2O4 etc. with Perovskite such as BaTiO3, PbTiO3 and CaTiO3 is of

technological importance. Because these ferrites based composites are results in

Multiferroic properties of higher magnetization in spintronics devices. Also the electric

behaviour of ferrites is highly usable in high frequency based devices.

1.1 Ferrite:

Based on the magnetic properties of high or low coercivity ferrites are

classified as soft and hard ferrites. Ferrites can be classified according to crystal

structure—that is, cubic vs. hexagonal ferrite or magnetic behaviour; that is, soft vs.

hard ferrite. Soft magnetic materials exhibit magnetism only when they are exposed to

a magnetic field, while hard magnetic materials retain magnetism when they are

removed from a magnetic field. Soft ferrites are easy to magnetize and demagnetize.

Hard ferrites are hard to magnetize and demagnetize. Hard magnetic materials are

commonly used for permanent magnetic applications [Srivastava and Yadav 2012].

5

More commonly it can be seen that magnetic heads of a tape deck are made up of

magnetically soft material, and the tape is made of magnetically hard material.

Because of low price and very good chemical stability ferrites are included in the

most important magnetic materials which cannot be easily replaced. One of the

factors used to express the properties of a magnetic material is coercive force. Ferrite

materials are broadly divided into those that do not have coercive force and those that

have high coercive force. Both soft and hard ferrite can store powerful magnetic

energy internally and play key roles in a wide range of electronic circuits and

electronic devices. There are many metallic ferromagnetic materials with strong

magnetic force, but ferrite is a type of ceramic, which means it has high electrical

resistance and maintains its excellent properties even when used with high-frequency

signals [Ohashi et al. 1993].

1.1.1 Soft ferrites:

Ferrites that are used in transformer or electromagnetic cores contain nickel,

zinc, or manganese compounds. They have a low coercivity and are called soft

ferrites. Due to their comparatively low losses at high frequencies, they are

extensively used in the cores of switched-mod power supply (SMPS) and radio-

frequency (RF) transformers and inductors [Srivastava and Yadav 2012].

Noteworthy is the recent rapid increase in the production of soft ferrites used

in transformers for switching regulators. Soft ferrites, compared with magnetic

metals, have such advantages as high electric resistivity, excellent magnetic properties

in the high frequency region, and superior corrosion resistance, but also such

disadvantages as low saturation magnetic flux density, low Curie point, and inferior

mechanical properties. The application of soft ferrites may be divided into two main

fields, one is the field where high permeability and low power loss are required as

represented by Mn-Zn and Ni-Zn ferrites with less than 300 MHZ, while the other is

the microwave region of 300 MHZ or higher where magnetic resonance is involved.

MN-Zn ferrites are used in a frequency region of several megahertz or less as

transformers for SWRs, flyback transformers and communication coils. Ni-Zn ferrites

on the other hand, are used in such applications such as rotary transformers at

frequencies higher than for Mn-Zn ferrites, and as intermediate-frequency

transformers and coils[ Ohashi et al. 1993].The soft ferrites are particularly important

6

since they are relatively inert and their properties can be tailored by chemical

manipulations. The magnetic characteristics of ferrites are strongly affected when the

particle size approaches the critical diameter below which each particle is a single

magnetic domain. The quantum size effects of the large surface area of these

nanometer ferrite particles dramatically changes some of the magnetic properties &

inhibit quantum tunnelling of magnetization .They have boosted up new electronic

technology and are widely used in electromagnetic cores of transformers, switching

circuits in computers and for motors and generators. Ferrites of Ni, Zn, Li, Mn, and

Cu as individual or in mixed compositions do have less value of coercivity causing

low hysteresis loss at high frequency, so are the best material for new technology.

Magnetic soft materials have low coercivity and also low value of remanent magnetic

induction Mr.

1.1.2 Hard ferrites:

In contrast, permanent ferrite magnets (or hard ferrites) which have a high

remanence after magnetization are composed of iron and barium or strontium oxides.

In a magnetically saturated state, they conduct magnetic flux well and have a high

magnetic permeability. This enables these so-called ceramic magnets to store stronger

magnetic fields than iron itself. Hard ferrites have a hexagonal structure and can be

classified as M-, W-, X-, Y-, and Z-type ferrites.

It is an integral property associated with material like Barium, Strontium

ferrite having a characteristic feature of having high value of retentivity and

coercivity. They retain magnetization even when magnetic field is taken off, so after

considered as permanent magnet. Hard ferrite magnets have a wide variety of

applications: Speakers magnets, DC Motors, Sweepers, Magnetic separators for

ferrous materials, Automotive Sensors, MRI’s, and Reed Switching, Hall Effect

devices, Refrigerators and Arts and Crafts as well as many other novel applications.

Magnetic hard ferrites have wide hysteresis loop and coercivity Hc > 2.5 kA/m. They

also express high value of remanent magnetic induction Mr and high value of

maximum energy product (BH) max. These ferrites with hexagonal structure and

strong magneto-crystalline anisotropy are suitable for producing of permanent

magnets.

1.2 Origin of thesis:

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The particles or grains in Nano size play a vital role in the improvement of

properties of Multiferroic materials as compared to bulk i.e. low leakage current and

show dielectric response up to higher frequency region apart from magnetic

properties. With the increasing demand of miniaturized/smaller, faster electronic

devices, sensitive detectors for biomedical and environmental applications, it has

become a necessity to synthesize materials in Nano range < 100 nm.

1.2.1 Importance of soft ferrites:

Nickel ferrite is an important member of the spinal family and it is found to be

the most versatile technological material switched for high frequency application due

to its high resistivity [Albuquerque et al. 2001].Spinal ferrites are good dielectric

materials and they have wide applications ranging from microwave frequency to radio

frequency. Nickel zinc ferrite nanoparticles have potential technological importance

in different applications such as storage media, biomedical fields, and high

performance microwave devices because of their high resistivity, high Curie

temperature, chemical stability, and good soft magnetic properties even at high

frequencies [Tsay et al. 2000; Harris 2012].

Generally, the high resistivity ferrite is possible by having very small size

nanoparticles, responsible for higher frequency dependent of dielectric properties

reasonably involving superparamagnetism which lower its magnetization. Therefore,

the large surface to volume ratio of ferrite nanostructures (Nanorods, nanowires etc.)

exhibits unique properties such as spin canting, surface anisotropy, high resistivity

etc. which may recover the required limitation of dielectric and magnetic properties.

Therefore, Nano size, high purity and uniform distribution of particles are

essential to get enhancement in tailoring various properties including ferromagnetic as

well as electrical properties of ferrites with low preparation cost and small device size.

Recently one of the challenges is to improve the magnetic properties of soft ferrites

such as saturation magnetization, magnetic hysteresis, demagnetizing force and

anisotropic energy.

For soft ferrite synthesis in the thesis work,, among various methods a simple

chemical combustion route and hydrothermal route are employed to synthesize

various compositions of pure and zinc substituted nickel ferrite nanoparticles using

Poly-ethylene glycol as the reducing and chelating agent, which neither requires

8

sophisticated instrument nor high sintering temperature and ethanol water mixture as

solvent cum surfactant in an autoclave.

1.2.2 Importance of Hard Ferrites:

Researchers are trying to produce hard and soft ferrite using simple methods.

In view this, many studies have focused on new systems, such as

CoFe2O4/ZnFe2O4[Masala et al. 2006], earth-iron-boron [Maeda et al. 2004] and

Fe/Z-type ferrite [Liu et al. 2006].The results suggest that coupling exchange exists

between the nanoparticles and the interaction significantly influences magnetization

and coercivity of the composite powders. Masala et al. [2006] reported that exchange

interaction between hard and soft magnetic phases improve the microwave absorption

and magnetic properties of Nano composites.

[Shen et al. 2012] investigated the magnetic properties of SrFe12O19/Ni0.5Zn0.5

Fe2O4 Nano composite in their research. They have pointed to this thread that the

Nano composite magnets combining a high saturation magnetization of the soft phase

and high coercivity of the hard phase will be recognized as the next generation of

permanents.

Recently, the demand for increasing information density and signal-to-noise

ratio and allowing writeability, for e.g. exchange - coupled composite media,

composite granular continuous media and percolated media [Verma and Kotnala

2011a], a composite of soft/hard ferrite layer proposes excellent properties. It is based

on direct exchange coupling across grain boundaries which makes an intimate mixture

of Nano size grains behave differently from a pure superposition of the grains

individual magnetic properties. The coupling could permit that the grain size of the

soft phase should not largely exceed the exchange length of the hard magnetic phase.

Otherwise, a domain wall can form in the anisotropic phase at sufficient distance from

the hard phase, and the so-initiated magnetization process will easily reverse the

whole magnet. To reduce bit size in magnetic recording require higher uniaxial

anisotropy, exchange coupling was proposed to achieve moderate coercivity and thus

write fields while maintaining the stability against thermal demagnetization at room

temperature [Eckert et al. 1996].

1.2.3 Multiferroic (ferrite /ferroelectric) composites:

Ferrites based composites have two advantages such as ferrite as well as

Multiferroic properties which are used in spintronics and high frequency electronic

9

devices. To enhance ferrite properties there is need to combine some hard ferrite

(CoFe2O4, SrFe2O4 etc.) with soft ferrite (NiFe2O4, ZnFe2O4, MnFe2O4 etc.).

Therefore it is important to study best quality of ferrites for improved electrical and

magnetic based applications and their composite with Perovskites for fabricating MF

materials.

Recent work on nanomaterial’s has revealed ME behaviour in

NiFe2O4/BaTiO3 systems, NiFe2O4/PZT, NiCoFe2O4/Ba(0.8)Pb(0.2)TiO3 [Kadam et al.

2003; Kothale et al. 2003] and NiFe2O4/Ba(0.8)Sr(0.2)TiO3 etc.BaTiO3 [Wang et al.

2008]which shows these systems have high permittivity, low dielectric loss and high

tenability whereas NiFe2O4 and NiZnFe2O4 are known for their chemical stability,

high resistivity and excellent electromagnetic properties [Costa et al.

2010].Composites of these materials and individually they need to be researched for

further improvement and their possible applicability in different fields.

Interest in nanoparticle materials permanently increases because of the

significant influence of large surface/volume ratio of nanoparticles on their physical

properties, compared to their bulk counterpart [Vetrone et al. 2004].Ferrite Nano

crystals are also of interest in various applications, such as inter-body drug delivery

[Li et al. 2007; Sun et al. 1995], bio separation, and magnetic refrigeration systems

[Chen and Zhang 1998], in particular due to their specific properties, such as

superparamagnetism. In addition, among ferrospinels zinc ferrites are used in gas

sensing [Niu et al. 2004; Ikenaga et al. 2004], catalytic application [Toledo-Antonio

et al. 2002], photo catalyst [Qiu et al. 2004; Fan et al. 2009],and absorbent materials

[Kobayashi et al. 2002].

Doping ferrite Nano crystals with various metals, such as chromium, copper,

manganese, and zinc are usually used to improve some of their electric or magnetic

properties [Gubbala et al. 2004; Saafan et al. 2010; Singhal and Chandra 2007].For

example, Zn/Ni ferrites have applications as soft magnetic materials with high

frequency (due to high electrical resistivity and low eddy-current loss [Tsay et al.

2000Along that line, (Cu, Zn)/Ni ferrites offer a further improvement as softer

magnetic materials [Aphesteguy et al. 2009].

Transition metal oxide Nanoparticles represent a broad class of materials that

have been investigated extensively due to their interesting catalytic, electronic, and

10

magnetic properties relative to those of the bulk counterparts, and the wide scope of

their potential applications[Raghasudha et al. 2013; Farhadi et al. 2013].Among these

materials, ferrites have attracted immense attention of the scientific community

because of their novel properties and technological applications especially when the

size of the particles approaches to nanometer scale [Chander et al. 2004].As magnetic

materials, Nano-sized ferrites cannot be replaced by any other magnetic material

because they are relatively inexpensive, stable, and have a wide range of

technological applications [Costa et al. 2010].

The spinel ferrites have remarkable magnetic and electrical properties. Among

them, CoFe2O4 is interesting because of its perfect chemical properties, thermal

stability, high electrical resistivity, and excellent magnetic properties [Múzquiz-

Ramos et al. 2010]. Nano crystalline CoFe2O4 with such properties have potential

applications in high frequency devices, memory cores, recording media, and in

biomedical field [Pervaiz and Gul 2012].

The first practical soft ferrite application was in inductors used in LC filters in

frequency division multiplex equipment. The combination of high resistivity and good

magnetic properties made these ferrites an excellent core material for these filters

operating over the 50-450 kHz frequency range. For four decades ferrite components

have been used in an ever widening range of applications and in steadily increasing

quantities.

From a technology development point of view, as global trend for higher

efficiency and miniaturization of the electronic devices, the application requirement

for soft ferrite is getting more and more demanding and challenging. For power

ferrite, it requires lower loss, higher saturation flux density, higher frequency and a

wider temperature range. For high permeability ferrite, wider temperature stability

and frequency stability, higher insert loss, higher impedance and lower THD (Total

Harmonic Distortion) are general requirements.

Soft ferrites are widely used in electronic devices as magnetic cores for high

frequency applications. The advantages of ferrites for these applications are higher

electronic resistivity as opposed to metals, high machinability, and ease of the

pressing, chemical stability and lower cost. Various performance characteristics of

11

ferrites are necessary for varied applications. However, basically high permeability,

high saturation magnetization, high Curie temperature, and low loss are expected.

1.3 Ferrite and Multiferroics nanostructures:

1.3.1 Nanoparticles:

Nano scaled materials are of scientific and technological interest due to their

unique optical, electric, and magnetic properties. Porous solids have higher surface

area, pore volume and tunable pore size compared to nonporous materials. These

properties make the materials interesting in different fields including catalysis,

sorption, separation, drug delivery, sensors, photonics and Nano-devices. As a

colouring and coating material, the iron oxides such as magnetite, hematite and

goethite are commonly used as pigments for black, red, brown and yellow colours

respectively. In general, particle size from 2 to 10 nm increases transparency 3-10

time when compared to the bulk form. These are strong absorbers of ultraviolet

radiation [Sreeram et al. 2006] and mostly used in automotive paints, wood finishes,

construction paints, industrial coatings, plastic, nylon, rubber and print ink. The

excellent weather fastness, UV absorption properties, high transparency and colour

strength makes them to enrich the colours, increase colour shades when combined

with organic pigments and dyes.

Many toxic cations (Co, Zn, Pb, Cd, Cs, U, Sr etc.) and anions like AsO43−

,

CrO42−

, PO43−

, CO32−

etc. are removed by using various phases of iron oxide

[Benjamin and Leckie 1981; Todorović et al. 1992; Ding et al. 2000; Zhou et al.

2001; Luengo et al. 2006; Mohapatra and Anand 2010].Use of iron oxide

nanoparticles is thus becoming very attractive in the area of adsorption or recovery of

metal ions from industrial wastes or natural water streams.

Nanoparticles of magnetic oxides, including most representative ferrites, have

been studied for many years for their application as magnetic refrigeration

[McMichael et al. 1992] photo anode for possible photo-electrochemical cells [Prosini

et al. 2002]. The Nano size of magnetic particle with large surface area change some

of the magnetic properties and exhibit superparamagnetic phenomena and quantum

tunnelling of magnetization which offer a high potential for several biomedical

applications [Reimer and Weissleder 1996; Bonnemain 1998; Pankhurst et al.

2003].Their super paramagnetic property, together with other intrinsic properties,

12

such as low cytotoxicity, colloidal stability, and bioactive molecule conjugation

capability, makes such Nano magnets ideal in both in-vitro and in-vivo biomedical

applications .In case of mitigation of anions/cations from aqueous solutions, iron

oxides in Nano form will have higher number of active sites for adsorption, thereby

reducing the amount required per litre of solution. The adsorption process involves

surface hydroxyl group interaction with adsorbents. Nano iron oxides exhibit very

different magnetic properties which can be used for soft ferrites and biomedical

applications including drug delivery and magnetic resonance imaging. Down to the

Nano scale, superparamagnetic iron oxide nanoparticles can only be magnetized in the

presence of an external magnetic field, which makes them capable of forming stable

colloids in a physio-biological medium. The Nano particles usually have much larger

surface area due to their smaller size and can reduce the volume required to achieve

same effect when used as a catalyst. Considering numerous applications of iron

oxides in various emerging fields, tremendous efforts on synthesis of Nano-dispersed

particles are continuing. The biggest challenge in this field is to economically produce

iron oxide Nano particles of desired characteristics for specific application in large

scale. There has been a lot of progress in understanding the basic science of Nano iron

oxides but evaluation of economic viability for commercial application needs much

more attention.

The adsorption properties of the iron oxide is due to combination of both

surface complexation by inner or outer sphere bonding with adsorbate and ion

exchange by Vander Wall forces. Again the small size of Nano particle also gives a

high surface area-to-volume ratio, which facilitates interaction with several kinds of

chemical species, both gaseous and aqueous [Hiemstra et al. 2004].At the Nano scale

these materials are potentially highly efficient for binding metal ions. By tailoring the

composition of the metal oxides, one can induce selective adsorption of different

metal ions.

When a magnetic field is applied, the particles acquire a certain magnetization

but, because of the high thermal energy, the long range order is lost when the field is

removed, and the particles have no remanent magnetization [Uheida et al. 2006]. This

makes magnetic nanoparticles excellent candidates for combining metal binding and

selective adsorption properties with ease of phase separation.

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Iron oxides have relatively high surface area and surface charge, therefore,

often regulate free metal and organic matter concentrations in soil or water through

adsorption reactions.

Iron oxide-based materials have been found to be good candidates as cheap

and efficient catalysts, especially in environmental catalysis. [Miyata et al. 1978]

studied the catalytic activity of several iron oxides and oxide hydroxides of various

particle sizes for the reduction of 4-nitrotoluene using hydrazine hydrate as reducing

agent, and found β-FeOOH was the most effective catalyst. Iron oxide (usually mixed

with other metal oxides) in particular, has been shown to be a very active (although

unstable) catalyst for the oxygen evolution process as well as other related processes,

such as water splitting, chlorine evolution, the oxidation of organic molecules, the

oxygen reduction process and for the hydrogen peroxide decomposition.

1.3.2 Nanorods:

In nanotechnology, Nanorods are morphology of Nano scale objects. Each of

their dimension ranges from 1-100 nm. Nanorods may be synthesized from metals or

semiconducting materials with ratios (length divided by width) are 3:5.One-

dimensional (1D) nanostructures represent a group of Nano-materials with highly

anisotropic morphologies and have received much attention since the discovery of

carbon nanotubes (CNTs) in 1991. [Iijima 1991] Controlled nucleation and growth in

a particular crystallographic direction is considered the basis for the formation of a 1D

nanostructure. However, the growth mechanisms by which anisotropic development

occurs can differ depending on the Nano-material and its method of production.

On the basis of experiments showing large variation in magnetic behaviour due

to size or morphology differences in nanoparticles as well as a few recent studies on

ferrite Nanorods and nanowires, 1D nanostructured ferrites are expected to exhibit

many properties unlike those of particles of the same phase. In contrast to spherical

nanoparticles, Nanorods with their inherent one-dimensional (1- D) shape anisotropy

may exhibit unique magnetic behaviour which is significantly different from that of

the bulk material. Few investigations of the magnetic properties of hematite Nanorods

have been reported.

For 1-D Nanorods with a high aspect ratio, the shape anisotropy may play an

important role in dictating the magnetic properties. The magnetic domain structures,

14

which determine the magnetic properties of the materials, have been reported to be

affected by the 1-D nanostructure. Ad atoms and Nano clusters adsorbed at the

Nanorods surfaces may also have an impact on the magnetic properties. Further, the

impurities, defects, and internal stresses of the Nanorods that are likely to strongly

depend on the preparation methods could also be important factors influencing the

magnetic properties.

Recently, some of the researchers have prepared nanostructured materials by

different synthesis methods. [Bousquet-Berthelin et al. 2008] have reported NiFe2O4

nanoparticles with elementary particle size close to 4–5 nm by flash microwave

synthesis and investigated their possible applications as cathode materials for lithium-

ion battery. [Kavas et al. 2009] prepared NiFe2O4 nanoparticles by surfactant assisted

hydrothermal process and their structural and magnetic properties were investigated in

detail.

The micron sized rod-like particles of nonstoichiometric Co and Ni ferrites

were synthesized by aging co-precipitated Fe(OH)2 and (NiOH)2 at 90 oC in the

presence of an external magnetic field and mechanism for the formation of rod-like

particles was investigated by the time-dependent observation of growing Ni ferrite

rods [Vereda et al. 2008].[ Wang et al. 2008]synthesized MFe2O4 (M = Co, Ni),

ribbons with Nano porous structure which were prepared by electro spinning

combined with sol–gel technology. [Liu et al. 2009] reported NiFe2O4 nanoparticles

and Nanorods synthesized by a facile hydrothermal treatment of Ni(DS)2, FeCl3 and

NaOH aqueous solution. [Zhang et al. 2005] synthesized Nanorods by polyethylene

glycol assisted route and investigated their structural and magnetic properties.

The exciting discovery of the fullerenes was followed closely by the discovery

of nanotubes of carbon. Nanotubes show tremendous promise as building blocks for

new materials. Because of their topology, nanotubes have no dangling bonds, and so

despite being very small, they do not exhibit ―surface effects.‖ As a consequence,

individual nanotubes exhibit nearly ideal electrical, optical, and mechanical

properties. Nanorods are also under extensive development and investigation.

Nanorods have wide applications; they find their applications in dye solar cells, for

oligonucleotide detection, applied electric field, for applied humidity sensitive.

1.3.3 Nanowires:

15

A nanowire is a wire of dimension of the order of nanometer (10-9

meters). At

these scales, quantum mechanical effects are important-hence such wires are also

known as ―quantum wires‖. The nanowires could be used, in near future, as

components of nanotechnology to create electrical circuits out of compounds that are

capable of being formed into extremely small circuits. Some early experiments have

shown that they can be used to build the next generation of computing devices. To

create active electronic elements, the first key step was to chemically dope a

semiconductor nanowire. This has already been done to individual nanowires to create

p-type and n-type semiconductors. Nanowires are not observed in nature and must be

produced in a laboratory.

These nanowires can be suspended, deposited or synthesized from the

elements. Nanowires show peculiar properties due to their size. Unlike carbon

nanotubes, whose motion of electrons can fall under the regime of ballistic transport,

nanowires conductivity is strongly influenced by edge effects. The edge effects come

from atoms that lay at the nanowire surface and are not fully bonded to neighbouring

atoms like the atoms that lay at the surface and are not fully bonded to neighbouring

atoms like the atoms within the bulk of the nanowires. The unbounded atoms are

often a source of defects within the nanowire, and may cause the nanowire to conduct

electricity more poorly than the bulk material. As a nanowire shrinks in size, the

surface atoms becomes more numerous compared to the atoms within the nanowire,

and edge effects become more important. Recent research has shown that the high

aspect ratio of magnetic nanowires can produce a larger magnetic moment than that

observed in particles of the same volume, providing significant benefits in numerous

applications. In fields such as local drug delivery, improved magnetic properties

would allow the ability to deliver drugs more quickly and accurately, enabling

treatment of smaller areas with lower dosages and decreased side effects. Novel

ferrite properties could likewise benefit current communications, defence, memory

storage, and energy technologies, among others.

The synthesis of transition metal doped BaTiO3 nanostructures are of great

importance [Kaur et al. 2012] and has attracted much attention due to its novel shape

and size dependent properties. For instance, the ferroelectric Curie temperature of the

zero dimensional BaTiO3 nanoparticles decreases progressively with particles size

[Verma et al. 2012].On the other hand one dimensional BaTiO3 nanowires still retain

16

their ferroelectric properties, and non-volatile polarization domains with dimensions

can be induced in the nanowires. This further open ups the possibility of fabricating

BaTiO3 nanowire-based non-volatile memory devices and for many more applications

[Yun et al. 2002; Mao et al. 2003].The polarization within individual ferroelectric

domains of the nanowire generally orient along the wire axis [Morber et al. 2006].

1.4 Different parameters responsible for fabrication of nanostructures:

It is well known that increasing or decreasing the concentration of the

chemical reactants will eventually influence the resultant products. The properties of

ferrites and their composite materials are sensitive to the grain size and also strongly

influenced by the distribution of metallic ions among crystallographic crystal lattice

sites. These in turn are sensitive to the method used to prepare those materials

[Mouallem-Bahout et al. 2005].As a whole there are Several factors are important in

the preparation of nanostructures, such as the nature of the cations, their ratio, and the

nature of the anions, pH, temperature, aging, fuel, solvent, surfactant and the

preparation method. Commercial applications of all these different nanostructures

required a high degree of control over the processing as well as the structure and

composition of the resulting materials. This requires a much better understanding of

the underlying mechanisms, chemistry and physics behind the processes occurring

during synthesis. As much more is dependent on synthesis methodology, so it should

be versatile, simple and rapid process which should allow effective synthesis of a

variety of Nano size materials. The development of this knowledge is currently still in

its infancy and clearly much more work needs to be done in this area in the near

future.

1.4.1 Surfactants:

In synthetic techniques fundamental goal is to produce atoms in solution

which quickly (rather spontaneously) formulate into nanoparticles and then to control

their size/shape by utilizing surfactants. Shape controlled Nano crystals possess well-

defined surfaces and morphologies because their nucleation and growth are controlled

at the atomic level. The recent years have seen tremendous progress in the preparation

of nanostructured materials using surfactant. Surfactant: The name "surfactant" refers

to molecules that are surface active, usually in aqueous solutions [Kanel et al.

2006].They are typically soluble in both organic solvents and water. There are

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hundreds of compounds that can be used as surfactants and are usually classified by

their ionic behaviour in solutions; anionic, cationic, non-ionic or amphoteric

(zwiterionic). Each surfactant class has its own specific properties.

A huge variety of different organic and inorganic compounds can be prepared

into a wide range of different nanostructures. A systematic study of commonly used

surfactants cationic or anionic and different pH values lead to surface induced

reactions that are responsible for fast decomposition of the raw material being used

for preparation and chemical reduction due to active groups [Kaczmarek and Ninham

1997]. Nanoparticles grow with increasing the temperature, while surfactant prevents

the particle growth under the same condition. Commonly used surfactants include,

PVA, PVP, PEG and CTAB etc. An ideal surfactant should have qualities like it

should be cheap, easily available and utmost requisite property it should act as solvent

and fuel. The properties of ferrite nanoparticles can be altered by controlling their

size, which can provide an advantage in formulating new composite materials with

optimized properties for various applications.

Thus, to control the growth of the spinel ferrite nanoparticles and their

corresponding Nanocomposites, organic stabilizers (polymers), e.g., polyvinyl alcohol

(PVA), polyethylene oxide (PEO), polymethacrylic acid (PMAA), and Poly vinyl

pyrrolidone (PVP), are added during the synthesis. New methods of synthesizing

Nano scale materials have also shown that in addition to size, a nanostructure’s shape

can also profoundly affect its physical properties. Non spherical architectures such as

one-dimensional (1D) wires and rods and shapes have demonstrated an enthralling

diversity of properties [Alarifi et al. 2009; Kavas et al. 2009].

1.4.2 Role of pH and temperature:

It has been found that both temperature and base play a very important role in

the formation of well-defined confined nanostructures. In general role of base is to act

as a mineralizer. High calcination temperature (above 450°C) is usually required to

form a regular crystal structure but it is also observed that above 700 oC crystal size

growth takes place. The properties of ferrite do change from expected ones with a

small change in sintering temperature. Temperature is a crucial factor and its role

starts from homogenous mixing of raw materials to till the formation of product. The

effect of the pH is crucial because hydroxide ions (OH−) are strongly related to the

18

reactions that produce nanoparticles. The pH of solution is one of the main factors on

which the final composition of the product depends, which can be varied to get the

desired final product.

It is well known that the morphology of the precipitation/solution synthesized

metal-oxides strongly depend on the amount of H+ or OH

- ions in the sol that

effectively determines the polymerization of the metal–oxygen bonds. Precursor

solution pH variation affects the hydrolysis and condensation behaviour of the

solution during gel formation, and hence influences the morphology. The pH of

solution appears to be critical parameter for the phase formation, particles size and

morphology of the structure during preparation method. The pH is particularly

important when some impurities are present in the growth medium because it

influences, for example, the formation of either zwitterions (ions having both positive

and negative charges) or complex ions. The presence of these various species during

the Nano crystal growth modifies the growth of certain crystal faces. The changes in

shape are due to the differences between the growths rates of the various

crystallographic faces.

1.4.3 Stoichiometric ratio of the compound, doping:

It is well known that increasing or decreasing the concentration of the

chemical reactants will eventually influence the resultant products. Substitutions in

simple, inverse and mixed ferrites and their composites have received a great deal of

attention over the past few years. The substitution of various magnetic and

nonmagnetic ions at different sub lattices in ferrites materials has provided interesting

magnetic structures and electrical properties.

The spinel ferrites and composites are very attractive among them, as

substitution/doping allow a variety of magnetic, electrical and structural disorders,

and also surface chemistry is altered which in turn introduces numerous novel

properties in them. As these substitutions have different sitting preferences for the two

sites (A and B) in the spinel structure and can change many properties as an effect of

modified cation distribution in the material. This may be due to the fact that in spinel

ferrite, the intra-sublattice interactions (A-A and B-B) are weaker than the inter-

sublattice interactions (A-B) as a result of the unsatisfied bonds in antiferromagnetic

phase.

19

The presence of unsatisfied bonds results in increase in magnetic dilutions,

which generate competition between the various exchange interactions. These

exchange interactions result in a variety of magnetic structures. Scientist are still

improving the property of different preparation technique and also doping with

impurities. Any change in surface chemistry may directly affect the gas sensing

properties of one-dimensional (1D) nanostructures which is governed by the

distribution of anions and cations in the structure. For example, the sensitivity of

magnetic mixed oxide-based sensors can be boosted by various doping schemes and a

number of different dopants such as Pd, Sn, Ti, Zn etc. have been used [Kanai et al.

1992; Neri et al. 2006; Reddy et al. 2002; Vasiliev and Polykarpov 1992; Gurlo et al.

1997; Korotcenkov et al. 2007; Tiemann 2007].

1.4.4 Synthesis routes:

Nano-scale science and engineering is likely to produce the strategic

technology breakthroughs of tomorrow. Our ability to work at molecular level- atom

by atom- to create something new, something we can manufacture from the ―bottom

up‖ and ―bottom down‖ opens huge vistas for many of us. The continuous study &

enhancement of nanofabrication techniques is a crucial activity in Nano-science

/technology. By choosing a method that leads to a reduction of the particle size, the

magnetic properties such as coercive field, Curie temperature, saturation

magnetization and increase in absorption coefficients may change significantly in

comparison with those of the bulk material.

In case of nanostructures, it is possible to control the crystallinity and

stoichiometry during the growth process which allows for the manipulation of the

crucial parameters that control their properties. There are several methods for

preparation of nanomaterial’s Viz. Chemical vapour deposition, Physical vapour

deposition, Sputtering, Hydrothermal, Co-precipitation, chemical combustion method

etc. [ Dube and Darshane 1993; Upadhyay et al. 2004; Shannon and Prewitt 1970].

Every method has its own impact on the properties of Nano materials which depend

upon various parameters. Among these we have selected/chosen Chemical

combustion and hydrothermal method for the preparation because of their individual

and distinctive features. Chemical combustion method produces nanoparticles with

much ease and comfort, simple calculation, homogenous and un-agglomerated

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powder, inexpensive raw material. On the other hand hydrothermal method is a low

temperature synthesis routes resulting in the fabrication of different nanostructures (in

the form of Nanorods, Nano particles, Nano wires, etc.) also provides ease in

optimization of process parameters and restricts size to remain in between 1-20 nm

which further enhances various properties of synthesized material.

Details of processing steps

(A) Processing steps used for preparation of materials by chemical combustion

method

1. Analysis, purification of raw materials.

2. Checking feasibility of Stoichiometric ratios of constituent of different

powders.

3. Processing of nanoparticles by chemical combustion method using PEG.

4. Crystallization and annealing of powdered samples, their washing and

purification.

5. Pellet formation by pressing crystallized powder by using PVA as binder.

(B) Processing steps used for synthesis of materials by hydrothermal method

1. Analysis, purification of raw materials.

2. Checking solubility of stoichiometric ratios of different raw materials.

3. Processing of materials by hydrothermal treatment in an autoclave.

4. Crystallization and annealing of powdered samples, also their washing and

purification.

5. Pellet formation by pressing crystallized powder by using PVA as binder.

1.5 Application of Ferrites and Multiferroics:

Due to advancement in all aspects of life the development in electronic

technology is directly coupled with the advances made in materials science. Within

the broad class of materials available today, functional materials provide exclusive

opportunity for developing novel components and devices as their physical and

chemical properties are sensitive to changes occurring in the environment such as

temperature, pressure, electric field and magnetic field. Ferromagnetic and

ferroelectric materials are presently utilized in a wide range of systems.

21

1.5.1 Ferrite:

Ferrite materials have wide range of applications. Among oxide compounds,

spinel ferrites emerge as subjects of intense research activity, mainly due to the

magnetic, electrical, chemical and other properties exhibited by this class of materials.

[Srivastava and Yadav 2012]Recently, ferrite materials have received extensive

applications in humidity sensors, gas sensors, catalysts, pigments, and anticorrosive

agents [Sun et al. 1995; Chakrabarti et al. 2005; Hotta et al. 1991; Gardner et al.

1966; Darshane et al. 2008; Jing 2006].Filter inductors, Antenna core, Flyback

Transformers, Magnetic Amplifiers, Magnetic memories and switches, IF

transformers and tuned inductor, Ceramic magnet as medical treatment.

Besides well-known applications related to data storage, new fields utilizing

magnetic Nano sized particles are emerging particularly in the biomedical

technologies development. Ferrites also play a significant role in thermochemical

hydrogen production from water-splitting cycles [Padella et al. 2005; Varsano et al.

2011]. Ferrites in Nano-scale have exhibited great potential for their applications as

catalytic materials, wastewater treatment adsorbents, pigments, flocculants, coatings,

gas sensors, ion exchangers, magnetic recording devices, magnetic data storage

devices, toners and inks for xerography, magnetic resonance imaging, bio separation

and medicine [Mohapatra and Anand 2010]. These nanostructures find applications as

catalysts, sorbents, pigments, flocculants, coatings, gas sensors, ion exchangers and

for lubrication [Lim et al. 2001; Sharrock and Bodnar 1985; Sestier et al. 1998; Choo

and Kang 2003].

Magnetic Nano-composites have potential applications in areas such as

magnetic recording, magnetic data storage devices, toners and inks for xerography,

and magnetic resonance imaging, wastewater treatment, bio separation, and medicine

[Raj and Moskowitz 1990; Pieters et al. 1991; Ziolo et al. 1992; Šafařík 1995; Häfeli

1997; Schütt et al. 1997; Denizli and Say 2001]. Ni-Zn substituted mixed ferrites have

properties like low coercivity, high resistivity values and little eddy current losses

which makes them excellent core materials for power transformers in electrical and

telecommunication properties [Costa et al. 2003].Magnetic nanoparticles get heated

on subjection to alternating magnetic fields, this can be utilized in destroying tumour

cells.

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1.5.2 Multiferroic:

Recently, ferroelectric–ferromagnetic composite materials that exhibit a large

ME effect at above room temperature and at low bias have attracted noteworthy

attention from the scientific community due to their capability of efficient energy

transfer between electric energy and magnetic energy as shown by the rising number

of publications in the last few years and their subsequent applications for their

significant usages such as, magnetic–electric sensors in radio-electronics [Petrov et al.

2007], oscillators, phase shifters, memory devices, transducers and also as compact

electrical filters for suppressing electromagnetic interference (EMI) and so onto the

next generation multifunctional devices that can be electrically written and

magnetically read [Kang et al. 2009].

The ME (Magneto electric) composites can be used as magnetic probe for

detecting ac or dc fields. Sensitive magnetic sensors can be obtained using the ME

composites with high ME coefficients. Due to novelty of the ME effect, these

composites may find applications in memory devices, a memory device so produced

will be accompanied with the combination of best functionalities of FeRAMs and

MRAMs (ferroelectric write and magnetic read operations) would efficiently improve

the writing speed and reduce the energy consumption. Moreover, device

miniaturization can further lead to reduced energy consumption and higher speeds

[Bichurin et al. 2012; Eerenstein et al. 2006].

1.6 Objective and present work:

The concept for the present plan of research work was aimed to undertake a

systematic study on synthesising parameters and possibility of multifunctional

properties i.e. structural, microstructural, magnetic, electric and dielectric properties

in nanostructured materials. Nanomaterial’s are of prodigious scientific interest as

they are an actual bridge between bulk materials and atomic or molecular structures.

The properties of bulk materials are only reliant on their chemical composition.

However at the Nano-scale, the properties of materials are not only determined by

chemical compositions, but also by sizes and shapes.

The survey of recent literature studies have showed that there still remains

scope of research for the production of Ni1-xZnxFe2O4/SrFe2O4 and

Ni1-xZnxFe2O4/BaTiO3 system in both bulk and Nano forms with low cost, efficient,

23

ease of processing and with desired properties and structure. Ni1-xZnxFe2O4, Ni1-

xZnxFe2O4/SrFe2O4, NiFe2O4/BaTiO3 and Ni1-xZnxFe2O4/BaTiO3 system by primarily

employing Chemical combustion method and then subjecting selected samples from

the chemical combustion method to be prepared by hydrothermal method. These

methods are cheap, simple and provide free choice of the composition of components.

So we have planned to involve both methods due to their unique features to

investigate changes in properties by applying these methods.

Further, these results can be useful to a large extent in giving new dimensions

to the emerging technologies. Because of exclusive physical, chemical properties and

numerous applications, a lot of work has been done in the field of ferrite

Nanomaterial’s. However, there are still challenges ahead that we intend to address

effectively in the thesis.

In view of various research challenges in the Ferrite materials, the objectives

of the present study were structured as.

1) To chemically synthesize the Nanomaterial’s of pure and

doped with Zn and composite materials with , and the

enhancement in ferrite properties of with hard ferrite

. All the compositions of these ferrite and Multiferroic

composite have been prepared by two methods: Chemical Combustion

and Hydrothermal.

2) For combustion PEG (Poly Ethylene Glycol) is used as an efficient

fuel and solvent and urea is used to create an overall redox system.

3) In hydrothermal synthesis, have been used as

a basic medium for pH adjustment.

4) Effects of particle size on the various properties of ferrite nanoparticles

and Multiferroic nanoparticles have been investigated.

5) Comparative analyses of preparation methodology on properties of

resulting materials have been investigated.

24

6) Effect of Zn concentration on the structural, Microstructural, dielectric

and magnetic properties of system have been

thoroughly investigated.

7) Effects of preparation of composite of and with

and have been investigated.

1.7 Outline of the thesis:

Chapter 1 gives an insight of the brief introduction of ferrites; composites of

hard and soft ferrites along with Multiferroic have been taken up. It also includes their

applications, role of different parameters in their synthesis part along with objectives

of the research work. In the Second chapter attempts have been made to

systematically classify the available information .This chapter incorporates

information to assist in understanding the aim and objective of the investigation ,

literature survey of Ferrite nanostructured materials their structure, history; general

methods of synthesis of nanomaterial and techniques used to analyse their various

parameters are explained. Third chapter enunciates with the detailed insight of the

material and experimental methods used to prepare samples where in quantity of

material, reactions involved and methodology is discussed. Fourth and Fifth chapter

comprises of results and discussions obtained from prepared samples. In chapter sixth

a summarized conclusion of all previous chapters is given. A complete list of

references has been given towards the end of the thesis. Finally a concise list of

publications based on research work has been presented at the end.

1.8 Future scope:

With the arrival of nanotechnology, an incredible rush in research on

miniaturization and high efficiency electronic devices is on growth. These materials

suit these demands and are considered to shape the future of advanced technology. In

recent years, nanotechnology has not only opened new vistas for the preparation of

various novel Nano size oxides and composites, but also prospered in continuous

synthesis methods of Nano powders and development of various supported catalysts

25

and coatings. As a result, conditions are developed for breakthroughs in these areas

over the next several years.

The author is interested in the study of the magnetic properties of the Nano

crystalline soft magnetic material, and the changes in the magnetic, electric, dielectric

and catalytic and other properties of Nano crystalline spinel ferrites and their

composites. The future challenges of nanoparticles and their applications are abundant

only a few of them are explored, for instance the preparation of ferrite samples with

low eddy current losses and a useful frequency of the order of gigahertz is a

challenging one. Also in the pursuit of high density recording the challenge is to have

unidirectional permanent magnetism. The magnetic relaxation which tends to destroy

magnetization has to be successfully overcome by novel methods of preparation of

samples. These nanostructured materials having large surface to volume ratio would

act as efficient catalyst for heavy and toxic metal separation from waste water and

also in degradation of coloured effluents from dye and related industries. Their

efficiency in various organic reactions is needed to be explored in the form of

extension of this work.