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Physica B 404 (2009) 2309–2314

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Comparative study of structural and magnetic properties of nano-crystallineLi0.5Fe2.5O4 prepared by various methods

Vivek Verma a,b, Vibhav Pandey a, Sukhveer Singh a, R.P. Aloysius a, S. Annapoorni b, R.K. Kotanala a,�

a National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, Indiab Department of Physics and Astrophysics, University of Delhi, New Delhi 110007, India

a r t i c l e i n f o

Article history:

Received 18 November 2008

Received in revised form

18 March 2009

Accepted 23 April 2009

Keywords:

Magnetic properties

X-ray diffraction

Nano-crystalline

Microstructure

Magnetization

26/$ - see front matter & 2009 Published by

016/j.physb.2009.04.034

esponding author. Tel.: +9111456 08266.

ail address: rkkotnala@mail.nplindia.ernet.in

a b s t r a c t

Lithium ferrite has been considered as one of the highly strategic magnetic material. Nano-crystalline

Li0.5Fe2.5O4 was prepared by four different techniques and characterized by X-ray diffraction, vibrating

sample magnetometer (VSM), transmission electron microscope (TEM) and Fourier transform infrareds

(FTIR). The effect of annealing temperature (700, 900 and 1050 1C) on microstructure has been

correlated to the magnetic properties. From X-ray diffraction patterns, it is confirmed that the pure

phase of lithium ferrite began to form at 900 1C annealing. The particle size of as-prepared lithium

ferrite was observed around 40, 31, 22 and 93 nm prepared by flash combustion, sol–gel, citrate

precursor and standard ceramic technique, respectively. Lithium ferrite prepared by citrate precursor

method shows a maximum saturation magnetization 67.6 emu/g at 5 KOe.

& 2009 Published by Elsevier B.V.

1. Introduction

Lithium ferrite belongs to a group of soft ferrite materialsand is extensively used in many applications such as microwavedevices, computer memory chip, magnetic recording media,radio frequency coil fabrication, transformer cores, rod antennasand many branches of telecommunication and electronic engi-neering [1–5]. Nano-crystalline materials are showing greatpromise in industry and technology. This is mainly due to theirsome unique properties which are not exhibited by bulk crystal-line materials [6,7].

The structural and magnetic properties of spinel ferritesdepend upon the method of preparation [8,9]. The conventionalway to produce these materials is the solid state reaction methodof oxides/carbonates followed by calcinations at high tempera-ture. The solid state reaction processing has some inherentdisadvantages such as chemical inhomogeneity, coarser particlesize and introduction of impurity during the mixing/ball milling.In this work we have made the comparative study of micro-structure and magnetic properties of lithium ferrite prepared bydifferent techniques. There are many methods to prepare lithiumferrite but we opted for commonly used methods like as ceramictechnique [10–12], sol–gel [13,14], citrate [15,16] and flashcombustion [17,18] techniques.

Elsevier B.V.

(R.K. Kotanala).

2. Experimental section

2.1. Sample preparation

The lithium ferrite was prepared by four different techniques:three unconventional routes like flash combustion (to be denoted asA), sol–gel (to be denoted as B), citrate precursor (to be denoted as C)and conventional standard ceramic technique (to be denoted as D). Inthis work we have compared the structural and magnetic propertiesof Li0.5Fe2.5O4 prepared by four different processing techniques.The flow chart of the synthesis route of each technique is shown inFig. 1.

2.1.1. Flash combustion

Fine particles of Li0.5Fe2.5O4 have been prepared by flashcombustion technique. Analytical grades of Fe(NO3)3 �9H2O andLiNO3 were mixed with urea in the ratio of 2.5:0.5 in an agatemortar for 20 min. Urea was added as a fuel. The mixture wasplaced in furnace at 400 1C. At this temperature the combustiontook place and reaction completed within 4–5 min. A foamy andhighly porous product obtained was grounded and annealed at700, 900 and 1050 1C for 5 h in a furnace whereas heating/coolingrate of 5 1C/min was maintained.

2.1.2. Sol–gel method

A stoichiometric amount of LiNO3 and Fe(NO3)3 �9H2O wasdissolved in minimum amount of water followed by the additionof ethylene glycol at about 60 1C. After heating the sol of metal

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Preparation Technique

A B C D

Flashcombustion

LiNo3,Fe(NO3)3.9H2O,

Urea

LiNo3,Fe(NO3)3.9H2O,

LiNo3,Fe(NO3)3.9H2O,

+ Citric acid

Standardceramic

Li2CO3, Fe2O3

Mixing, Grinding& Pre-Annealing

at 750°°C,8 h.

Mixing, Grinding& Final-Annealingat 900, 1050°C, 5 h.

Annealing at700, 900,

1050°C, 5 h.Annealing at

700, 900,1050°C, 5 h.

Annealing at700, 900,

1050°C, 5 h.

Mixing,thermolysis

at 400°C, 1 h.

Mixing inethylene glycol +deionized water

at 40°C, 6 h.

Stirrer at80°C, 24 h.

Deionizedwater

Evaporatedat 40°C

Sol-gel Citrateprecursor

Fig. 1. Flow chart of different preparation routes.

V. Verma et al. / Physica B 404 (2009) 2309–23142310

compounds to around 40 1C a wet gel is obtained. The obtained gelwhen dried at 80 1C self-ignites to give a highly voluminous andfluffy product of lithium ferrite. As-prepared sol–gel powder wasgrounded and annealed separately at 700, 900, and 1050 1C for 5 hin a furnace with a heating and cooling rate of 5 1C/min.

2.1.3. Citrate precursor

Fine particles of lithium ferrite was prepared by citrateprecursor method. A stoichiometric amount of Fe(NO3)3 �9H2Owas dissolved in deionized water and mixed with the aqueoussolution of citric acid in 1:1 molar ratio of cation to citric acid.A stoichiometric amount of lithium nitrate was added in thesolution drop by drop under continuous stirring. The resultantmixture was slowly evaporated and then dried at 40 1C for 1

2 h withstirring until brown agglomerates was obtained. The obtainedpowder was annealed separately at 700, 900 and 1050 1C for 5 hwith a heating and cooling rate of 5 1C/min.

2.1.4. Standard ceramic technique (solid state)

In this method sample was prepared by conventional doublesintering ceramic technique. According to stoichiometric compo-sition Li0.5Fe2.5O4 specified molar ratio of iron oxide (Fe2O3) andlithium carbonate (Li2CO3) were milled by wet grinding. Themixture was heated in air at 750 1C for 8 h maintaining theheating/cooling rate at 5 1C/min. Further, pre-annealed powderwas grounded and mixed properly and final annealing was carriedout at 900 and 1050 1C in air for 5 h with a heating and coolingrate maintained at 5 1C/min.

2.2. X-ray diffraction studies

Phase analysis of lithium ferrites samples prepared fromdifferent preparation methods were investigated by X-ray diffrac-tion (XRD) (model) with Cu Ka radiation of wave lengthl ¼ 1.5443 A. The scanning range was from 201 to 801 in step of0.03.

The average grain size was determined from the mea-sured width of their diffraction curves using Debye Scherrer’srelation:

D ¼ 0:9l=b cos y

where l is the wavelength of the Cu Ka radiation (l ¼ 1.5443 A), bis the full width half maxima in radians. Debye Scherer’s formulaassumes approximations and gives the average grain size. Thecontribution due to instrumental broadening has to be taken intoaccount in order to obtain the accurate grain size.

The lattice constant was calculated by using the relation

2d sin y ¼ nl

where d ¼ a/(h2+k2+l2)1/2 for fcc system.

2.3. Magnetic measurements

Magnetic measurements were carried out using vibratingsample magnetometer (Lake Shore 7305) for all samples at roomtemperature. The magnetic properties such as saturation magne-tization (Ms), coercivity (Hc) and retentivity (Mr) were calculatedfrom M–H curves.

2.4. TEM measurement

The particle size of as-prepared samples A (flash combustion),B (sol–gel) and C (citrate) were observed from (JEM-200CX)TEM.

2.5. FTIR measurement

FTIR spectra of all as-prepared samples A, B, C and D wererecorded in KBr medium in the wave number range400–4000 cm�1 with a Perkin Elmer FTIR (spectrum BX).

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Fig. 2. X-ray diffraction patterns of as-prepared Li0.5Fe2.5O4 samples by technique

of (A) flash combustion, (B) sol–gel, (C) citrate precursor and (D) solid state.

Fig. 3. X-ray diffraction patterns of 700 1C annealed Li0.5Fe2.5O4 samples by

technique of (A) flash combustion, (B) sol–gel, (C) citrate precursor and (D) solid

state.

Fig. 4. X-ray diffraction patterns of 900 1C annealed Li0.5Fe2.5O4 samples by

technique of (A) flash combustion, (B) sol–gel, (C) citrate precursor and (D) solid

state.

Fig. 5. X-ray diffraction patterns of 1050 1C annealed Li0.5Fe2.5O4 samples by

technique of (A) flash combustion, (B) sol–gel, (C) citrate precursor and (D) solid

state.

V. Verma et al. / Physica B 404 (2009) 2309–2314 2311

3. Results and discussion

3.1. Crystalline structure

Figs. 2–5 exhibit the XRD patterns of lithium ferrite preparedby different techniques and annealed at different temperatures. Itis evident that the as-prepared powders of lithium ferriteprepared by four different techniques are not in single phases.As-prepared samples A, B and D have lithium ferrite as well asa-Fe2O3 phase but in the case of C (citrate method) there is noclear evidence of any phase/peaks.

Lower annealing temperature leads to a poor crystallinity,whereas much better crystallized powder is obtained above700 1C. As the annealing temperature is increased above 700 1C,Li0.5Fe2.5O4 is formed confirming all the peaks in the patternmatching well with JCPDS card (17-0115). As the annealing

temperature increases, the width of the central maxima (311)decreases and the intensity of the peaks increases. This is dueto the increase in the grain size of the ferrite particles asthe annealing temperature increases. The average grain sizeand saturation magnetization of all samples of lithium ferriteare listed in Table 1. Lattice constant value 8.3556 A on an averageremains constant and grain size increases with annealingtemperature. The average particle size of as-prepared samplewas found different from one method of preparation toanother. The minimum particle size 22 nm of lithium ferrite wasobtained by citrate processing technique. More uniform particlesize distribution was found by sol–gel technique compared tocitrate and flash combustion technique as evident from TEMphotographs in Fig. 6. The grain size of samples annealed at1050 1C was not calculated due to limitation of Debye Scherrer’srelation.

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Table 1Saturation magnetization and grain size for all samples of lithium ferrite.

Preparation techniques Saturation magnetization Ms (emu/g) Grain size (nm)

As-prepared 700 1C 900 1C 1050 1C As-prepared 700 1C 900 1C

Flash 40.00 56.22 57.39 61.56 40.60 94.18 106.40

Sol–gel 26.00 54.42 60.96 62.54 31.07 87.16 97.61

Citrate 00.50 54.65 55.30 67.66 22.20 95.73 102.92

Solid state 00.85 57.00 61.84 63.34 93.96 94.89 99.64

Fig. 6. Transmission electron micrographs of Li0.5Fe2.5O4 samples (a) flash combustion, (b) sol–gel and (c) citrate method.

Fig. 7. FTIR spectra of Li0.5Fe2.5O4 samples prepared by (A) flash, (B) sol–gel, (C)

citrate and (D) solid state method.

V. Verma et al. / Physica B 404 (2009) 2309–23142312

Fig. 7 shows the FTIR spectra of as-prepared lithium ferritesamples prepared by different techniques. The presence of wavenumber bands in the range of 400–600 cm�1 in the spectra confirmthe formation of lithium ferrite [19]. As-prepared samples A, B and Cshowed the characteristic bands at about 1300 and 2900 cm�1

corresponding to the NO3� ion and O–H group, respectively. The

existence of the characteristics bands of NO3� indicated that the NO3

�

as a group exists in the structure. NO3� band in citrate prepared

sample is very dominant in comparison to other synthesis routes oflithium ferrite samples. Combustion can be considered as thermallyinduced anionic redox reaction of the gel where in the citrate ions actas a reductant and the nitrate ions as an oxidant. Because the nitrateions provide an in situ oxidizing component, the rate of the oxidationreaction relatively increases. From Fig. 7 it is confirmed that very lessformation of spinel phase is in sample C but for the samples A and Bsome spinel phase begins to form. FTIR curve for sample prepared bysolid state technique shows the formation of spinel structure.

3.2. Magnetic properties of lithium ferrites

The hysteresis curves (M–H curves) for the lithium ferritesprepared by different techniques and annealed at different

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Fig. 8. Hysteresis curve for the flash combustion synthesized as-prepared

Li0.5Fe2.5O4 powder and annealed at 700, 900 and 1050 1C.

Fig. 9. Hysteresis curve for the sol–gel synthesized as-prepared Li0.5Fe2.5O4

powder and annealed at 700, 900 and 1050 1C.

Fig. 10. Hysteresis curve for the citrate precursor synthesized as-prepared

Li0.5Fe2.5O4 powder and annealed at 700, 900 and 1050 1C.

Fig. 11. Hysteresis curve for the solid state synthesized as-prepared Li0.5Fe2.5O4

powder and annealed at 700, 900 and 1050 1C.

V. Verma et al. / Physica B 404 (2009) 2309–2314 2313

temperatures are shown in Figs. 8–11. Parameters such assaturation magnetization (Ms) and coercivity (Hc) weredetermined from the hysteresis graphs. The saturation magneti-zation of all samples increases with annealing temperatureincrease. The highest saturation magnetization is 67.6 emu/gfor 1050 1C annealed lithium ferrite prepared by citrateprecursor method is shown in Fig. 12. But as-preparednano lithium ferrite shows paramagnetic behavior. The areawithin a M–H loop represents a magnetic energy loss; thisenergy loss is defined as heat that is capable of rising specimentemperature. The relative area for all samples is small, andthe loop is thin and narrow which is a specific criterion for softferrite. It can be clearly noticed that the annealing temperaturehas a pronounced effect on the size of the hysteresis loop.An increase in annealing temperature from 700 to 1050 1C causesan increase in the loop size and so the magnetic propertiessuch as saturation magnetization, coercivity and retentivity are

greatly affected [20,21]. The increase in saturation magnetizationwith annealing temperature is may be due to pure phaseformation of ferrites. Table 2 shows the effect of sinteringtemperature on magnetic properties of samples prepared bydifferent routes. The coercivity decreases with increase insintering temperature and least (19.67 Oe) for the sampleprepared by citrate route (annealed at 1050 1C). This is a verynatural characteristic for magnetic materials. For multi-domainregion the variation of coercivity with grain size is defined by therelation

Hc ¼ aþ b=D

where ‘a’ and ‘b’ are constant and ‘D’ is the diameter of particle.As annealing temperature increases the size of the particleincreases, therefore in the multi-domain region the coercivitydecreases as the particle diameter increases [22].

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Fig. 12. The variation of saturation magnetization with respect to preparation

technique and annealing temperature.

Table 2Coercivity and retentivity for all samples of lithium ferrite.

Preparation

techniques

Coercivity Hc (Oe) Retentivity Mr (emu/g)

As-

prepared

700 1C 900 1C 1050 1C As-

prepared

700 1C 900 1C 1050 1C

Flash 108.27 84.81 43.36 25.13 5.74 6.02 3.42 1.96

Sol–gel 104.60 119.79 50.25 22.59 4.19 10.37 4.57 2.01

Citrate 35.28 93.02 56.03 19.67 6.96�10�3 6.52 4.30 1.67

Solid state 02.54 98.46 33.68 19.87 1.69 7.99 2.75 1.53

V. Verma et al. / Physica B 404 (2009) 2309–23142314

4. Conclusions

A comparative study of spinel lithium ferrite prepared bydifferent routes has been carried out. The structural and magneticproperties of resultant particles were investigated. The resultsreveal that single phase of lithium ferrites can be achieved when

the dried gel is annealed at and above 700 1C. Sample prepared bycitrate route gives minimum particle size and better magneticproperties after annealing. The coercivity value is least in citrateroute prepared lithium ferrite sample. One may conclude that thestructural and magnetic properties of lithium ferrite can befurther improved by unconventional preparation methods likeflash combustion, sol–gel, citrate precursor method, etc.

Acknowledgments

The authors are grateful to Dr. Vikram Kumar, Director,National Physical Laboratory, New Delhi, for providing constantencouragement and motivation to carry out this work. We arethankful to Dr. Pran Kishan for valuable suggestions rendered timeto time.

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