Journal of Magnetism and Magnetic Materials 324 (2012) 2711–2716

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0304-88

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Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite

G.C.P. Leite a, E.F. Chagas a,n, R. Pereira a, R.J. Prado a, A.J. Terezo b, M. Alzamora c, E. Baggio-Saitovitch c

a Instituto de Fısica, Universidade Federal de Mato Grosso, 78060-900 Cuiaba-MT, Brazilb Departamento de Quımica, Universidade Federal do Mato Grosso, 78060-900 Cuiaba-MT, Brazilc Centro Brasileiro de Pesquisas Fısicas, Rua Xavier Sigaud 150 Urca, Rio de Janeiro, Brazil

a r t i c l e i n f o

Article history:

Received 14 October 2011

Received in revised form

8 February 2012Available online 5 April 2012

Keywords:

Exchange coupling

Ferrite

Nanocomposite

(BH)max product

Coercivity

53 & 2012 Elsevier B.V.

x.doi.org/10.1016/j.jmmm.2012.03.034

esponding author. Tel.: þ55 65 3615 8747; f

ail address: efchagas@fisica.ufmt.br (E.F. Chag

Open access under the El

a b s t r a c t

In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe2O4 and

ferrimagnetic oxide/ferromagnetic metal CoFe2O4/CoFe2 nanocomposite. The latter compound is a

good system to study hard ferrimagnet/soft ferromagnet exchange coupled. Two steps were followed to

synthesize the bimagnetic CoFe2O4/CoFe2 nanocomposite: (i) first, preparation of CoFe2O4 nanoparti-

cles using a simple hydrothermal method, and (ii) second, reduction reaction of cobalt ferrite

nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures,

particle sizes, morphology, and magnetic properties of CoFe2O4 nanoparticles were investigated by

X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and

vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50 K.

The mean diameter of CoFe2O4 particles is about 16 nm. Mossbauer spectra revealed two sites for Fe3þ .

One site is related to Fe in an octahedral coordination and the other one to the Fe3þ in a tetrahedral

coordination, as expected for a spinel crystal structure of CoFe2O4. TEM measurements of nanocompo-

site showed the formation of a thin shell of CoFe2 on the cobalt ferrite and indicate that the

nanoparticles increase to about 100 nm. The magnetization of the nanocomposite showed a hysteresis

loop that is characteristic of exchange coupled systems. A maximum energy product (BH)max of

1.22 MGOe was achieved at room temperature for CoFe2O4/CoFe2 nanocomposites, which is about 115%

higher than the value obtained for CoFe2O4 precursor. The exchange coupling interaction and the

enhancement of product (BH)max in nanocomposite CoFe2O4/CoFe2 are discussed.

& 2012 Elsevier B.V. Open access under the Elsevier OA license.

1. Introduction

The figure of merit for permanent magnet material, thequantity (BH)max to the ideal hard material (rectangular hysteresisloop), is given by (BH)max¼(2pMS)2. For materials with highcoercivity (HC) the magnetic energy product is limited by thesaturation magnetization (MS). Seeking to overcome this limita-tion, and in order to obtain a material with high (BH)max product,Kneller and Hawig [1] proposed a nanocomposite formed by theexchange coupling of both hard (high HC) and soft (high MS)magnetic materials. These materials, called exchange spring orexchange-hardened magnets, combine the high coercitivity of thehard material with the high saturation magnetization of the softmaterial, making possible the increase of the (BH)max product ofthe nanocomposite when compared with any one of the indivi-dual phases that form the nanocomposite [1–6].

The increase of the MS is caused by the exchange couplingbetween grains of nanometer size. Kneller and Hawig [1] deriveda relationship that predicts how to reach a significant remanence

ax: þ55 65 3615 8730.

as).

sevier OA license.

enhancement. This enhancement is achieved using the micro-structural and magnetic properties of this new kind of material,thus allowing the development of nanostructured permanentmagnetic materials.

According to the one-dimensional model of Kneller and Hawig,the critical dimension (bcm) for the m-phase (soft material)depends on the magnetic coupling strength of the soft phase Am

and the magnetic anisotropy of the hard phase Kh, according tothe following equation

bcm ¼ pAm

2Kh

� �1=2

ð1Þ

To obtain a sufficiently strong exchange coupling, the grainsize of the soft phase must be smaller than 2bcm. In a general way,a good magnetic coupling of the hard and soft components isachieved in materials with grain sizes of about 10–20 nm [7], theapproximate value of the domain wall width in the hard magneticmaterials.

Cobalt ferrite, CoFe2O4, is a hard ferrimagnetic material that hasinteresting properties like high HC [8,9], moderate MS [10,11], highchemical stability, wear resistance, electrical insulation, and ther-mal chemical reduction [12,13]. The last property allows thetransformation of CoFe2O4 into CoFe2 (a soft ferromagnetic material

G.C.P. Leite et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2711–27162712

with a high MS value of about 230 emu/g [14]) in moderate/hightemperatures. This property was used by Cabral et al. [13] to obtainthe nanocomposite CoFe2O4/CoFe2 and by Scheffe et al. [12] inhydrogen production. Also, the CoFe2O4/CoFe2 nanostruturedbimagnetic material was previously studied as layered thin filmsby Jurca et al. [15] and Viart et al. [16].

In this work we describe an original process of chemicalreduction used for the synthesis of the CoFe2O4/CoFe2 nanocom-posite materials, as well as the magnetic and structural character-ization of both precursor and nanocomposite materials. Finally,the enhancement obtained for the (BH)max product of the CoFe2O4/CoFe2 nanocomposite compared with that of the CoFe2O4 pre-cursor is reported.

Fig. 1. XRD diffraction pattern of the CoFe2O4 sample. Rietveld refinement profile

(solid line) is also displayed.

Fig. 2. XRD diffraction pattern of the CoFe2 sample produced by reduction

reaction of cobalt ferrite nanoparticles blended with activated charcoal in the

molar ratio 1:10.

2. Experimental procedure

2.1. Synthesis of CoFe2O4

The hydrothermal method was used to synthesize cobaltferrite. This method provides different classes of nanostructuredinorganic materials from aqueous solutions by means of smallTeflon autoclaves. It also has a number of benefits such as a cleanproduct with a high degree of crystallinity at a relative lowreaction temperature (up to 200 1C). All the reagents used in thissynthesis are commercially available and were used as receivedwithout further purification. Appropriate amounts of analytical-grade ammonium ferrous sulfate (NH4)2(Fe)(SO4)2 �6H2O (0.5 g,1.28 mmol) and sodium citrate Na3C6H5O7 (0.86 g, 4.72 mmol)were dissolved in 20 ml of ultra-pure water and stirred togetherfor 30 min at room temperature, then stoichiometric CoCl2 �6H2O(0.15 g, 0.64 mmol) was added and dissolved, followed by theaddition of an aqueous solution of 5 M NaOH. The molar ratio ofCo (II) to Fe (II) in the above system was 1:2. The mixtures weretransferred into an autoclave, maintained at 120 1C for 24 h andthen cooled to room temperature naturally. A blackish precipitatewas separated and washed several times with ultra-pure waterand ethanol.

2.2. Synthesis of CoFe2O4/CoFe2 nanocomposite

To obtain the nanocomposite, we mixed the nanoparticles ofcobalt ferrite with activated charcoal (carbon) and subjected themixture to heat treatment at 900 1C for 3 h in an inert atmosphere(Ar), promoting the following chemical reduction

CoFe2O4þ2CD!CoFe2þ2CO2

The symbol D indicates that thermal energy is necessary in theprocess.

A similar process was used by Cabral et al. [13] to obtain thesame nanocomposite, and by Scheffe et al. [12] to producehydrogen.

Theoretically, by varying the molar ratio between activatedcarbon and cobalt ferrite we can control the fraction of CoFe2 inthe nanocomposite. However, the process is difficult to controldue the residual oxygen in the inert atmosphere.

Two samples were prepared using the process described here:one full and another partially reduced. The molar ratio betweenactivated charcoal and cobalt ferrite was 2:1 (partially reducedsample) and 10:1 (fully reduced sample).

2.3. Structural and magnetic measurements

The crystalline phases of the calcined particles were identifiedby the powder X-ray diffraction (XRD) patterns of the magnetic

nanoparticles obtained on a Siemens D5005 X-ray diffractometerusing Cu–K radiation (0.154178 nm).

Magnetic measurements were carried out using a VSM(VersaLab Quantum Design) at room temperature and 50 K.57Fe Mossbauer spectroscopy experiments were performed intwo temperatures, 4.2 K and 300 K to CoFe2O4 samples.

The morphology and particle size distribution of the sampleswere examined by direct observation via transmission electronmicroscopy (TEM) (model JEOL-2100, Japan).

3. Results and discussion

XRD analysis of the synthesized powder after calcination(Fig. 1) showed that the final product was CoFe2O4 with theexpected inverse spinel structure (JCPDS no. 00-022-1086), pre-senting the Fd3m spatial group with a lattice parametera¼8.403 A70.0082 A. A value is close to that is expected forthe bulk CoFe2O4 (a¼8.39570) [17]. The XRD pattern also revealstraces of Co and Co7Fe3 crystalline phases (indicated in Fig. 1).

Fig. 2 shows the diffraction profile obtained for the completelyreduced sample. The XRD profile was similar to that to the CoFe2

(JCPDS no. 03-065-4131), indicating that the expected chemical

G.C.P. Leite et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2711–2716 2713

reduction had occurred. Due to the small quantity of the partiallyreduced sample obtained, XRD measurements could not beperformed.

Mossbauer spectroscopy experiments were performed onnanoparticles of CoFe2O4 at room temperature and 4.2 K (seeFig. 3). About 7% of the total area of the spectrum at bothtemperatures corresponds to the Fe metallic with small quantitiesof Co. The hyperfine magnetic field for magnetic iron is 330 kOe,but in this case the Bhf obtained for the fitting was a little bitlarger, indicating the presence of Co. At 4.2 K, where all theparticles are blocked, the Mossbauer spectra were fitted withtwo sextets corresponding to the Fe3þ in a tetrahedral andoctahedral coordinate (A and B-site respectively), as expectedfor a spinel crystal structure of CoFe2O4. At room temperature thespectra are properly fitted if we add a hyperfine field distributioncentered at 460 kOe (this could be due to the size distribution ofparticles). The hyperfine fields values of A and B patterns at roomtemperature are 48572 and 50072 kOe, respectively, which aretypical values for Fe3þ . This characteristic of the Fe ions inCoFe2O4 was also manifested by the value nearly zero withinexperimental error of the electrical quadrupole splitting (hencethe possibility of Fe2þ is also excluded) [18]. The hyperfineparameters obtained from the fit of Mossbauer spectra areshowed in the Table 1.

Fig. 3. Mossbauer spectra of CoFe2O4 at room temperature (CoFe2O4-RT) and 4.2 K

(CoFe2O4-HE).

Table 1Hyperfine parameters obtained from the fit of Mossbauer spectra at room temperature

Temperature (K) A (Tetrahedral)

d (mm/s)a DEQ (mm/s)b Bhf (

295 0.29 0.04 48.5

4.2 0.27 0.02 51.5

a Isomer shift.b Quadruple splitting.c Effective magnetic field.

Both the morphology and the dimension of nanoparticles wereanalyzed by TEM measurements. The measurement of the cobaltferrite sample (precursor material) shows the formation ofaggregates. This result is expected for samples prepared byhydrothermal method [19,20]. Fig. 4 shows a TEM image of cobaltferrite particles. The TEM measurement reveals that the CoFe2O4

nanoparticles form a polydisperse system with nearly sphericalnanoparticles. The particle size distribution indicates that ferritecobalt particles have a mean diameter of 16 nm and a standarddeviation of about 4.9 nm. The particle size histogram obtained byTEM measurements of the cobalt ferrite sample is shown in Fig. 5.

The TEM measurements of the nanocomposite (CoFe2O4/CoFe2) are shown in Figs. 6 and 7. In Fig. 6a one can see thatthere is roughness on the surface of the nanoparticle. Note thatsimilar roughness was not observed on the surface of theprecursor material (see Fig. 4). In addition, Fig. 6b shows thatthe superficial material connects the nanoparticles, and that themajority of this material is at the interface of the nanoparticles. InFig. 7a one can see that the nanoparticle is composed of twoparts: a big core and a thin shell (thickness about 1.5 nm). Similarpictures are observed for other nanoparticles (not shown).As previously mentioned, the shell does not cover each nanopar-ticle, but rather aggregates of nanoparticles. We attribute the coreto the CoFe2O4 (hard material) and the shell to the CoFe2 (softmaterial). Thus the nanocomposite obtained is composed ofspheres of magnetically hard material in a soft matrix.

and liquid helium temperature for CoFe2O4 sample.

B (Octahedral)

kOe)c d (mm/s)a DEQ (mm/s)b Bhf (kOe)c

0.37 0.02 50

0.37 0.02 54

Fig. 4. Transmission electron microscopy image of as-prepared CoFe2O4.

Fig. 5. Particle size histogram fitted with a log-normal distribution (solid line). Particles have a mean diameter of 16 nm.

Fig. 6. Transmission electron microscopy image of CoFe2O4/CoFe2 nanocomposite. View of (a) one nanoparticle and (b) the region between two nanoparticles.

G.C.P. Leite et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2711–27162714

The inserts in Fig. 7a and b show details of the interplanardistance of both core and shell, respectively. The interplanardistance observed in the core is about 0.49 nm (insert ofFig. 7a). This value is the same as that obtained by Chen andGao [21] for the (111) plane of CoFe2O4. The insert in Fig. 7bshows an interplanar distance of about 0.3 nm obtained for theshell, but due to the small thickness of the shell we considermeasurements by high-resolution TEM (HRTEM) as necessary inorder to obtain more precise results.

TEM analysis indicates that the nanoparticles of the nanocom-posite are larger than the original nanoparticles, indicating thatthe reduction process increases the mean size (diameter) of thenanoparticles to about 100 nm. Also, TEM measurements showedthat the dimension of the soft phase (CoFe2) is smaller than thecritical size (2bcm) obtained by Eq. (1) (see Fig. 6b). Using themagnetic parameters available for CoFe2O4 and CoFe2 (Am�1.7�10�11 J/m [22,23], Kh�2.23�105 J/m3)[24], the calculated quan-tity bcm for the soft CoFe2 phase is about 20 nm.

The cobalt ferrite sample studied in this work has showedcoercivity of about 1.69 kOe, at room temperature (see Fig. 8).This result is higher than the coercivity obtained by Cabral et al.(1.32 kOe) [13], but lower than that reported by Ding et al. [8] andby Liu and Ding [9] for samples treated by thermal magneticannealing and mechanical milling, respectively.

The hysteresis loop at 50 K shows a strong increase of coercivityto a value of 8.8 kOe as compared with the value obtained at roomtemperature (see Fig. 8). Similar behavior of the coercivity wasobserved by Maaz et al. [25] and Gopalan et al. [26]. Another effectobserved by these authors and also observed in this work was theincrease of the remanence ratio (Mr/MS). The saturation magnetiza-tion (MS) and remanent magnetization (Mr) obtained here were,respectively, 445 emu/cm3 (82 emu/g) and 181 emu/cm3 (33 emu/g)at room temperature, while at 50 K they were 477 and 323 emu/cm3

(88 and 60 emu/g). These values indicate an increase in the rema-nence ratio (Mr/MS), from 0.42 to 0.68, when the temperature isdecreased from 300 K to 50 K. These results contain two importantfacts: first, the increase of Mr/MS value; and second, the Mr/MS valueobtained at room temperature is close to the theoretical valueexpected (0.5) for non-interacting single domain particles withuniaxial anisotropy [27] even though the cobalt ferrite has a cubicstructure. Kodama [28] attributes the existence of an effectiveuniaxial anisotropy in magnetic nanoparticles to the surface effect.The strong anisotropy that produces a high coercivity can also becaused by a surface effect [28]. Golapan et al. [26] suggested that theincrease in the value of the Mr/MS ratio is associated with anenhanced cubic anisotropy contribution at lower temperatures.

Fig. 9 shows the hysteresis loop of the partially reduced sample(CoFe2O4/CoFe2) at room temperature and 50 K. The hysteresis

Fig. 7. TEM measurements of CoFe2O4/CoFe2 nanocomposite. (A) Shows an

interplanar distance of CoFe2O4 the insert shows details of the marked area.

(B) Shows an interplanar distance of CoFe2 the insert shows details of the

marked area.

Fig. 8. Hysteresis loops for CoFe2O4 at room temperature (300 K) and 50 K for

maximum applied field of 30 kOe.

Fig. 9. Hysteresis loops for CoFe2O4/CoFe2 nanocomposite at room temperature

(300 K) and 50 K for maximum applied field of 20 kOe.

Fig. 10. Hysteresis loops for CoFe2O4/CoFe2 and CoFe2O4 nanocomposites at room

temperature (300 K).

G.C.P. Leite et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2711–2716 2715

curves of the nanocomposite can be described by a single-shapedloop (no steps in the loop) similar to that of a single phase indicatingthat magnetization of both phases reverses cooperatively.

The same behavior of the coercivity for the cobalt ferrite wasalso seen for the nanocomposite. The coercivity increased from1.34 kOe (at 300 K) to 6.0 kOe (at 50 K) as shown in Fig. 9. Thisenormous increase of coercivity deserves more investigation.

The MS obtained at room temperature was about 146 emu/g, avalue higher than the MS obtained for the precursor material andlower than that expected for pure CoFe2 (230 emu/g) [14].

The CoFe2O4/CoFe2 nanocomposites demonstrate inter-phaseexchange coupling between the magnetic hard phase and themagnetic soft phase, which leads to magnets with improvedenergy products. We obtained an energy product (BH)max of1.22 MGOe for the nanocomposite. This value is about 115%higher than the value obtained for CoFe2O4. At room temperaturewe obtained 0.568 MGOe for the product (BH)max, assuming thetheoretical mass density for CoFe2O4 [29]. The value of (BH)max forthe nanocomposite is higher than the best value obtained byCabral et al. [13] (0.63 MGOe) to the same nanocomposite (butwith a different molar ratio). Also, the precursor sample preparedin this work showed higher coercivity and saturation magnetiza-tion than the precursor sample of Cabral et al. [13]. This observa-tion suggests that the (BH)max product depends on the magneticproperties of the precursor material.

Considering the nanocomposite formed only by the mixture ofCoFe2O4 and CoFe2, we expect that the value of MS is the sumof the individual saturation magnetizations of these twocompounds.

Using the values of MS¼230 emu/g for CoFe2 [14] and 82 emu/gfor cobalt ferrite (our sample), the saturation magnetization of the

G.C.P. Leite et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2711–27162716

nanocomposite (146 emu/g) suggests that the content of CoFe2 inthe nanocomposite is about 40% while that of CoFe2O4 isabout 60%.

To better visualize the improvement obtained in magneticproperties, Fig. 10 shows the hysteresis curves of both nanocom-posite CoFe2O4/CoFe2 and cobalt ferrite at room temperature.The small decrease in coercivity and the increase in both Mr andMS were expected. These behaviors can be qualitatively explainedby the simple one-dimensional model proposed by Kneller andHawig [1].

4. Conclusion

We synthesized a nanocomposite of hard ferrimagnetic CoFe2O4

and soft ferromagnetic CoFe2 with exchange coupling behavior atroom temperature. This assertion is confirmed by the hysteresis curveof the nanocomposite, which does not show steps in the loop. Thethermal treatment at 900 1C used in the synthesis increases the meansize of nanoparticles to about 100 nm (indicated by TEM measure-ments). However, the thermogravimetric analysis (not shown) indi-cates that the same treatment can be used at a temperature of about600 1C, but is necessary increase the processing time.

The chemical reduction process described in this work is agood pathway to obtain CoFe2O4/CoFe2 with exchange couplingbehavior, but the residual oxygen in the argon commercial gasmakes control the CoFe2 molar ratio in the nanocompositedifficult.

The magnetic energy product was greatly improved in thenanocomposite when compared with the ferrite precursor. How-ever, other studies show that the coercivity of this precursormaterial can be increased by thermal annealing, thermal andmagnetic annealing, or mechanical milling. This increase ofcoercivity may also improve the magnetic energy product of thenanocomposite, but this assumption deserves more investigation.

Acknowledgments

This work was supported by CAPES Brazilian funding agency(PROCAD-NF project #2233/2008). The authors would like to thankthe LME/LNLS for technical support during electron microscopy work.

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