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The effect of cobalt substitution on magnetic hardening of magnetite

M. Mozaffari a,n, Y. Hadadian b, A. Aftabi c, M. Oveisy Moakhar b

a Department of Physics, Faculty of Science, University of Isfahan, Isfahan 81746-73441, Iranb Physics Department, Razi University, Taghebostan, Kermanshah, Iranc Department of Physics, University of Kurdistan, Sanandaj 66177-15175, Iran

a r t i c l e i n f o

Article history:

Received 4 July 2013

Received in revised form9 October 2013Available online 7 November 2013

Keywords:

Substituted spinel ferriteMagnetiteMagnetic hardeningCoercivityCurie temperature

a b s t r a c t

In this work cobalt-substituted magnetite (CoxFe1xFe2O4,x0, 0.25, 0.50 and 0.75) nanoparticles weresynthesized by coprecipitation method and their structural and magnetic properties were investigatedX-ray diffraction was carried out and the results show that all of the samples have single phase spinestructure. Microstructure of the samples was studied using a eld emission scanning electron microscopeand the results show that particle sizes of the prepared nanoparticles were uniform and in the 5055 nmrange. Room temperature magnetic properties of the nanoparticles were measured by an alternatinggradient force magnetometer and the results revealed that substituting cobalt for iron in magnetitestructure, changes the magnetite from a soft magnetic material to a hard one. So that coercivity changesfrom 0 (a superparamagnetic state) to 337 Oe (a hard magnetic material), which is a remarkable changeCurie temperatures of the samples were determined by recording their susceptibility-temperature (Tcurves and the results show that by increasing cobalt content, Curie temperature of the samples alsoincreases. Also T curves of the samples were recorded from above Curie temperature to roomtemperature (rst cooling), while the curves in the second heating and second cooling have the samebehaviour as the rst cooling curve. The results depict that all samples have different behaviour in therst cooling and in the rst heating processes. This shows remarkable changes of the cation distributionin the course ofrst heating.

&2013 Elsevier B.V. All rights reserved

1. Introduction

Magnetic nanoparticles have received much attention in recentyears due to their underlying applications in targeted drugdelivery, medical diagnostic, genetic screening [15], biosensors[6], ferrouids[7], gas sensing[8], catalyses[9,10], high-densityinformation storage devices [1113], etc. Magnetic properties ofthe nanoparticles can be more complicated than those of theirbulk counterparts. In fact, all relevant magnetic properties (coer-civity, blocking temperature, saturation and remanent magnetiza-tions) are functions of particles size and shape and of surface

chemistry[14,15]. For example as size reduces to a denite size,nanoparticles exhibit the so-called superparamagnetic regime,which is of great interest in macroscopic quantum tunnelling ofspin states [16,17]. Among the various magnetic nanoparticles,spinel ferrites have been extensively studied because of theirinteresting magnetic properties. Specically magnetite and cobaltferrite, which both have spinel structure belonging to the spacegroup (Fd3m), play a key role in magnetic applications.

Magnetite with the formula Fe3O4is the most strongly magne-tized material found in nature. At room temperature (RT) magne-tite has a cubic inverse spinel structure which can be visualized asa face centred cubic arrangement of the oxygen anions [18]Structurally, the cations in Fe3O4 are distributed between twosites, as (Fe3)A[Fe

3Fe2]BO4, where A and B indicate tetrahedraand octahedral sites respectively [19]. In case of cobalt ferritewith formula (CoFe1)[Co1Fe1]O4, tetrahedral (A-site) andoctahedral (B-site) sites are occupied randomly by Co3 /2 andFe3 /2 , respectively[20]. This type of cation distribution is calledpartially inverse spinel structure and as the degree of inversion

which represents the ratio between divalent and trivalent ions onboth A and B sites, depends mostly on the thermal history of thesample[21]. Also it has been reported that count upon synthesisconditions, cobalt ferrite can form both normal and invers spinestructure [22]. Cobalt ferrite has been extensively investigatedowing to its interesting properties such as high coercivity, mod-erate saturation magnetization, high chemical stability, wearresistant, and electrical insulation[23]. These benecial character-istics make it as a suitable candidate for many applicationsmentioned above.

Each application of spinel ferrite nanoparticles requires asomewhat different set of magnetic properties in nanocrystalsResearching on a widespread range of several compositions and

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Journal of Magnetism and Magnetic Materials

0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jmmm.2013.10.039

n Corresponding author. Tel.: 98 311 793 4741.E-mail addresses:mozafari@sci.ui.ac.ir,mmozafari@hotmail.com (M. Mozaffari).

Journal of Magnetism and Magnetic Materials 354 (2014) 119124

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sizes, reveals an existence of a potential for tuning the magneticproperties and then vastly developing the range of applications.For example by preparation and characterisation of various spinelferrites, MeFe2O4 (MeFe, Co, Mg, Mn, Zn, etc.) changing theamount of metals which exist in their composition, substitutingone or more for other one, changing the synthesis method[11]oreven changing the synthesis conditions, a desire characteristic canbe achieved.

Both Fe3O4 and CoFe2O4 in bulk form have inverse spinelstructures and their magnetocrystalline anisotropy constants (K)are 14 and 380 kJ/m3 respectively. This huge difference in Kvaluesallows the possibility of controlling the magnetic properties of ironoxide nanoparticles by substituting different amounts of cobalt foriron[19]. Wide range applications of ferrites with small particlesleads to great expansion of methods for their preparation, such asmicrowave plasma[24],Host template[25], microemulsion[26],hydrothermal synthesis [27], solgel[28], sonochemical reaction[29], forced hydrolysis [30], citrate precursor techniques [31],electrochemical synthesis[32], solid-state reaction[33], combus-tion methods[34], mechanical alloying[35], and coprecipitation[5,11,3638]. As it is well known, chemical coprecipitation methodis an economical [39] and the most frequent way to produce

ultra

ne powders. This method is quite simple, fast and cheapsince it does not involve intermediate decomposition and does notneed calcining steps. In this work, a series of CoxFe1xFe2O4

(x0.00, 0.25, 0.50 and 0.75) nanoparticles was synthesized bycoprecipitation method and the effect of Co substitution on themagnetic properties of magnetite was investigated.

2. Experimental

As mentioned before Co substituted magnetite nanoparticles

with nominal formula of CoxFe1xFe2O4 (x0.00, 0.25, 0.50 and0.75) were prepared by the coprecipitation method. The startingmaterials were corresponding stoichiometric quantities of highpurity FeCl3, FeCl2 4H2O, CoCl2and NaOH (all of analytical grades)from Merck Company, Germany. Stoichiometric ratios (1:2)((Fe2Co2):Fe3) of FeCl3, FeCl2 4H2O and CoCl2 were dis-solved in deionized double distilled water and a 5 M sodiumhydroxide (NaOH) solution, with a ratio (1:5) (metal chlorides:sodium hydroxide) was added as fast as possible. Black precipi-tates were obtained after the rapid addition of NaOH solution. Theprecipitates were washed several times with deionized doubledistilled water and were then dried at room temperatureover days.

To study the effect of annealing on magnetic properties of

the samples, sample with x0.50 (Co0.5Fe0.5Fe2O4) has beenchosen as a typical Co substituted magnetite and heated fromroom temperature to 800 1C for 2 h by a rate of 8 1C/min and then

Fig. 1. (a) XRD patterns of the as-precipitated samples with x0.00 and 0.50. (b) XRD pattern of the sample with x0.50 annealed at 800 1C for 2 h in air.

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free-cooled to room temperature. Phase identication of theprepared nanoparticles was carried out at room temperature byX- ray diffraction method, using a BRUKER diffractometer D8ADCANCED model and CuK radiation (1.5406 ). Magneticproperties of the samples were investigated by an alternatinggradient force magnetometer (AGFM) of Meghnatis Daghigh KavirCo., Kashan, Iran. AsMHcurves of the samples were not saturatedup to the maximum measuring eld (79 kOe), their saturation

magnetization were determined by extrapolating of the high eldparts of the curves to innity eld. In this order M1/Hcurves ofthe high eld (H45000 Oe) parts of theMHcurves were plottedand extrapolated to 1/H-0. Morphology of samples was studiedby a eld emission scanning electron microscope (FESEM), usinga Hitachi S-4160 FESEM unit. Temperature dependence of theAC susceptibility () of the samples was measured by a BartingtonMS2 system.

3. Results and discussion

Fig. 1a shows XRD patterns of the as-precipitated samples with

x0.00 and 0.50. As it can be seen main peaks are related to(2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes, which arecorresponded to the spinel structure. Also it is clear that diffractionpeaks are fairly broad, which is attributed to the small crystallitesize of the samples[11,16].Fig. 1b illustrates the XRD pattern ofthe sample with x0.50 annealed at 800 1C for 2 h in air. Thispattern depicts that all peaks are more intense and thinner thanthose of the as-precipitated sample. It shows that heat treatmentincreases crystallinity and crystallite size.

Fig. 2shows FESEM images of the as-precipitated samples withx0.00 and x0.50 as typical images of the samples. Othesamples have the same morphology too. These images illustratefairly homogeneous nanoparticles with mean particle sizes in therange of 5055 nm.

MHcurve of the as-precipitated sample with x0.00 (mag-netite) is shown in Fig. 3. As can be seen, this sample shows nocoercivity, which is a superparamagnetic behaviour resulted fromthe small particle size. AlsoFig. 4shows the hysteresis loops of thesamples withx0.25, 0.5 and 0.75 in addition to MHcurve of thesample with x0.00. As can be seen when Co is substituted for

iron in magnetite structure, coercivity is not zero now and thesamples dont show superparamagnetic behaviour. The obtainedmagnetic parameters, including coercivity (Hc), magnetization amaximum measuring eld, extrapolated saturation magnetizationfromFig. 5, remanence (Mr) and squareness (Mr/Ms) are tabulatedin Table 1. A comparison between these data shows that withincreasing Co content, coercivity and squareness are increasedregularly, while the saturation magnetization and remanence donot possess regular variations. The increase in coercivity is a causeof increase in magnetocrystalline anisotropy which is in turn dueto cobalt substitution [40]. Magnetocrystalline anisotropy is themain key factor that determines superparamagnetic behaviouof nanocrystals. Magnetocrystalline anisotropy serves as anenergy barrier (EA) to block the spin relaxation, which changes

the magnetic state from ferromagnet to superparamagnet. The

Fig. 2. The FESEM images of the as-precipitated samples with x0 (up) andx0.50 (down).

-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000

-150

-100

-50

0

50

100

150

M

H(Oe)

Fig. 3. Room temperature MHcurve of the sample with x0.0.

-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000

-150

-100

-50

0

50

100

150

M

H(Oe)

x=0

x=0.25

x=0.75

x=0.50

Fig. 4. Room temperature M

H curves of the samples with x0.0, 0.25, 0.50and 0.75.

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magnetic moment can be agitated in to superparamagnetic relaxa-tion by thermal energy (kBT). The height of EA determines theblocking temperature in which the thermal activation can over-comeEAand the nanocrystals transfer in to the superparamagneticstate [41]. According to StonerWohlfarth single domain theory[42] both of the magnetocrystalline anisotropy constant (K) andthe volume of nanocrystals (V) control the magnetic anisotropybarrier, EA[41]. As mentioned above the mean particle sizes of thenanoparticles are approximately similar, therefore disappearanceof superparamagnetic properties by Co substitution in magnetite,can be due to the increase in magnetocrystalline anisotropy. Alsoaccording to the one-ion crystalline eld model[40], by increasingCo ion content, due to the increases of magnetocrystalline aniso-tropy, the coercivity and squareness of the spinel ferrites increasetoo[43].

The overall magnetization in spinel ferrites depend on (a) themagnetic moment of each ion, (b) distribution of the ions betweenA- and B-sites and (c) the fact that the exchange interactionbetween ions in A and B-sites is usually negative and also thestrongest one [44]. Since bulk magnetite has a complete inversspinel structure, the numbers of Fe3 on the A- and B-sites areequal and consequently their moments would be cancelled.Thereby its overall magnetization is just resulted from the Fe2

moments on the B-sites. As X. Li and C. Kutal reported from theirMssbauer investigation [16], the cation distribution of the Cosubstituted magnetite changes with the Co concentration. Cobaltferrite retains its invers spinel structure, even though the cationdistribution changes. The irregular variation of the saturationmagnetization and remanence can be related to the change ofCo2 and Fe3 ions distribution between the A- and B-sites.

Fig. 6shows the hysteresis loops of as prepared and annealedsample with x0.50. As it can be seen the coercivity increasessignicantly (from 197 to about 1000 Oe which means it changesmore than ve times) after annealing, while saturation magneti-zation decreases. High magnetic anisotropy of cobalt ferrite ismainly because of presence of Co2 ions on the B sites of thespinel structure [22,45]. Many researchers have reported that

cation distribution of cobalt ferrite changes by heat treatment

[36,38,4548]. If one supposes initially all Co2 ions were on theB-sites (complete invers spinel structure), any heat treatments orcation substitutions would result in the movement of some Co2

ions out of the B-sites and this will reduce the level of anisotropy.On the other hand, if initially some Co2 ions are on the A-sites(partial invers spinel structure), heat treatments or cation sub-stitutions may lead to some being moved to the B-sites resulting inan increase in anisotropy[46]. The increase in coercivity observedafter annealing is mainly a consequence of an increase in aniso-

tropy of the samples due to the cation redistribution, maybemigration of some Co2 ions from A- to B-sites. This migrationalso can cause the decrease in the saturation magnetization afterannealing, as is illustrated in Fig. 6.

Fig. 7 illustrates temperature dependency of the ac suscept-ibility of the as-precipitated samples. General trend of all Tcurves is nearly the same, except for x0, which is magnetite.Apart from Tcurve of magnetite, all other Tcurves achieve asignicant broad peak just below their Curie temperatures, knownas Hopkinson peak [49]. Although values of TC depend on thecomposition,Table 2, the verticality of the susceptibility drop justbelow Curie temperature indicates the degree of compositionhomogeneity of the sample [50,51]. As mentioned previously itis proved that any heat treatment can change cation distribution.

So it might be a cause for broadening of the observed Hopkinson

y = -70391x + 116.4

y = -125340x + 141.1

y = -84558x + 93.791

y = -128399x + 158.76

75

85

95

105

115

125

135

145

155

1.0E-04 1.2E-04 1.4E-04 1.6E-04 1.8E-04 2.0E-04 2.2E-04

x=0.00

x=0.25

x=0.75

x=0.50

M (emu/g)

1/H (1/Oe)

Fig. 5. Variation ofMwith respect to 1/Hof all samples and linear tting.

Table 1

Magnetic parameters of the samples.

x Values Hc(Oe) M (@9 kOe)(emu/g) M(extrapolated)(emu/g) Mr(emu/g) Mr/Ms

0.00 0 108 116 0 00.25 64 119 141 6 0.0420.50 198 84 93 9 0.960.75 337 144 158 24 0.15

-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000

-120

-80

-40

0

40

80

120

M

H(Oe)

unannealed

annealed

Fig. 6. Hysteresis loops of the as-precipitated and annealed sample with x0.50.

Fig. 7. Variation of ac susceptibility with respect to temperature of the sampleswith different x values, as labelled on the curves. Inset shows variation of Curietemperature with respect to Co contents.

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peaks. To clarify this issue, Tmeasurement was repeated.Fig. 8shows the rst heating and the rst cooling T curves of thesample with x0.50 andFig. 9shows the same measurement forthe second time. As can be seen the rst cooling and both secondheating and cooling curves are the same. This similarity showsthat cation distribution of the sample has changed during the rstheating and nally cation distribution of the sample becamestable. Also it can be seen that Hopkinson peaks have been morevertical than that of the rst heating curve.

Magnetite is unstable at temperatures above 250 1C andchanges to -Fe2O3 rst and then to -Fe2O3, which is a para-magnet at room temperature that is above its Nel temperature(200 K)[52]. This can be clearly seen in its Tcurves (Fig. 10), inwhich during the course of cooling measurement Tcurve has

been a horizontal line.

The Curie temperatures of all sample tabulated in Table 2

shows that as cobalt content increases the Curie temperature wilincrease consequently.

4. Conclusions

Results of this work show that it is possible to get single phaseCo substituted magnetite nanoparticles with a mean particle sizeof 50 nm by coprecipitation method directly. The nanoparticlesobtained by this method have different cation distribution incomparison with that of the bulk counterparts and it is possibleto change this cation distribution by annealing. Co substitutionincreases coercivity of the magnetite, so it changes from a softferrimagnetic to a hard one.

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Fig. 8. First heating and rst cooling Tcurves of the sample with x0.50.

Fig. 9. Second heating and second cooling Tcurves of the sample with x0.50.

Fig. 10. First heating and rst cooling T curves of the sample with x0.00(Magnetite).

Table 2

Curie temperature of the samples.

x Values TC(1C)

0.00 4310.25 5230.50 5340.75 544

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