Journal of Magnetism and Magnetic Mat

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Cobalt ferrite (CoFe2O4) is a well-known hard magnetic

fabrication processes that are relatively simple and yieldcontrolled particle sizes.

size distribution is not possible [1]. In order to overcome

assisted synthesis [6]. In this paper, we have presented thesynthesis of cobalt ferrite (CoFe2O4) nanoparticles by wetchemical method (coprecipitation) along with heat treat-

ARTICLE IN PRESSment at 600 1C. The size and size distribution wascontrolled by controlling the nucleation and growth rates.Smaller and uniformly disturbed particles were obtained if

0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmmm.2006.06.003

Corresponding author. Tel./fax: +92 51 9210256.E-mail address: arif@qau.edu.pk (A. Mumtaz).material with high coercivity and moderate magnetization.These properties, along with their great physical andchemical stability, make CoFe2O4 nanoparticles suitablefor magnetic recording applications such as audio andvideotape and high-density digital recording disks, etc.[1,2]. The magnetic character of the particles used forrecording media depends crucially on the size, shape andpurity of these nanoparticles. These particles should besingle domain, of pure phase, having high coercivity andmedium magnetization. Hence the need for developing

these difculties, nanometer size reactors for the formationof homogeneous nanoparticles of cobalt ferrite are used.To protect the oxidation of these nanoparticles from theatmospheric oxygen and also to stop their agglomeration,the particles are usually coated and dispersed in somemedium like sodium dodecyl sulfate (NaDS) or oleic acid[3,4].In general, the preparation methods for CoFe2O4

nanoparticles have been quite involved requiring specialtechniques to prevent agglomeration [5] or microwave(10.5 kOe) is observed on cooling down to 77K while typical blocking effects are observed below about 260K. The high eld moment is

observed to be small for smaller particles and approaches the bulk value for large particles.

r 2006 Elsevier B.V. All rights reserved.

PACS: 75.50Gg; 75.50Tt; 75.70Rf

Keywords: Ferrite nanoparticles; Surface anisotropy; High coercivity magnetic materials

1. Introduction

Recently metal-oxide nanoparticles have been the subjectof much interest because of their unusual optical, electronicand magnetic properties, which often differ from the bulk.

Conventional techniques for preparation of nanoparti-cles include solgel processing, hot spraying, evaporationcondensation, matrix isolation, laser-induced vapor phasereactions and aerosols. Generally, in most types of nano-particles prepared by these methods, control of size andwith annealing temperature and time while the coercivity goes through a maximum, peaking at around 28 nm. A very large coercivitySynthesis and magnetic propernanoparticles prepared

K. Maaza, Arif Mumtaza,, S.KaDepartment of Physics, Quaid-i-A

bDepartment of Physics and Astronom

Received 10 September 2005; re

Available on

Abstract

Magnetic nanoparticles of cobalt ferrite have been synthesized b

acid as the surfactant. X-ray Diffraction (XRD) and Transmission

cobalt ferrite nanoparticles in the range 1548 nm depending on therials 308 (2007) 289295

es of cobalt ferrite (CoFe2O4)y wet chemical route

Hasanaina, Abdullah Ceylanb

University, Islamabad, Pakistan

niversity of Delaware, Newark, USA

ed in revised form 2 May 2006

5 July 2006

et chemical method using stable ferric and cobalt salts with oleic

ctron Microscope (TEM) conrmed the formation of single-phase

nnealing temperature and time. The size of the particles increases

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the nucleation rate was higher than the growth rate. LargepH values in the range (1114) were used in accordancewith predictions where high production yields are expectedfor large pH values. The advantage of this method over theothers is that the control of production of ferrite particles,its size and size distribution is relatively easy and there is noneed of extra mechanical or microwave heat treatments.The size and size distribution of the particles prepared by

this method was studied by X-ray Diffraction (XRD) andTransmission Electron Microscope (TEM). The depen-dence of the particle size on the annealing temperature andannealing time was also studied. Finally, various magneticproperties of the particles have been studied as functions ofeld, temperature and size.

2. Experimental procedure

2.1. Materials

Ferric chloride and cobalt chloride (98 +% purity) andsodium hydroxide were used. Oleic acid of HPCL gradewas used as surfactant. All the materials were reagent grade

sodium hydroxide was prepared and slowly added to thesalt solution dropwise. The pH of the solution wasconstantly monitored as the NaOH solution was added.The reactants were constantly stirred using a magneticstirrer until a pH level of 1112 was reached. A speciedamount of oleic acid was added to the solution as asurfactant and coating material [1]. The liquid precipitatewas then brought to a reaction temperature of 80 1C andstirred for 1 h. The product was then cooled to roomtemperature. To get free particles from sodium andchlorine compounds, the precipitate was then washed twicewith distilled water and then with ethanol to remove theexcess surfactant from the solution. To isolate the super-natant liquid, the beaker contents were then centrifuged for15min at 3000 rpm. The supernatant liquid was thendecanted, and then centrifuged until only thick blackprecipitate remained. The precipitate was then driedovernight at 100 1C. The acquired substance was thengrinded into a ne powder. At this stage the product(CoFe2O4) contains some associated water (upto 10wt%),which was then removed by heating at 600 1C for 10 h. Thenal product obtained was then conrmed by XRD, etc. to

ARTICLE IN PRESSK. Maaz et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 289295290and used without further purication. Double distilled, de-ionized water was used as a solvent.

2.2. Procedure

0.4M (25ml) solution of iron chloride and a 0.2M(25ml) of cobalt chloride solutions were mixed in doubledistilled, de-ionized water. Deionized distilled water wasused as a solvent in order to avoid the production ofimpurities in the nal product. 3M (25ml) solution ofFig. 1. X-ray diffraction pattern (Cu Ka-radiation) of CoFe2O4 nanoparticles

average crystallite size of about 21 nm.be magnetic nanoparticles of cobalt ferrite (CoFe2O4) withinverse spinel structure (details below).

3. Results and discussion

The X-ray diffraction pattern (Fig. 1) of the calcinedpowder synthesized using this route shows that the nalproduct is CoFe2O4 with the expected inverse spinelstructure. No other phase/impurity was detected. The sizeof the particles was determined by Scherrer formula usingprepared by wet chemical method, after calcination at 600 1C for 10 h with

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rst two strongest peaks. The average sizes of the particlescalcined at 600 1C were found to be 15, 17.5, 2173 nm withvariation in sizes being introduced by controlling the rateof mixing of NaOH with the salt solution. Slow rates ofmixing resulted in larger size particles as the growth ratebegins to exceed the nucleation rate. By annealing 15 nmparticles at 800, 900, 850 and 1000 1C respectively for 10 h,particle sizes of 24, 26, 32 and 3873 nm were obtained.Finally, on further annealing the 17.5 nm particles,described above, at 1000 1C for 6 and 10 h respectively,42 and 4873 nm particles were obtained. Thus the sizevariation and control has been achieved by both the rate ofreaction and the annealing conditions.Fig. 2 shows the TEM images of CoFe2O4 nanoparticles

calcined at 600 1C for 10 h (with average crystallite size ofabout 21 nm as determined by XRD). The size distributionof these nanoparticles as observed in TEM images is shown

in Fig. 3. The distribution seems to be symmetric(Gaussian) about 21 nm, with particles of sizes 1626 nmfor this specimen. The maximum number lie between 20and 22 nm, peaking at 21 nm, in good agreement with XRDcrystallite size. Most of the particles appear spherical inshape, however, some elongated particles are also presentas shown in the TEM images. Some moderately agglom-erated particles as well as separated particles are present inthe images. Agglomeration is understood to increaselinearly with annealing temperature and hence some degreeof agglomeration at this temperature (600 1C) appearsunavoidable.Fig. 4 shows the correlation between the particle size and

annealing temperature. The size of the particles is observedto be increasing linearly with annealing temperature. Itappears that that increase in size with temperature becomesrapid between 800 and 900 1C and appears to be slowingdown above 900 1C. While annealing generally decreases

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Fig. 2. TEM micrograph of CoFe2O4 nanoparticles prepared, after

calcination at 600 1C for 10 h with average crystallite size of 21 nm.

K. Maaz et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 289295 291Fig. 3. Size distribution (histogram) of CoFe2O4 nanoparthe lattice defects and strains, however, it can also causecoalescence of crystallites that results in increasing theaverage size of the nanoparticles [7]. The inset in Fig. 4shows the dependence of particle size on annealing time ata xed annealing temperature of 600 1C. The particle sizeappears to increase almost linearly with annealing time,most likely due to the fact that longer annealing timeenhances the coalescence process resulting in an increase inthe particle size. Thus it appears that particle size may becontrolled by varying either of the two parameters, i.e.annealing temperature and time.Magnetic characterization of the particles was done

using vibrating sample magnetometer (VSM), betweenroom temperature and 77K, with maximum applied eldupto 15 kOe (see Fig. 5 forMH loops). For the 24 nm sizeparticles the coercivity at room temperature was 1205Oewhile at 77K it had increased to 11 kOe. The saturationmagnetization (MS) obtained at room temperature wasticles from TEM images (600 1C calcination for 10 h).

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ARTICLE IN PRESSnd MK. Maaz et al. / Journal of Magnetism a292found to be 68 emu/g and remanent magnetization (Mr)was 31.7 emu/g, while at 77K the values for the sameparameters were 40.8 and 34.4 emu/g, respectively. The

Fig. 4. Particle size (nm) as a function of annealing temperature (1C) for CoFe2annealing time, in hours (at T 600 1C).

Fig. 5. Hysteresis loops for 24 nm CoFe2O4 nanoparticles at room teagnetic Materials 308 (2007) 289295very large coercivity and low saturation magnetization at77K are consistent with a pronounced growth of magneticanisotropy inhibiting the alignment of the moment in an

O4 nanoparticles. The inset shows the correlation between particle size and

mperature (300K) and 77K at maximum applied eld of 15 kOe.

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applied eld. The remanence ratios at these temperaturesindicate the same feature, rising from 0.47 to 0.84 at 77K.The value of remanence ratio of 0.47 is very close to thatexpected (0.5) for a system of nonintracting single domainparticles with uniaxial anisotropy [8] even though cobaltferrite itself has a cubic structure. The existence of aneffectively uniaxial anisotropy in magnetic nanoparticleshas been attributed to surface effects [9] as evidenced bysimulations of nanoparticles. Surface effects also tend tolead to large anisotropies. Kodama et al. [9] have arguedthat due to the disorder near the surface, the typical twosublattice picture for antiferromagnetic or ferrimagneticnanoparticles appears to break down and multiplesublattice picture appears to hold. There appears to be asituation where several different spin congurations e.g. 2,4, 6 sublattice models have very similar energies and hencemultiple ground states are possible, e.g. in a spinglass. Theinteraction between the core and surface spins leads to avariety of effects including large anisotropy and exchangebias effects. Such effects are observed in these particles andare to be reported separately.It is common to nd higher effective anisotropy values in

magnetic nanoparticles as compared to their bulk counter-

cobalt ferrite, which has a room temperature coercivity of750980Oe [11].The coercivity of the nanoparticles was also studied as a

function of particle size. Fig. 6 shows the coercivity as afunction of particle size at room temperature (300K). TheGaussian t to the data shows the coercivity increases withsize rapidly, attaining a maximum value of 1250Oe at28 nm and then decreases with size of the particles. Thisdecrease at larger sizes could be attributed to either of tworeasons. Firstly, it may be due to the expected crossoverfrom single domain to multidomain behavior with increas-ing size. Secondly, such an effect can arise from acombination of surface anisotropy and thermal energies.The former effect is expected in CoFe2O4 particles for a sizeclose to 50 nm [12,13] that is signicantly higher than thecritical size of 28 nm that we observed. The latter source ofthe effect therefore is considered as more likely explanationfor the peak. We understand the initial increase of thecoercivity with decreasing size as being due to the enhancedrole of the surface and its strong anisotropy, as opposed tothe weaker bulk anisotropy. This rise is followed by adecline at small enough sizes when the product of theanisotropy energy and volume becomes comparable to the

ARTICLE IN PRESSK. Maaz et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 289295 293part. From a t of the magnetization data at high eldsusing the Law of Approach to Saturation,

w qM=qH aK2effMsH

3,

we obtained the value of the anisotropy constantKeff 3.8 106 ergs/cm3, where a 0.533 for uniaxialanisotropy [8]. This value is somewhat but not very muchhigher than the value of 1.83.0 106 ergs/cm3 [10] for bulkFig. 6. Correlation between the coercivity (HC) and mean particle dthermal energy, leading to thermally assisted jumps overthe anisotropy barriers. It is also likely that the twoprocesses are operating simultaneously and the singledomain effects may not be excluded, however, thedominant role will be of the surface effects for smallerparticles. The decrease of Hc at dX40 nm may very wellhave a contribution from the development of domain wallsin the nanoparticles. These aspects shall be discussed iniameter (nm), at room temperature and applied eld of 15 kOe.

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ole

nd MFig. 7. Temperature dependence of magnetic susceptibility for zero-eld co

dependence of blocking temperature (TB) on particle size (nm).

K. Maaz et al. / Journal of Magnetism a294detail elsewhere [14]. The magnetization of different sizenanoparticles is shown as a function of temperature in Fig.7. The samples were cooled to 77K without application ofexternal magnetic eld (zero-eld cooled (ZFC)). Follow-ing the cooling a eld of 5 kOe was applied andmagnetization was recorded as function of temperatureup to 300K. A peak in the magnetization is evident in eachcase with the exact position of the peak depending on thesize. It is understood that in the ZFC mode themagnetization of a collection of nanoparticles may gothrough a peak as the particles moments become blockedalong the anisotropy axes. The temperature at whichmaximum magnetization is achieved, is dened as theblocking temperature (TB). This temperature is a functionof applied eld and typical time scale of measurement.The inset in Fig. 7 shows the effect of size on the

blocking temperature. While there is a clear increase in theblocking temperature with size, it is also apparent that thisincrease is very rapid in the beginning (at smaller sizes) andthereafter the increase becomes very slow appearing toreach a maximum at T270K (for H 5 kOe). The largerparticles seem to be blocked at high temperatures ascompared to the smaller particles at the same eld. Forlarger particles, the larger volume causes increasedanisotropy energy, which decreases the probability of ajump across the anisotropy barrier and hence the blockingis shifted to a higher temperature. From the data it appearsthat above about 24 nm the particle blocking becomesrelatively insensitive to size.d (ZFC) CoFe2O4 nanoparticles at applied eld of 5 kOe. The inset is the

agnetic Materials 308 (2007) 289295Fig. 8 shows the dependence of saturation magnetizationon particle size. The MS values obtained for our samplesvary between 53 and 79.5 emu/g. The maximum value is79.5 emu/g for 48 nm particles close to the bulk value of80.8 emu/g for CoFe2O4.The saturation magnetization increases consistently with

size. For small particles the value of MS is signicantlylower than the bulk value of 80 emu/g while for the size of48 nm the magnetization has attained the bulk value.Once again we see a very sharp increase in the magnetiza-tion between the sizes of 1528 nm while there is a slowerincrease thereafter, as in the case of coericivity andblocking temperature. The decrease in MS at small sizesis attributed to the effects of the relatively dead or inertsurface layer that has low magnetization. This surfaceeffect becomes less signicant with increasing sizes andabove 48 nm seems to be no longer relevant to the bulkmagnetization.

4. Conclusions

In this paper, we have presented the synthesis ofCoFe2O4 nanoparticles in the range 1548 nm. The sizeof the nanoparticles were measured both by XRD andTEM and were in very good agreement with each otherindicating that there was no agglomeration and that thesize distribution of the prepared nanoparticles was small.The size of the nanoparticles appeared to increase linearlywith annealing temperature and time most probably due to

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f pa

nd Mcoalescence that increases with increasing temperature of

Fig. 8. Saturation magnetization (MS) as function o

K. Maaz et al. / Journal of Magnetism aanneal. It is evident that particle size and its distributionmay be controlled both by controlling the rate of reactionand the annealing temperature and time period. The verylarge coercivity and low saturation magnetization at 77Kin comparison with room temperature appear to be due toa pronounced growth of magnetic anisotropy at lowtemperatures. The observed magnetization remanence ratioof 0.47 at room temperature (very close to the value of 0.5typical of a system of noninteracting single domainparticles) suggests that CoFe2O4 nanoparticles exhibit aneffective anisotropy that is uniaxial. The effective uniaxialanisotropy in magnetic nanoparticles has been explained asarising from surface effects that also lead to largeanisotropy energy in nanoparticles. The coercivity showsa peak with particle size at a value much smaller than thesingle domain limit and is attributed to the onset of thermaleffects at small enough particle sizes. We nd that forsmaller particles the saturation magnetization had a valuethat was signicantly lower than the bulk value while thelarger size particles have values approaching those of thebulk. The smaller value of MS in smaller particles isattributed to the greater fraction of surface spins in theseparticles that tend to be in a canted or spin glass like statewith a smaller net moment.

Acknowledgments

K.M and A.M wish to acknowledge the HigherEducation Commission (HEC) for providing Ph.D. fellow-ship and Research Grant (no. 20-74/Acad(R)/03) for

rticle size (nm) at maximum applied eld of 15 kOe.agnetic Materials 308 (2007) 289295 295ARTICLE IN PRESSenabling this work. S.K.H also acknowledges the H.E.Cfor Research Grant (no. 20-80/Acad(R)/03) on ferromag-netic nanomaterials.

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Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical routeIntroductionExperimental procedureMaterialsProcedure

Results and discussionConclusionsAcknowledgmentsReferences