J Supercond Nov Magn (2014) 27:1293–1298DOI 10.1007/s10948-013-2440-9

O R I G I NA L PA P E R

Magnetic Properties of Mechanically Alloyed Cobalt-ZincFerrite Nanoparticles

Muhammad Aizat Noor Ismail · Mansor Hashim · Abdollah Hajalilou ·Ismayadi Ismail · Muhammad Misbah M. Zulkimi · Norhapishah Abdullah ·Wan Norailiana A. Rahman · Mutia Suhaibah Abdullah · Masni Manap

Received: 30 July 2013 / Accepted: 5 November 2013 / Published online: 24 November 2013© Springer Science+Business Media New York 2013

Abstract In this work, the physical and the magneticproperties of cobalt-zinc ferrite nanoparticles, synthesizedvia high-energy ball milling (HEBM), were examined.X-Ray diffraction (XRD), field emission scanning electronmicroscopy (FeSEM), and scanning transmission electronmicroscopy (STEM) were used to study changes of thepowder structure and morphology analyses. Hysteresis andpermeability measurement were carried out using a BHhysteresisgraph system and an impedance analyzer, respec-tively. The results suggested improved magnetic propertiesof Co0.5Zn0.5Fe2O4 with increasing sintering temperaturefrom 950 °C up to 1200 °C. However, the variations of themagnetic responses were consistent with the varying volumeconcentration of the ferrite composites. Unlike the highlycrystalline pure ferrite which showed magnetic resonancewithin the measured frequency, the crystallineamorphouscomposites showed no visible resonance peak. This provedthat the resonance peak shifted to higher frequency as a re-sult of the single domain spin behavior in the absence ofdomain walls movement.

Keywords Mechanical alloying · Magnetic properties ·Cobalt-zinc ferrite · Nanoparticles

M.A. Noor Ismail (B) · M. Hashim · A. Hajalilou · I. Ismail ·M.M.M. Zulkimi · N. Abdullah · W.N.A. Rahman ·M.S. AbdullahMaterial Synthesis and Characterization Laboratory,Institute of Advanced Technology (ITMA),Universiti Putra Malaysia, 43400 UPM Serdang, Selangor,Malaysiae-mail: aizat1389@gmail.com

M. Hashim · M. ManapDepartment of Physics, Faculty of Science,Universiti Putra Malaysia, 43400 UPM, Serdang, Malaysia

1 Introduction

Cobalt-zinc ferrite is a soft ferrite. Because of its excellentmagnetic and electrical properties, it is extensively used inmany electrical devices such as generators and transformercores [1].

Annealing CoFe2O4, and any spinel structure with theCo2+ ions in the B position and Fe3+ ions dispersed simi-larly among the A and B sites, would yield anisotropic be-havior with high coercivity and remanence after the sinter-ing process. In the normal spinel of ZnFe2O4, all the Fe3+ions are on the B sites and Zn2+ in the A places [2, 3].Hence, Co0.5Zn0.5Fe2O4 has the blend spinel structure ofthem.

Magnetic properties of spinel ferrites are influenced bymorphology, especially by their crystallite size; hence, inbiomedical and modern data storage application we need toclosely control the particles morphology and decrease theirsize to the single domain and superparamagnetic size [4].

Magnetic particles have been synthesized via conven-tional methods including cold or hot pressing and sinteringat high temperature [1]. These methods have strong limita-tions because of the inhomogeneous mix of initial powders,grain growth during sintering and weak interfaces betweenparticles [5, 6]. Therefore, low temperature techniques suchas chemical coprecipitation [7] and template-assisted hy-drothermal [8] were attempted by some researchers andwere expected to be inexpensive methods to control thegrain size and morphology. However, they caused the pre-cipitation of nanoparticles with a fairly wide size distribu-tion [9]. In this work, mechanical alloying (MA) accom-panied by the self-propagating high temperature synthesis(SHS), together called MASHS as a simple and efficienttechnique was used for synthesizing cobalt-zinc spinel fer-rite nanoparticles.

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2 Materials and Methods

In order to synthesize Co0.5Zn0.5Fe2O4 nanoparticles, cobaltoxide (CoO), iron (III) oxide (Fe2O3), and zinc oxide (ZnO)powders were selected as raw materials. Powders weremixed based on the mass ratio and the targeted proportionaccording to Eq. (1).

0.5CoO + 0.5ZnO + Fe2O3 → Co0.5Zn0.5Fe2O4 (1)

The powder mixture was milled for 12 hours in a plan-etary ball mill, model SPEX8000D, with a ball-to powderweight ratio (BPR) of 10:1. For making samples, 2 % PVAand zinc stearate were added to the powder and pressedinto toroid shape of an external diameter of 20 mm, usinga hydraulic press and applying a force of 3 tones in eachcase. The resulting specimens were sintered from 950 °Cup to 1200 °C. Physical properties of as-prepared sampleswere studied using X-ray diffraction (XRD), scanning trans-mission electron microscopy (STEM), and field emissionscanning microscopy (FeSEM). The permeability and lossfactors of the samples were measured using an HP 4291Bimpedance or material analyzer from 1 MHz up to 1 GHz.The magnetic behavior such as the coercivity, Hc, maxi-mum magnetic induction, Bmax and some other measure-ment details of the sintered Co0.5Zn0.5Fe2O4 samples wereexamined with a MATS-2010S hysteresis graph tester. Thetoroid sample was wound with two wires. One side of thetoroid sample was wound with 8 turns (secondary) and theother side was wound with 4 turns (primary) by a ratio 2:1.The end parts of the wires were connected to the MATS-2010S dynamic hysteresisgraph test analyzer. The analyzerwas connected to a computer and run by software. The resultwas shown by the software and recorded. Density measure-ment was carried out based on Archimedes’ principle.

3 Results and Discussion

3.1 Phase Evolution

X-ray diffraction patterns of the powder mixture after a 12hours milling time and sintering in the range of 950 °C upto 1200 °C are shown in Fig. 1. The results indicated thatafter 12 hours milling the only distinguishable phase waszinc iron oxide and no presence of Co0.5Zn0.5Fe2O4 wasobserved. Meanwhile, other phases had become amorphousconsequent to the milling process. Therefore, for determin-ing the synthesis temperature and recrystallization of amor-phous phases, 12 hours-ball-milled sample was heat treatedat about 950 °C for 16 hours duration in the furnace. In otherwords, there were about 3 hours to increase the temperatureto 950 °C, about 10 hours to maintain the temperature, and

Fig. 1 XRD pattern for 12 hr milled (950 °C–1200 °C)

then about 3 hours to decrease it back to the room temper-ature. For the sintering at 950 °C, peaks corresponding toa cobalt-zinc ferrite phase appeared on the XRD pattern in-dicating the degree of structural order. All the peaks werematched with indexed reference patterns: 89-1012 for zinciron oxide (ZnFe2O4) and 22-1086 for cobalt iron oxide(CoFe2O4) [4]. The peaks can be indexed to (2 2 0), (3 1 1),(2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (533) planes of acubic unit cell that corresponds to the cubic spinel structure[4, 11]. Increasing the sintering temperature from 950 °C upto 1000 °C caused the intensity related to all the resultingpeaks to increase, their width to decline due to the crystal-lite growth and the lattice strain to shrink. However, for thestep from 1000 °C to 1200 °C although the intensity wasexpected to increase, it decreased instead. This could be aresult of some phase changes produced due to zinc loss atthe high temperatures.

Applying Williamson–Hall’s equation [10] and the highscore software to compute the mean crystallite size and lat-tice strain of Co-ZnFe2O4 reveals growth of grains from 35to 81 nm size when the samples were sintered from 950 °Cto 1200 °C, respectively. Meanwhile, the lattice strain de-creased from 0.88 to 0.75 percent. This indicates that, withmore sintering, the crystallite size of the zinc cobalt ferritesingle phase increased as a result of rising atomic mobil-ity and higher reaction temperature as well as disappearingstructural defects and dislocations while the lattice strain de-cline due to decrease in defect densities.

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Fig. 2 Real permeability, μ′and loss factor permeability, μ′′

In fact, after sintering at higher temperature, the surfaceof a nanometer-sized particle of the amorphous phase wasconverted to the crystalline phase with micron-size grainswhich obey Williamson–Hall’s equation, Eq. (2);

b cos θ = 0.9λ

d+ 2ε sin θ (2)

where d is the mean crystallite size, θ is the scattering angle,b is the diffraction peak width at half maximum intensity,ε is the mean lattice strain, and λ is the wavelength of theradiation applied.

3.2 Permeability

Permeability is one of the factors utilized in evaluating mag-netic materials. It exists due to the spin rotation in a mag-netic domain and the motion of domain walls. It is stronglyrelated to the microstructure, crystal structure, temperature,composition of materials, and numerous other features. Themagnetic permeability, μ, is the ratio of induction, B , to themagnetizing filed, H . If the magnetizing filed is very low,approaching zero, the ratio is called the initial permeability,μi Eq. (3).

μi = 1

μo

(B

H

); H → 0 (3)

From Fig. 2, it can be observed that the real permeabilitygenerally increases with the increase of sintering tempera-ture. The real permeability is highly influenced by the degreeof crystallinity, phase purity, and grain size. The increasingsintering temperature will increase the density, crystallinity,phase purity, and grain size thus reducing the hindrance ofthe domain wall motion.

For the loss factor, it can be seen that at lower sinteringtemperatures (950 °C and 1000 °C) the loss factor responsesare quite low compared to that of 1100 °C sintering temper-ature. The change happens because of the increase of aver-age grain size and crystallinity due to the increased sintering

Fig. 3 Hysteresis loops of Co0.5Zn0.5Fe2O4 sintered from (a) 950 °Cto 1000 °C (b) 1100 °C to 1200 °C

temperature. At 1100 °C, the larger grain size makes the do-main wall easier to move, hence resulting in higher energyloss.

3.3 Hysteresis Loop

Figures 3a and 3b give the room temperature hysteresisloops of Co0.5Zn0.5Fe2O4 after being sintered at various

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temperatures. It showed that at low applied magnetic field,domains and domain walls are perturbed from their equi-librium positions. The magnetic moment is forced to lineup in the direction of the field. With higher externally ap-plied field, there is increasing arrangement of the momentsthrough a complex process where some domains grow whileothers vanish; the domain walls move, and domains rotate.This process continues until all moments are lined up in thedirection of the field.

The sigmoid shape can well be seen for the sample sin-tered at 1100 °C onward. This indicates that the ferromag-netic behavior (ordered magnetism) started to occur duringthe stage. Even though a single phase of cobalt-zinc ferriteappeared to have been achieved at 950 °C sintering tem-perature, it is believed that at this stage the sample exhib-

Fig. 4 Coercivity vs average grain size of Co0.5Zn0.5Fe2O4

ited weak ordered magnetism because the sample was stilla mixture of small crystallites and significant amounts ofamorphous phase. To obtain the sigmoid shape, it must beassociated mainly with magnetocrystalline anisotropy field,Ha, and optional addition of defects like grain boundariesand pores besides the contribution of a large enough numberof grains with grain size > 0.1 µm.

The coercivity, Hc, as a function of grain size forCo0.5Zn0.5Fe2O4 is shown in Fig. 4. The coercivity in-creases with average grain size until the critical size (attemperature 1200 °C). Figure 5 shows the average grainsize increases with increasing sintering temperature. TheHc is controlled by anisotropy below critical size. The mag-netic anisotropy related to the increasing of the Hc is theshape anisotropy. For the single domain, since part of do-main wall is in the grain boundary, this would make thedomain walls hard to move. This case is correlated with theshape anisotropy, when particles are not perfectly spherical,the demagnetizing field will not be equal in all directions,thus creating one or more easy axes.

Since ZnFe2O4 is a normal spinel, with the Zn2+ ionsin the “A” site and both Fe3+ ions in the “B” site. Instead,CoFe2O4 is an inverse spinel with Co2+ in B site and Fe3+distributed evenly between A and B sites. In CoFe2O4 fer-rites, one-half of Fe3+ is placed in the A sites and anotherhalf in the B sites. The magnetic moments of Fe3+ are mu-tually compensated and the resulting moment of the ferriteis due to the magnetic moments of bivalent cations Co2+in the B positions. Substitution of Co2+ with Zn2+ leads to

Fig. 5 FeSEM micrographs forCo0.5Zn0.5Fe2O4 sintered attemperature (a) 950 °C(b) 1000 °C (c) 1100 °C(d) 1200 °C

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Fig. 6 Particle size ofCo0.5Zn0.5Fe2O4 as milled

introduction of non-magnetic Zn2+ ions into A sites, thusincreasing the saturation magnetization, Ms, leading to anincreased magnetization

This variation with crystallite size is also explained on thebasics of domain structure, diameter of particles, and crystalanisotropy. Since sintering temperature causes changes bydecomposition or transformation of phases. These, in turn,brings about increase in grain size, change in pore shape,pore size, and number. The starting materials, being in nano-size are not likely to have any grain boundary, however, sin-tering introduces the effects of grain boundary, in additionto some microstructural defects; thereby forming a sourceof flux B, thereby resulting to higher values of coercivityand retentivity.

After analyzing the STEM image as shown in Fig. 6, theaverage particle size is 63.4 nm. From the theoretical methodfor the mechanical alloying, we obtained the nanoparticlesize.

4 Conclusion

In conclusion, Co0.5Zn0.5Fe2O4 nanoparticles were success-fully synthesized via the mechanical alloying method. Thepermeability and hysteresis-loop characteristic were foundto depend on the degree of crystallinity, phase purity, andgrain size.

Acknowledgements The authors have benefited from discussionswith Dr. Mansor Hashim and Dr. Ismayadi Ismail. This work was sup-ported by the Institute of Advanced Technology and Faculty of ScienceUPM.

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