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Materials Chemistry and Physics 122 (2010) 133–137

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

agnetic properties of nano-crystalline Li0.35Cd0.3Fe2.35O4 ferrite prepared byodified citrate precursor method

ivek Vermaa,b, M. Abdullah Dara, Vibhav Pandeya, Anterpreet Singhc, S. Annapoornib, R.K. Kotnalaa,∗

National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, IndiaDepartment of Physics and Astrophysics, University of Delhi, Delhi 110007, IndiaBaba Kuma Engineering College, Amritsar, India

r t i c l e i n f o

rticle history:eceived 26 June 2009eceived in revised form 22 January 2010

a b s t r a c t

Single phase nano-crystalline lithium cadmium ferrite Li0.35Cd0.3Fe2.35O4 is synthesized by a modifiedcitrate gel precursor technique in different pH media. The modified citrate precursor technique reducesthe formation of the impurity phase �-Fe2O3 in the inverse spinel phase of lithium ferrite. X-ray diffraction

ccepted 23 February 2010

eywords:ano-crystalline-ray diffractionransmission electron microscopy

(XRD) and transmission electron microscopy (TEM) confirmed the average crystallite size. As-preparedsample shows the paramagnetic behaviour of M–H curve measured by vibrating sample magnetometer(VSM). The coercivity (Hc) and magnetization (M) both increase with decrease in temperature from 300 Kto 80 K. Temperature dependent magnetic properties below the Curie point are defined by the Bloch’s lawand Neel relaxation relations. The effect of annealing on magnetic properties at different temperature is

agnetizationelaxation

studied.

. Introduction

Lithium ferrite belongs to a group of soft ferrite materi-ls extensively used in many applications such as microwaveevices, computer memory chips, magnetic recording media, radiorequency coil fabrication, transformer cores, rod antennas and

any branches of telecommunication and electronic engineer-ng [1–5]. This is due to its attractive features such as high Curieemperature, high saturation magnetization and square hystere-is loop. Nano-crystalline materials are showing great promisen industry and technology. This is mainly due to their uniqueroperties that are not exhibited by bulk crystalline materials6,7].

The structural and magnetic properties of spinel ferrites dependpon the method of preparation [8,9]. The conventional way toroduce these materials is the solid state reaction method ofxides/carbonates followed by calcination at high temperature. Theolid state reaction processing has some inherent disadvantagesuch as; chemical inhomogeneity, coarser particle size and intro-

uction of impurities during the mixing/ball milling. The citrateel precursor method is a well-established method for the syn-hesis of nano spinel type ferrites [10]. There are very few reportsn the synthesis of single phase lithium cadmium ferrite by the

∗ Corresponding author. Tel.: +91 11 45608599.E-mail addresses: rkkotnala@nplindia.org, rkkotnala@mail.nplindia.ernet.in

R.K. Kotnala).

254-0584/$ – see front matter © 2010 Published by Elsevier B.V.oi:10.1016/j.matchemphys.2010.02.057

© 2010 Published by Elsevier B.V.

citrate precursor technique. In this work we have studied the mag-netic properties of nano Li0.35Cd0.3Fe2.35O4 prepared by a modifiedcitrate gel precursor technique at two different pH media. X-raydiffraction confirms the pure phase formation of lithium cadmiumferrite sample prepared in a solution of pH 7. We have studied theeffect of chemical control on pure phase formation and magneticproperties of lithium ferrites.

2. Experimental technique

Cadmium substituted lithium ferrite with composition Li0.35Cd0.3Fe2.35O4 wasprepared by a citrate gel precursor technique at two different pH media say pH 1and pH 7 sample. In this technique an oxidation–reduction reaction takes place inwhich the NO3

− ion is the oxidant and the carboxyl group is the reductant. All highpurity (more than 99% purity) chemicals Fe(NO3)3·9H2O, Cd(NO3)2·4H2O, Li(NO3)were used to synthesize the required compound. Previous studies have shown that a1:1 mole ratio of metal and citric acid is the best ratio for the synthesis of nanosizedferrite particles [11]. The homogeneous aqueous solutions of each metal nitrate saltwere formed in distilled water separately. Then we mixed all these solutions in citricsolution drop by drop with constant stirring at room temperature. The homogeneoussolution was obtained after mixing the aqueous solutions of the metal salts andcitric acid with its natural pH equal to one. The solution was slowly evaporatedfor 24 h at 40 ◦C to form a viscous gel. Subsequently gel was placed in furnace at100 ◦C to obtain porous powder followed by self combustion. In another case, thepH of the mixed solution was adjusted to 7 by adding ammonia solution and thefinal porous powder was obtained by following the same process. The powdersof Li0.35Cd0.3Fe2.35O4 prepared at above mentioned pH were annealed at different

temperatures viz. 500, 700, 800, 900 and 1000 ◦C.

2.1. X-ray diffraction studies

Phase analysis of ferrite samples prepared from different preparation methodsroute was investigated by X-ray diffraction (XRD Rigaku Miniflex II) with Cu K�

1 istry and Physics 122 (2010) 133–137

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34 V. Verma et al. / Materials Chem

adiation of wave length � = 1.5406 Å. The scanning range was from 20◦ to 80◦ intep of 0.02◦ .

The average crystalline size was determined from the measured width of theiriffraction curves using the Debye Scherrer’s relation:

= 0.9�

ˇ cos �(1)

here � is the wavelength of the CuK� radiation (� = 1.5406 Å) and ˇ is the fullidth half maximum (FWHM) in radians calculated using Gaussian fitting. The con-

ribution due to instrumental broadening has been taken into account in order tobtain the accurate crystallite size. In the present work instrumental broadening isorrected by using standard silicon sample.

The lattice constant was calculated using the relation:

d sin � = n�

here d = a/(h2 + k2 + l2)1/2 for fcc system.

.2. Magnetic measurements

Magnetic measurements were carried out using a vibrating sample magnetome-er (Lake Shore 7305) for all samples over the temperature range from 300 K to 80 K.he magnetic properties such as saturation magnetization (Ms), coercivity (Hc) andetentivity (Mr) were calculated from M–H curves.

.3. TEM measurement

The particle size of as-prepared samples was observed using a JEM-200CX trans-ission electron microscope.

.4. FTIR measurement

FTIR spectra of all as-prepared samples were recorded in KBr medium in theave number range 400–4000 cm−1 using a PerkinElmer FTIR (spectrum BX).

. Results and discussion

.1. Crystalline structure

In Li0.35Cd0.3Fe2.35O4 ferrite samples an inverse spinel phase ishowing the main diffraction planes [2 2 0], [3 1 1], [4 0 0], [4 2 2],5 1 1] and [4 4 0] with maximum diffraction intensity from [3 1 1]lane as shown in Fig. 1. A small amount of a �-Fe2O3 is alsobserved in pH 1 sample as depicted from plane [1 0 4] in XRD pat-ern. The average crystalline size of the as-prepared sample pH 1s smaller in comparison to the pH 7 sample, which may be due toigher temperature during combustion. The larger crystallite size

f the pH 7 sample may be due to generation of high temperatureuring the last stage of reaction. At pH 7 as-prepared sample showshe pure phase formation compared to pH 1 as-prepared sample ashown in Fig. 1. The crystallite sizes are about 10 nm and 30 nm forH 1 and pH 7 sample respectively, measured by Debye Scherrer’s

Fig. 1. X-ray diffraction patterns of as-prepared Li0.35Cd0.3Fe2.35O4 samples.

Fig. 2. Typical TEM image and diffraction pattern of as-prepared nanoLi0.35Cd0.3Fe2.35O4 (a) pH 1, (b) pH 7 samples.

relation and the value of lattice constant of pH 1 sample (8.365 Å) ishigher than pH 7 sample (8.359 Å). The size of these nano particles isalso confirmed by TEM images as shown in Fig. 2. The crystallite dis-tributions were found to be in the range of 7–15 nm and 15–35 nmfor pH 1 and pH 7 samples respectively. The crystallite size of sam-ples, observed from XRD is in good agreement for both as-preparedsamples. Most of the crystallite appear spherical in shape for pH 1sample however some elongated crystallite are also present in thepH 7 sample. The bigger crystallite size in the pH 7 sample may bedue to the presence of ammonia in solution, which increases thetemperature during the exothermic self-propagating combustion.This increase in the temperature during the exothermic reactionfor pH 7 sample can be defined on the basis of Kissinger’s equa-tion which suggests that the formation of the chelation complexesdepends on the pH of the solution [12]. The addition of ammonia inreaction mixture decreases the activation energy, which is due tothe higher enthalpy of the gel precursor that allow the easy ruptureof the metal–oxygen bond in the gel precursor.

Fig. 3 shows the FTIR spectra of as-prepared samples in therange of (400–4000) cm−1. As-prepared samples showed the char-acteristic band at about 3445, 1627 and 1394 cm−1 correspondingto vibrations of the O–H group, carboxyl group and NO3

− ions,respectively. The presence of the characteristic bands of NO3

− at1384.6 cm−1 indicates that the NO − exists as a group in the struc-

3ture of citrate gel during the gelation of mixed solution formedfrom nitrates and citric acid. In FTIR spectra, there is one charac-teristic peak at 585 cm−1 which confirm the formation of ferrite inas-prepared samples.

V. Verma et al. / Materials Chemistry and Physics 122 (2010) 133–137 135

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Fig. 7. While the value of saturation magnetization of pH 7 sam-ple is less than that we reported for bulk Li0.35Cd0.3Fe2.35O4 in ourprevious work [15], but here we are able to prepare pure nanocrystallite (40 nm) by modified citrate technique. The results ofmagnetic measurements are M = 37.5 emu g−1, Hc = 83.3 Oe at room

ig. 3. FTIR spectra of Li0.5Fe2.5O4 samples prepared in pH 1 and pH 7 medium.

.2. Magnetic properties

Normally, when the crystallite size of ferrite magnetic mate-ials becomes nanosized the nanostructure magnetic moment istrongly affected by thermal fluctuations. This is because of thenfluence of thermal energy over the magnetic moment order-ng, originating from the paramagnetic relaxation phenomenon. Inrder to understand the magnetic behaviour of nanostructured fer-ite particles, the temperature dependence of magnetization andoercivity in range of 80 K to room temperature (300 K) can beefined on the basis of temperature and time-dependence wellnown Neel relaxation relation (2);

= �0 exp(

KV

KBT

)(2)

here V is the particle volume, KB is the Boltzmann constantnd �0 is an intrinsic time constant of the order of 10−9 s to0−12 s. KV represents the energy barrier between two easy direc-ions. For identical, isolated and mono domain particles, a criticalemperature, called the blocking temperature TB is given byelation (3);

B = KV

ln(t/�0)KB(3)

here t is the characteristic time of the experiment.The coercivity response of the cubic crystal below the Curie

emperature is given by the relation (4);

c = a 3K

2�0Ms

[1 −

(T

TB

)1/2]

(4)

Or

c = a 3K

2�0Ms−

{a 3K

2�0TB

(T1/2

Ms

)}

here Ms is the saturation magnetization and the constant ˛ = 1, ifhe crystallite easy axes are aligned or ˛ = 0.48, for randomly ori-nted. The coercivity field of Li0.35Cd0.3Fe2.35O4 is expected to decayinearly with the square root of temperature as shown in Fig. 4.oth samples of lithium cadmium ferrite follow the nearly linear

ehaviour as given by the relation (4). A small deviation from linearehaviour is predicted in pH 1 that may be due to the presence of

mpurity phase of Fe3O4. Therefore, one can calculate the value ofhe blocking temperature of soft ferrites by considering the valuef anisotropic constant K = −1.35 × 105 erg cm−3. In our studies we

Fig. 4. Variation of coercivity Vs T1/2/Ms for as-prepared Li0.35Cd0.3Fe2.35O4 samples.

also found that the curve between coercivity and absolute tem-perature is steeper for pH 1 sample than pH 7 sample. The valueof saturation magnetization increases with respect to temperaturefrom room temperature to 80 K for both as-prepared samples asshown in Fig. 5. The temperature dependent magnetization at tem-perature below the Curie point generally follows Bloch’s law [13]given by the relation (5);

M(T) = M(0) (1 − BTb), (T << Tc) (5)

where B is the Bloch constant and b is the Bloch exponent. Thevalues of B and b for samples can be obtained by fitting the mea-sured data. It is reported that the Bloch constant B increases withdecreasing crystallite size.

The curve between saturation magnetization (Ms) and coerciv-ity (Hc) at different temperature (below curie) shows a physicalsignificance of nanosized particles [14]. The gradual change in sat-uration magnetization with respect to coercivity is observed forsmaller crystalline size as shown in Fig. 6. This further confirms thesmaller crystallite formation at pH 1 sample. Sample pH 7 exhibitsmore magnetization than pH 1 at room temperature as shown in

Fig. 5. Variation of saturation magnetization with temperature for as-preparedLi0.35Cd0.3Fe2.35O4 samples.

136 V. Verma et al. / Materials Chemistry and Physics 122 (2010) 133–137

Fig. 6. Variation of saturation magnetization and coercivity for as-preparedLi0.35Cd0.3Fe2.35O4 samples.

Ft

tsa

3

aeim5m1

TM

Fig. 8. X-ray diffraction patterns of annealed Li Cd Fe O (a) pH 1, (b) pH 7

ig. 7. M–H curve of for as-prepared Li0.35Cd0.3Fe2.35O4 samples at room tempera-ure.

emperature and M = 46.96 emu g−1, Hc = 186.6 Oe at 80 K, for pH 1ample and M = 60.2 emu g−1, Hc = 118.2 Oe at room temperaturend M = 68 emu g−1, Hc = 151.7 Oe at 80 K for pH 7 sample.

.3. Effect of annealing temperatures

Table 1 shows the magnetic properties of annealed samplest different temperatures. The variation of magnetization param-ters with annealing temperature is not only due to variationn crystallite size but also due to several other phenomena. The

agnetization of the pH 7 sample annealed at 500 ◦C drops to0.8 emu g−1 and the sample annealed at 700 ◦C** has a saturationagnetization of 42.6 emu g−1. Samples annealed at 800, 900 and

000 ◦C show increase in magnetization value. This may due to the

able 1agnetic properties of annealed samples.

Annealing temperature (◦C) Ms (emu g−1) Hc (Oe)

pH 1 pH 7 pH 1 pH 7

As-prepared 37.5 60.0 124.3 118.6500 39.9 50.8 110.2 118.4700 45.7 42.6 104.6 110.9800 58.8 43.9 76.9 91.40900 58.0 45.8 53.4 70.401000 60.0 50.2 38.1 52.00

0.35 0.3 2.35 4

samples.

volatization of lithium and oxygen [16]. The loss of Li2O results information of �-Fe2O3, thereby reducing the saturation magneti-zation. The formation of �-Fe2O3 in pH 7 sample with annealingtemperatures is observed from XRD patterns as shown in Fig. 8(b).The peak [3 1 0] corresponding to �-Fe2O3 increased with annealingtemperature, which confirms the volatization of Li2O and formationof �-Fe2O3. But in case of pH 1 sample the peak [3 1 0] correspond-ing to �-Fe2O3 decreases with annealing temperature as shownin Fig. 8(a), which attributed to an enhancement of magnetizationvalues.

The values of coercivity for both samples are observed todecrease with annealing temperatures as shown in Table 1. Thisis a very natural characteristic for magnetic materials. For multidomain region the variation of coercivity with grain size is definedby the relation (6),

Hc = a + b

D(6)

where ‘a’ and ‘b’ are constants and ‘D’ is the diameter of parti-

cle. As annealing temperature increases the size of the crystalliteincreases, therefore in the multi domain region the coercivitydecreases as the crystallite diameter increases.

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[15] Vivek verma, Vibhav Pandey, S.P. Gairola, R.K. Kotnala. Magnetic materials:International Conference on Magnetic Materials (ICMM-2007); Synthesis AndCharacterization Of Cadmium Doped Lithium Ferrite By Sol-gel Technique, AIP

V. Verma et al. / Materials Chem

. Conclusions

A pure inverse spinel phase of nano-crystalline sample ofi0.35Cd0.3Fe2.35O4, without annealing, was prepared by a modi-ed citrate technique. In this technique an oxidation–reductioneaction takes place in which the NO3

− ion is the oxidant and thearboxyl group is the reductant. The combustion rate can be con-rolled by the pH value of the solution. Using this modified citrateechnique, Li0.35Cd0.3Fe2.35O4 ferrite powder with crystalline sizef less than 30 nm were synthesized. Analysis of XRD pattern con-rms the sample has a cubic spinel structure with lattice constant= 8.365 Å and 8.359 Å for pH 1 and pH 7 samples respectively.emperature dependent magnetic properties below the Curie pointre well defined by the Bloch’s law and Neel relaxation relations.nnealing improves the magnetization for pH 1 sample but reducess for pH 7 samples due to the loss of Li2O results in formation of-Fe2O3.

cknowledgements

The authors are grateful to Director, National Physical Lab-ratory; New Delhi for providing constant encouragement andotivation to carry out this work. We are thankful to Dr. Pran

ishan for valuable suggestions rendered time to time.

[

nd Physics 122 (2010) 133–137 137

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