J Supercond Nov MagnDOI 10.1007/s10948-014-2520-5

ORIGINAL PAPER

Trends of Parallel Microstructure and Magnetic PropertiesEvolution in Co0.5Zn0.5Fe2O4

Muhammad Misbah Muhammad Zulkimi · Mansor Hashim · Ismayadi Ismail ·Raba’ah Syahidah Azis · Samikanu Kanagesan · Muhammad Syazwan Mustaffa ·Mohd Khairul Ikhwan Mohd Zawawi

Received: 10 December 2013 / Accepted: 14 February 2014© Springer Science+Business Media New York 2014

Abstract The present paper reports on an effort to exposeand scientifically explain the microstructure–magneticproperties relationship as they evolve with increasing sinter-ing temperature. Mechanical alloying was used to preparecobalt–zinc ferrite nanoparticles with sintering temperaturefrom 800 to 1,350 ◦C with 50 ◦C increment. The microstruc-ture of the samples was observed using a field emissionscanning electron microscope, and the magnetic parameters,such as the real permeability and loss factor, were measuredat room temperature in the frequency range from 10 MHz to1.0 GHz using an Agilent 4291B impedance/material ana-lyzer. The B–H hysteresis of the samples was investigatedusing a MATS-2010SD Static Hysteresisgraph. From theresults, the real permeability and loss factor were observedto increase up to 1,250 ◦C. These increases correspondedto increases in grain size and are mainly due to easierdomain wall movement. However, due to zinc loss, μ′and μ′′ as well as the saturation induction decreased from1,300 to 1,350 ◦C. The coercivity increased up to 850 ◦Cand decreased with increasing temperature. This increasing-to-decreasing coercivity trend corresponded well with thesingle- to multi-domain grain size transition marked bycritical grain size at about 0.13 μm.

M. M. Muhammad Zulkimi (�) · M. Hashim · I. Ismail ·S. Kanagesan · M. S. MustaffaMaterials Synthesis and Characterisation Laboratory,Institute of Advanced Technology (ITMA),Universiti Putra Malaysia, 43400 Serdang,Selangor, Malaysiae-mail: muhammadmisbah1215@gmail.com

R. S. Azis · M. K. I. Mohd ZawawiPhysics Department, Faculty of Science,Universiti Putra Malaysia, 43400 Serdang,Selangor, Malaysia

Keywords Magnetic properties · Microstructure ·Mechanical alloying · Spinel

1 Introduction

In recent years, there has been a considerable interestin the study of the properties of nanosized ferrite par-ticles because of their importance in the fundamentalunderstanding on the physical processes as well as theirproposed applications for many technology purposes [1].Of the ferrites, ferromagnetic cubic spinel ferrites pos-sess properties of both a magnetic material and elec-tric insulator. These properties make such ferrites impor-tant materials in many technological applications. How-ever, recently, new focus at attention has been paid tothe investigation of nanophase spinel ferrite particles dueto their technological importance in application areas ofmicrowave devices, high-speed digital tape, disk record-ings, magneto-fluids, catalysis, and magnetic refrigerationsystems. Among spinel ferrites, Zn2+-substituted CoFe2O4

nanoparticles (Cox−1ZnxFe2O4) exhibit improved prop-erties such as excellent chemical stability, high corro-sion resistivity, magneto-crystalline anisotropy, magneto-striction, and magneto-optical properties [2]. For Zn2+substitution rate x = 0.5, maximum magnetization can beachieved [2, 3].

To date, cobalt–zinc ferrite (Cox−1ZnxFe2O4) nanopow-ders have been fabricated by many techniques such asthe co-precipitation method [4–6], the sol–gel method [7],the hydrothermal method [8], etc. The properties of ferritematerials are known to be strongly influenced by their com-position and microstructure that, in turn, are sensitive to theprocessing method to synthesize them. However, for severaldecades, studies of the relationship between morphological

J Supercond Nov Magn

and magnetic properties of soft ferrites have been focus-ing only on the product of the final sintering temperature,largely neglecting the parallel evolutions of morphologicaland magnetic properties [9].

In this study, to enable the parallel evolutions to beclosely observed, cobalt–zinc ferrite was prepared bymechanical alloying. The specific purpose of this study wasto expose and explain experimental microstructure prop-erty relationship as they evolve with increasing sinteringtemperature.

2 Materials and Method

A targeted composition of Co0.5Zn0.5Fe2O4 powder wasprepared using the mechanical alloying method. Allreagents used were of analytical grade from Alfa Aesarwithout any further purification. The starting powder ofFe2O3 (99.95 %), CoO (99.9 %), and ZnO (99.99 %) wasweighed according to the composition formula. The chemi-cals were mixed with chosen molar ratio 1:0.5:0.5, and theball-to-powder mass charge ratio (BPR) was chosen to be10:1. The powder mixture was milled for 12 h using a SPEX8000D shaker mill in ambient air atmosphere. The resultingparticles were divided into 12 samples and were granu-lated using 1 % PVA and lubricated using zinc stearate andthen molded into samples of toroidal shape. The sampleswere sintered with different sintering temperatures rangingfrom 800 to 1,350 ◦C with 50 ◦C increments at selectedtemperatures for a 10-h holding time.

The phase identification for as-prepared powder andsintered samples was examined with X-ray diffraction(Philips Expert Pro PW3040) using CuKα radiation.The microstructure was observed through an FEI NovaNanoSEM 230 machine, and the grain size was measured bythe mean linear intercept method involving over 200 grains.

The density of the samples was measured using theArchimedes principle with water as the fluid medium. Thepercentage theoretical density (% Dth) was calculated usingthe following formula:

%Dth = (Measured density/Theoretical density)× 100 %

(1)

The theoretical density was calculated using the followingequation:

Theoretical density = 8M/Na3 (2)

where M is the molecular weight of a sample, N is the Avo-gadro’s number, and “a” is the lattice constant which wascalculated by indexing the X-ray diffraction (XRD) pattern.

The percentage of porosity of the sample was calculatedusing the relation

P = (1 − ρexp/ρtheoretical

) × 100 % (3)

where ρexp is the measured density of the sample.An Agilent Model 4291B Network/Spectrum Analyzer

in the frequency range of 1 MHz to 1.0 GHz was usedto measure the real permeability and the loss factor. Themagnetic B–H hysteresis properties of the samples wereinvestigated using a MATS-2010SD Static Hysteresisgraph.

3 Results and Discussion

3.1 Phase Analysis

Figure 1 shows XRD spectra of the as-prepared sample andsamples sintered from 800 to 1,350 ◦C after the high-energyball milling. The as-prepared sample showed the phase ofFe2O3 at 2θ = 33.19◦, ZnO at 2θ = 35.95◦, and CoO at2θ = 36.95◦. A complete phase of cobalt–zinc ferrite wasobserved to form at the 800 ◦C sintering temperature withall diffraction peaks being indexed to zinc–cobalt ferritewith spinel structure (PDF no. 65-3107) [5].

As the sintering temperature increased, the average grainsize would also increase, yielding more crystallization ofthe sample. At the same time, the magnetic phase would beincreased due to the increasing crystalline mass having thecubic spinel structure. The increase of magnetic phase inthe samples caused the A–B interaction contribution in thespinel to be increased. Therefore, a higher sintering temper-ature leads to a higher value of saturation magnetization.The density measurement for samples sintered from 800to 1,350 ◦C, as shown in Table 1, showed that the den-sity was increased with sintering temperature but slightlydropped after being sintered at 1,300 ◦C. The sample sin-tered at 1,300 ◦C showed existence of pores which causeda decrease in density. The formation of pores was sub-jected to the probability of the zinc loss and rapid graingrowth at high sintering temperature, causing the pores tobe trapped inside the grains. This is shown in Table 1, wherethe porosity decreased generally up to the 1,250 ◦C sin-tering temperature and increased at the 1,300 ◦C sinteringtemperature.

3.2 Particle Size Analysis

The scanning transmission electron microscopy (STEM)micrograph shows the as-milled particles (Fig. 2) aftermilling. The size of a particle varied from 13 to 283 nm withan average size around 75.21 nm. This inhomogeneous dis-tribution might be due to the non-uniformity of the force ofmilling media (steel balls and vials) to the powders during

J Supercond Nov Magn

2θ20 30 40 50 60

Intensity

(a,units)

a

Iron

oxi

de

Iron

oxi

de

Zin

c ox

ide

Cob

alt

oxid

e

Iron

oxi

de

Iron

oxi

de

Iron

oxi

de

Iron

oxi

de

2θ (degree)

20 30 40 50 60

Intensity

(a.u)

850oC

900oC

950oC

1000oC

1050oC

1100oC

1150oC

1200oC

1250oC

1300oC

1350oC

(311)

(220) (222) (400) (422)

(511)(440)

800oC

b

Fig. 1 XRD spectra of a as-milled raw powder and b sintered cobalt–zinc ferrite at 800 to 1,350 ◦C

high-energy milling process. However, this average particleis considered to be smallest for a starting powder, comparedto that normally use in the ferrite production industry andeven in much of the past research on ferrites. Particle sizeof the sample is important in order to obtain uniform grainsize. Also, the bigger the surface area of the powder, thegreater driving force for densification so that higher den-sities could be achieved. According to Goldman [10], theoptimization control of starting particle size is important inorder to control the evolution of microstructure of the sam-ple such as grain size, pores size, and density because themagnetic properties are sensitive to these parameters.

3.3 Microstructure and Magnetic Properties

The surface microstructure of toroidal bulkCo0.5Zn0.5Fe2O4 compacts with sintering temperature from800 to 1,350 ◦C is shown in Fig. 3. After the sintering pro-cess, the samples were directly viewed under field emissionSEM (FESEM) without polishing to study the microstruc-tures. The grain size distribution and average grain size areshown in Fig. 4 and Table 1. These show that the averagegrain size increased from 0.12 to 3.34 μm, and this increaseindicates the microstructural evolution due to grain growthof the samples.

Table 1 Detail of samples with sintering temperature, measured density, theoretical density, porosity, and average grain size

Temperature Density Theoretical density Porosity (%) Average grain

(◦C) measured (±0.01) (%) (±0.01) (±0.01) size (μm) (±0.01)

800 4.41 83.08 16.92 0.122

850 4.41 86.24 13.76 0.126

900 3.87 72.79 27.21 0.183

950 4.42 87.43 12.57 0.217

1,000 4.70 88.36 11.64 0.258

1,050 4.51 84.93 15.07 0.273

1,100 4.73 88.92 11.08 0.319

1,150 4.83 90.63 9.37 0.509

1,200 4.78 90.00 10.00 0.925

1,250 4.83 90.87 9.13 2.403

1,300 4.81 90.11 9.89 2.764

1,350 4.00 74.81 25.18 3.341

J Supercond Nov Magn

Fig. 2 a STEM image. b Grainsize distribution of as-milledraw powder

Figure 4 shows the grain size distribution for all the sin-tered samples from 800 to 1,350 ◦C. Sintering temperaturesof 800 to 950 ◦C produced average grain size distributionin the range of 0.12 to 0.22 μm. The samples sintered from800 to 950 ◦C mostly showed the process of necking, indi-cating the growth of grains over the sintering. The realpermeability, μ′, and the loss factor, μ′′, showed low val-ues of the magnetic properties (Figs. 5 and 6) which weredue to spin rotation. According to Kingery [11], for suffi-ciently small grains, there should be no domain walls within

the grains, and thus, the permeability was given solely bythe rotational processes. The samples sintered at 1,000 to1,250 ◦C (Fig. 3e–j) showed increasing grain size, from 0.25to 2.40 μm. At this stage, grain boundaries (and grains) wereformed. It showed a well-established behavior of strongordered magnetism and was attributed to significantly eas-ier domain wall displacement, which was believed to givea major contribution to the μ′ and μ′′ values. The sam-ples sintered at 1,300 and 1,350 ◦C (Fig. 3k, l) showed theexistence of intragranular pores. Intragranular pores were

Fig. 3 FESEM micrographs of cobalt–zinc ferrite sintered at a 800 ◦C, b 850 ◦C, c 900 ◦C, d 950 ◦C, e 1,000 ◦C, f 1,050 ◦C, g 1,100 ◦C, h1,150 ◦C, i 1,200 ◦C, j 1,250 ◦C, k 1,300 ◦C, and l 1,350 ◦C

J Supercond Nov Magn

05

1015202530

0-10

21-30

41-50

61-70

81-90

101-11

012

1-13

014

1-15

016

1-17

018

1-19

020

1-21

022

1-23

0Per

cent

age

(%)

Size Range (nm)

800oCa

05

1015202530

0-10

21-30

41-50

61-70

81-90

101-11

012

1-13

014

1-15

016

1-17

018

1-19

020

1-21

022

1-23

024

1-25

0Per

cent

age

(%)

Size Range (nm)

850oCb

0

5

10

15

0-10

0

121-13

0

151-16

0

181-19

0

211-22

0

241-25

0

271-28

0

301-31

0

331-34

0

361-37

0Per

cent

age

(%)

Size Range (nm)

900oCc

0

5

10

15

20

0-10

012

1-13

015

1-16

018

1-19

021

1-22

024

1-25

027

1-28

030

1-31

033

1-34

036

1-37

039

1-40

042

1-43

045

1-46

0Per

cent

age

(%)

Size Range (nm)

950oCd

0102030405060

Per

cent

age

(%)

Size Range (nm)

1000oCe

0

10

20

30

40

Per

cent

age

(%)

Size Range (nm)

1050oCf

05

1015202530

0-10

0

151-20

0

251-30

0

351-40

0

451-50

0

551-60

0

651-70

0

751-80

0

851-90

0

951-10

00

Per

cent

age

(%)

Size Range (nm)

1100oCg

0

5

10

15

20

100-15

0

201-25

0

301-35

0

401-45

0

501-55

0

601-65

0

701-75

0

801-85

0

901-95

0

Per

cent

age

(%)

Size Range (nm)

1150oCh

01020304050

200-30

0

301-40

0

401-50

0

501-60

0

601-70

0

701-80

0

801-90

0

901-10

00

1001

-200

0

Per

cent

age

(%)

Size Range (nm)

1200oCi

020406080

100

600-70

070

1-80

080

1-90

090

1-10

0010

01-200

020

01-300

030

01-400

040

01-500

050

01-600

060

01-700

0

Per

cent

age

(%)

Size Range (nm)

1250oCj

01020304050607080

Per

cent

age

(%)

Size Range (nm)

1300oCk

0102030405060

800-90

090

1-10

0010

01-200

020

01-300

030

01-400

040

01-500

050

01-600

060

01-700

070

01-800

080

01-900

090

01-100

00perc

enta

ge (%

)

Size Range (nm)

1350oCl

Fig. 4 Grain size distribution of cobalt–zinc ferrite sintered at a 800 ◦C, b 850 ◦C, c 900 ◦C, d 950 ◦C, e 1,000 ◦C, f 1,050 ◦C, g 1,100 ◦C, h1,150 ◦C, i 1,200 ◦C, j 1,250 ◦C, k 1,300 ◦C, and l 1,350 ◦C

trapped pores in the grains due to rapid grain growth. Intra-granular pores are known to be bad inclusions becausethe presence of these pores would pin down the magneticmoment in grains, thus reducing the magnetization.

Figures 5 and 6 show the real permeability (μ′) andthe loss factor (μ′′), respectively, for the sintered samplesat 800 to 1,350 ◦C. As shown in Fig. 5, the real per-meability increased with an increasing temperature up to

1,250 ◦C and decreased with further increasing sinteringtemperatures from 1,300 to 1,350 ◦C. At higher sinteringtemperature, with increased grain size, a fewer number ofthe grain boundaries were present, giving rise to very mobiledomain walls, thus increasing the permeability values ofthe Co0.5Zn0.5Fe2O4. Moreover, during grain growth, manypores would be removed, thus reducing the hindrance tothe motion of domain walls because pores provide stress

Fig. 5 Real permeability, μ′, ofcobalt–zinc ferrite sintered at800 to 1,350 ◦C

J Supercond Nov Magn

Fig. 6 Loss factor, μ′, ofcobalt–zinc ferrite sintered at800 to 1,350 ◦C

concentration that may affect the easy direction of magne-tization. The porosity that existed at 1,300 and 1,350 ◦Cof sintering temperature would hinder the motion of thedomain walls, causing a decrease in the real permeabil-ity values. The permeability was very stable in the lowerfrequency range, while at higher frequencies, there was asmall rise due resonance. The parameters μ′ and μ′′ wereprominent at the resonance frequency for higher sinter-ing temperature, and the resonance frequency decreasedwith an increase in permeability. This follows Snoek’s law,which suggests that the resonance frequency is inverselyproportional to the magnetic permeability [9].

As we can see from Fig. 6, the loss factor was observedto increase with a rise of the frequency from 1 MHz andattained the maximum value at a particular frequency anddecreased with a further increase in frequency. The loss fac-tor increased as the sintering temperature increased for thesintered samples. As the sintering temperature increased,the grain size increases; thus, more domain walls wereinvolved. It is suggested that the domain walls movementbecomes easier in the larger grain.

Figure 7 shows the magnetic induction, B, against themagnetic field strength, H (B–H hysteresis loop) plotted for

different sintering temperatures. The shape of the hysteresisloop depends on the grains in the sample [12]. Based on thehysteresis loop in Fig. 7 for the sample sintered at 800 ◦C,a small hysteresis was observed, indicating the weak ferro-magnetic nature of the material. The linear-looking loopshave a very low saturation induction, Bs, indicating a verysmall amount of ferromagnetic phase. However, a signif-icant coercivity, Hc, with somewhat elongated B–H loopshape was due to size–shape anisotropy. Even though theXRD pattern for sintered sample at 800 ◦C showed onlycobalt–zinc ferrite peaks that were present, the Bs valuewas still very low, indicating that the crystalline phase wasnot yet dominant. The samples sintered at 850 to 1,050 ◦Cexhibit a mixture of single- and multi-domain grains. Thehigher Bs values but falling Hc values (Fig. 8) indicatethe presence at higher ferromagnetic phase crystallinity,starting at the dominance of multi-domain magnetization–demagnetization process. The samples sintered at 1,100 to1,350 ◦C have a well-defined sigmoid shape because of veryhigh crystallinity, large grain size, and high density whichallow domain walls to move with great ease in the mag-netization and demagnetization process. Table 2 shows thevariation trend in the coercivity, Hc, values for the sintered

Fig. 7 B–H hysteresis ofcobalt–zinc ferrite sintered at800 to 1,350 ◦C

J Supercond Nov Magn

Fig. 8 Coercivity value as afunction of grain size

samples. The increasing variation of Hc values with increas-ing grain size, D, was observed for the samples sinteredat 800 and 850 ◦C. Meanwhile, for the samples sinteredat 900 ◦C and above, Hc values were decreasing withincreasing grain size, D. From a previous study by Sanchezet al. [13], the Hc is proportional to 1 / D; therefore, the Hc

decreases with increasing sintering temperature. The rea-son for this is that in small particles, the formation of aclosed magnetic flux becomes energetically less favorableso that the magnetic domain size with uniform magnetiza-tion become more and more identical with the grain size.This grain size is defined as the critical grain size thresholdwhere a multi-domain material changes to a mono-domainmaterial [12]. From Fig. 8, a similar trend was observedfor the sintered samples; when the grain size was reduced,the Hc increased to a maximum value and then decreasedtoward 0. The sintered samples showed the maximum Hc

value when sintered at 850 ◦C which indicated that thecritical grain size corresponded to the grain size transitionfrom being single-domain to being multi-domain. Based

Table 2 Saturation induction and coercivity values of cobalt–zincferrite sintered at 800 to 1,350 ◦C

Temperature (◦C) Coercivity (Oe) Saturation induction, Bs

(±0.01) (Gauss) (±0.01)

800 23.10 138.20

850 24.93 259.50

900 23.84 336.50

950 20.26 482.00

1,000 19.48 671.80

1,050 17.40 929.60

1,100 15.26 1,215.00

1,150 11.70 1,330.00

1,200 11.12 1,503.00

1,250 9.47 1,965.00

1,300 7.03 1,367.00

1,350 7.02 1,486.00

on our study, the critical size for the sintered sample wasabout 0.13 μm. The grain sizes larger than the critical sizeare multi-domain; meanwhile, grain sizes with smaller thancritical sizes are single-domain.

4 Conclusions

Polycrystalline Co0.5Zn0.5Fe2O4 samples with nanometergrain size have been prepared using the mechanical alloy-ing method followed by sintering. The B–H loop showeda nearly linear-to-slanted sigmoid-to-erect sigmoid progres-sive trend as the grain size and crystallinity degree wereincreasing. The threshold grain size at about 0.13 μm,observed by correlating the coercivity values and the aver-age grain sizes, marked the single- to multi-domain grainsize transition. From the results, the real permeability, μ′,and the loss factor, μ′′, were observed to increase up to1,250 ◦C primarily due to grain size increase, giving rise toeasier domain wall movement. For the sintering temperatureincrease from 1,300 to 1,350 ◦C, the sample’s μ′, μ′′, andsaturation induction showed a decrease due to zinc loss.

Acknowledgments The researchers wish to thank the UniversitiPutra Malaysia, Malaysia, for providing a grant under the gradu-ate research fellowship and MNPEG (Magnetics and NanostructurePolycrystal Evolution Group) for supporting this project.

References

1. Sharma, R.K., Suwalka, O., Lakshmi, N., Venugopalan, K.,Banerjee, A., Joy, P.A.: Synthesis of chromium substituted nanoparticles of cobalt zinc ferrites by coprecipitation. Mater. Lett. 59,3402–3405 (2005)

2. Vaidyanathan, G., Sendhilnathan, S., Arulmurugan, R.: Structuraland magnetic properties of Co1−xZnxFe2O4 nanoparticles by co-precipitation method. J. Magn. Magn. Mater. 313(2), 293–299

3. Hou, C., Yu, H., Zhang, Q., Li, Y., Wang, H.: Preparationand magnetic property analysis of monodisperse Co–Zn ferritenanospheres. J. Alloy. Compd. 491, 431–435 (2010)

J Supercond Nov Magn

4. Sharifi, I., Shokrollahi, H.: Nanostructural, magnetic andMossbauer studies of nanosized Co1−xZnxFe2O4 synthesized byco-precipitation. J. Magn. Magn. Mater. 324, 2397–2403 (2012)

5. Gul, I.H., Abbasi, A.Z., Amin, F., Anis-ur-Rehman, M.,Maqsood, A.: Structural, magnetic and electrical propertiesof Co1−xZnxFe2O4 synthesized by co-precipitation method. J.Magn. Magn. Mater. 311(2) (2007)

6. Pandya, P.B., Joshi, H.H., Kulkarni, R.G.: Bulk magnetic proper-ties of Co-Zn ferrites prepared by the co-precipitation method. J.Mater. Sci. 26(20), 5509–5512 (1991)

7. Singhal, S.: Effect of Zn substitution on the magnetic proper-ties of cobalt ferrite nano particles prepared via sol-gel route. J.Electromagn. Anal. Appl. 02(06), 376–381 (2010)

8. He, H.Y.: Comparison study on magnetic property ofCo0.5Zn0.5Fe2O4 powder by template-assisted sol–gel and

hydrothermal methods. J. Mater. Sci. Mater. Electron. 23, 995–1000 (2012)

9. Ismail, I., Hashim, M.: Sintering temperature dependenceof evolving morphologies and magnetic properties ofNi0.5Zn0.5Fe2O4 synthesized via mechanical alloying. J.Superconduct. Novel Magn. 25(5), 1551–1561

10. Goldman, A.: Modern Ferrite Technology, 2nd edn. Springer,Pittsburg (1990)

11. Kingery, W.D.: Introduction to Ceramic, 2nd edn. Wiley, NewYork (1976)

12. Bean, C.P.: Hysteresis loops of mixtures of ferromagnetic microp-owders. J. Appl. Phys. 26, 1381–1383 (1955)

13. Sanchez, R.D., Rivas, J., Vaqueiro, P., Caeiro, D.: Particle sizeeffects on magnetic properties of yttrium iron garnets prepared bya sol–gel method. J. Magn. Magn. Mater. 247, 92–98 (2002)