Processing and Application of Ceramics 12 [3] (2018) 248–256

https://doi.org/10.2298/PAC1803248T

Synthesis of novel hard/soft ferrite composites particles with improvedmagnetic properties and exchange coupling

Faezeh Tavakolinia1, Mohammad Yousefi1,∗, Seyyed Salman Seyyed Afghahi2,Saeid Baghshahi3, Susan Samadi4

1Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran2Department of Engineering, Imam Hossein University, Tehran, Iran3Department of Materials Engineering, Faculty of Engineering, Imam Khomeini International University,

Qazvin, Iran4Department of Chemistry, Yadegar-e-Imam Khomeini (RAH), Shahre Rey Branch, Islamic Azad University,

Tehran, Iran

Received 21 February 2018; Received in revised form 18 June 2018; Accepted 14 August 2018

Abstract

SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4 hard/soft ferrite composite particles with 20, 40, 60 and 80 wt.% of soft phasewere prepared by one-pot sol-gel auto-combustion and physical mixing methods. Fourier transform infraredspectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and vi-brating sample magnetometer (VSM) were used to characterize the structural and magnetic properties of thesamples. XRD spectrum revealed the formation of mixed ferrite phases in the composite particles. The hys-teresis loops of the samples showed the presence of exchange coupling between the hard and soft ferrites. Thecomposite particles with 20 and 60 wt.% of the soft phase demonstrated the highest Mr/Ms ratio, i.e. 0.29 and0.28, respectively. In addition, the highest Ms, Mr and Hc were achieved in the composite particles with 40,60 and 20 wt.% of the soft phase, respectively. Compared to the physical mixing method (PM), the compositeparticles prepared by the sol-gel auto-combustion method (OP) demonstrated better magnetic properties. Theexchange coupling interaction between the hard and soft ferrite phases was similar in both methods. Thesecomposite particles exhibited magnetically single phase behaviour, however, the saturation magnetization waslower in the physical mixing pared to that of the one-pot method.

Keywords: sol-gel auto-combustion, physical mixing, hard/soft composite ferrites, magnetic properties,

exchange coupling

I. Introduction

Magnetic composites are being used as magneticfluids, microwave devices, biomedicines and perma-nent magnets in various applications [1]. Compositewith hard (SrFe12O19) and soft (Zn0.4Co0.2Ni0.4Fe2O4)phases can improve the magnetic properties becauseof the high exchange coupling of both phases. Conse-quently, with high exchange coupling between hard andsoft phases, the high saturation magnetization of the softphase and high coercivity of the hard phase can increaseall magnetic properties rather than hard and soft phase

∗Corresponding authors: tel: +98 912 525 2435,e-mail: myousefi50@hotmail.com

itself [2]. Ferrite composites composed of spinel softand hard ferrites are good candidates for advanced per-manent magnets, because of their low cost, excellentcorrosion resistance, relatively high Curie temperatureand high electrical resistivity [3].

The hard/soft ferrite composites have been syn-thesized with good exchange spring interactionbetween two phases. Hard/soft composite pow-ders such as BaFe12O19/Mn0.2Ni0.4Zn0.4Fe2O4[4], BaFe12O19/NiFe2O4 [5,6], BaFe12O19/

Ni0.65Zn0.35Fe2O4 [7], Co0.6Zn0.4Fe2O4/SrFe10.5O16.75[8], SrFe12O19/Ni0.7Zn0.3Fe2O4 [9], SrFe10Al2O19/

Co0.8Ni0.2Fe2O4 [10], BaFe12O19/Ni0.5Zn0.5Fe2O4 [11],SrFe12O19/Ni0.5Zn0.5Fe2O4 [12], BaFe12O19/Y3Fe5O12[13], BaFe12O19/Fe3O4 [14] and BaFe12O19-

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Ni0.5Zn0.5Fe2O4 [15] were prepared, but they couldnot reach the aim of intresting high exchange cou-pling between hard/soft ferrite nanocomposites [16].Hard hexagonal and soft spinel ferrites with highcorrosion resistance, good electrical resistivity andcost-effective materials can be used as a permanentmagnet [5,6,12,17–22].

In the present research, the ferrite composite particlesof SrFe12O19 as a hard phase and Zn0.4Co0.2Ni0.4Fe2O4as a soft phase were synthesized to obtain the highexchange coupling. SrFe12O19 is a hexagonal ferritethat has the highest coercivity among the other simi-lar ferrites. CoFe2O4 has high saturation magnetization(Ms ∼ 80 emu/g) [22], and this value could be changedby Ni and Zn doping. SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4hard/soft composite ferrites with various amount of thesoft phase (20, 40, 60 and 80 wt.%) have been synthe-sized by the one-pot sol-gel auto-combustion and phys-ical mixing methods to investigate exchange couplingbehaviour.

II. Experimental

2.1. Sample preparation

In the present work, SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4hard/soft ferrite composite particles with 20, 40,60 and 80 wt.% of soft phase were prepared by

two different processing methods: i) one-pot sol-gelauto-combustion and ii) physical mixing. Iron ni-trate nonahydrate (Fe(NO3)3 · 9 H2O), strontium nitrate(Sr(NO3)2), zinc chloride (ZnCl2), cobalt nitrate hex-ahydrate (Co(NO3)2 · 6 H2O), nickel nitrate hexahydrate(Ni(NO3)3 · 6 H2O) and citric acid (C6H8O7 ·H2O) (allfrom Merck) were used as precursors.

In one-pot sol-gel auto-combustion method, the stoi-chiometric amounts of Sr and Fe nitrates were dissolvedin deionized water at 80 °C to get a brown solution.Then citric acid was added to the solution, with the mo-lar ratio of metal ions to citric acid of 1 : 1.5. In anotherbeaker, stoichiometric amounts of Fe(NO3)3 · 9 H2O,Co(NO3)2 · 6 H2O, Ni(NO3)3 · 6 H2O, ZnCl2 and citricacid were mixed and dissolved in deionized water. Af-ter obtaining a light brown clear solution, the solu-tion was added to the first beaker containing stron-tium hexaferrite salts. The molar ratio of soft metallicions to citric acid was 1 : 1. After 2 h, ammonia so-lution was added dropwise to adjust pH at 7 and themixed solution was heated at 120 °C. The solution be-came a viscous gel while evaporating. Then gel ignitedin the microwave oven device and the black-colouredpowder was prepared. The powders were calcined at500 °C for 5 h and then at 1200 °C for 2 h to formSrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4 composite particles.

In physical mixing method, SrFe12O19 and

Figure 1. Synthesis procedure of SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4 composites by sol-gel auto-combustion andphysical mixing methods

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Zn0.4Co0.2Ni0.4Fe2O4 were synthesized separatelyby the sol-gel auto combustion method and then mixedwith stoichiometric amount physically. The preparedsamples were calcined at 1200 °C for 2 h (Fig. 1).

2.2. Characterization

X-ray diffraction measurements (XRD) were per-formed using a diffractometer (PW-1730, The Nether-lands) with Cu Kα (1.54Å) radiation operated at 40 kVand 30 mA The morphology and composition of thecomposites were investigated by field emission scan-ning electron microscope (FE-SEM; TESCAN; ModelMIRA3) and energy-dispersive spectroscopy spectrom-eter with 15 kV voltage. FTIR spectra were obtained byFourier transform infrared spectrometer (NEXUS870).A vibrating sample magnetometer (VSM; ZVK, R&S)was used to measure the magnetic properties of the pow-ders.

III. Results and discussion

3.1. Structural characterization - XRD analysis

XRD patterns of SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4composite particles prepared by one-pot and physicalmixing methods (calcined at 1200 °C) are presented inFigs. 2a and 2b, respectively. There are only XRD peaksof the hard SrFe12O19 phase (JCPDS card No. 01-084-1531) and soft Zn0.4Co0.2Ni0.4Fe2O4 phase (JCPDS cardNo. 01-087-2336), without any impurity peak, such asNiO, ZnO, SrFe2O4 or α-Fe2O3. Due to the fact that ho-mogeneity of hard and soft phase is related to the syn-thesis method, the intensity of all peaks changes.

Table 1. Average crystallite size of(SrFe12O19)1-x(Zn0.4Co0.2Ni0.4Fe2O4)x (x = 0.2, 0.4, 0.6, 0.8)

composite powders prepared by one-pot sol-gelauto-combustion method and calcined at 1200 °C

SampleCrystallite size [nm]

* (114) plane # (311) planePure soft – 21480% soft 83 3460% soft 78 8040% soft 53 5820% soft 64 47Pure hard 71 –* For SrFe12O19

# For Zn0.4Co0.2Ni0.4Fe2O4

Based on the Scherrer’s equation [23] and XRD 114and 311 peaks of SrFe12O19 and Zn0.4Co0.2Ni0.4Fe2O4phases, the average crystallite sizes were calculated tobe about 71 nm and 214 nm, respectively (Table 1). Byincreasing the amount of the soft phase, the averagecrystallite size is reduced to 69 nm and 53 nm for hardand soft phases, respectively. Cobalt-nickel-zinc ferriteinhibits the growth of strontium ferrite and improves thegrowth of soft phase rather than hard phase [10].

3.2. Structural characterization - FTIR analysis

Figure 3 shows the IR spectra of theSrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4 composite ferritesprepared by one-pot and physical mixing method. Thespectra show absorption peaks at 438, 545, 596, 1128,1633 and 2355 cm-1. The bands around 570–600 and438 cm-1 correspond to the characteristic bonds of(SrFe12O19)1-x(Zn0.4Co0.2Ni0.4Fe2O4)x (x = 0.2, 0.4,

(a) (b)

Figure 2. XRD patterns of (SrFe12O19)1-x(Zn0.4Co0.2Ni0.4Fe2O4)x (x = 0.2, 0.4, 0.6, 0.8) composites prepared by:a) one-pot sol-gel auto-combustion route and b) physical mixing method

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(a) (b)

Figure 3. FT-IR spectra of SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4 composites prepared by: a) physical mixing and b) one-pot methodwith various weight percentages of soft phase in the composite

0.6, 0.8) representing stretching vibration of iron andstrontium bonds with oxygen. These are metal-oxidebonds in the strontium ferrite crystal lattice whichreveal the formation of strontium ferrite. The bandsat 1040 and 1128 cm-1 are related to the C–N bondformation during the synthesis process of strontiumferrite composites The band at 1625 cm-1 is related toasymmetric stretching of carbonyl groups (C−−O). Also,the bands around 2350 cm-1 confirm the presence ofcarbonyl groups. The FTIR spectra of these compoundssuggest that the hydroxyl stretching vibration of ironand strontium bonds with oxygen. In the FTIR spectraof all samples within the range from 800 to 400 cm-1,the typical metal-oxygen absorption bonds confirmthe formation of the hexagonal structure of strontiumferrite [21,24,25].

3.3. Structural characterization - FE-SEM results

The pure SrFe12O19 and Zn0.4Co0.2Ni0.4Fe2O4 arepresented in Figs. 4a and 4b, respectively. It can beseen from Fig. 4a that the pure hard phase calcined at1200 °C has hexagonal platelet-like morphology. Fig-ures 5a-h show FESEM micrographs of the hard/softcomposites calcined at 1200 °C. The morphology of

Zn0.4Co0.2Ni0.4Fe2O4 particles is roughly spherical inshape. During one-step auto combustion process, igni-tion causes generation of huge amounts of gases that arereleased and highly porous nanoparticles are formed ini-tially, but later only a few of them remain which grow insize at the cost of smaller ones. The larger grains corre-spond to the SrFe12O19, while smaller ones represent theZn0.4Co0.2Ni0.4Fe2O4 phase. Microstructure of the pureSrFe12O19 (Fig. 4a) shows large elongated grains withsubmicrometer size. Zn0.4Co0.2Ni0.4Fe2O4 grains (Fig.4b) are nearly spherical in shape with an average size of∼2 µm.

Comparing the microstructural features of the physi-cally mixed samples (Figs. 5b,d,f h) with the ones fromone-pot (Figs. 5a,c,e,g), it is obvious that the compos-ite particles synthesized by one-pot method possess bet-ter homogeneous mixing of Zn0.4Co0.2Ni0.4Fe2O4 andSrFe12O19 phase than the composite particles preparedby physical mixing method. This shows that the pro-cessing method plays an important role in defining themicrostructure of composite. In addition, it is knownthat magnetic particles could induce their agglomerationand, thus, contribute to microstructural changes [25].This difference in the microstructure of the samples may

(a) (b)

Figure 4. FESEM micrographs of SrFe12O19 and Zn0.4Co0.2Ni0.4Fe2O4 samples prepared byone-pot sol-gel auto-combustion route

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(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 5. FESEM micrographs of (SrFe12O19)1-x(Zn0.4Co0.2Ni0.4Fe2O4)x (x = 0.2, 0.4, 0.6, 0.8) composites prepared byone-pot and physical mixing method

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Table 2. EDX element composition of(SrFe12O19)0.8(Zn0.4Co0.2Ni0.4Fe2O4)0.2 and

(SrFe12O19)0.2(Zn0.4Co0.2Ni0.4Fe2O4)0.8 compositesprepared by one-pot method

Sample Fe O Ni Zn Co Sr80% soft 45.3 21 12 6.4 4.9 1.520% soft 64.6 23.5 2.2 3.2 1.1 5.5

play an important role in the magnetic and microwaveabsorption properties of the composites.

EDX analysis (Table 2) of the calcined compositeparticles indicated the presence of all the elements (e.g.Co, Ni, Zn, Fe, Sr, and O). EDX analysis showed that inthe hard/soft composites, there was a good compromisewith chemical stoichiometry.

The range of the exchange-coupling interaction be-tween the grains of hard and soft magnetic phases,i.e., the exchange length, Lex, can be expressed as[20,24,26]:

Lex =A2

K2D3(1)

where A and K denote the exchange stiffness and themean amplitude of the random effective anisotropy con-stant, respectively, and D is the diameter of the grain.So, larger grain size reduces the exchange length andconsequently the coupled regions.

3.4. Magnetic properties

Room temperature magnetic loops for the pureSrFe12O19 and Zn0.4Co0.2Ni0.4Fe2O4 samples calcinedat 1200 °C are given in Fig. 6. Hc = 5813 Oe,Ms = 46 emu/g and Mr = 26 emu/g were measuredfor SrFe12O19 powder. The high Hc value confirmsthat SrFe12O19 belongs to the hard magnetic mate-rials. The saturation magnetization and coercivity ofZn0.4Co0.2Ni0.4Fe2O4 are 67 emu/g and 64.7 Oe, respec-tively, confirming that Zn0.4Co0.2Ni0.4Fe2O4 belongs tothe soft magnetic materials.

Room temperature magnetic loops for the compositesamples calcined at 1200 °C are given in Fig. 7. The M-

H behaviour of the composite particles prepared by one-

Figure 6. Hysteresis loops of SrFe12O19 andZn0.4Co0.2Ni0.4Fe2O4 powders

pot and physical mixing method shows a smooth curvewithout any step, meaning that the spinel and magneto-plumbite phase exchanged coupled with each other [27].The magnetization and demagnetization force have sin-gle ferrimagnetic property, because of the external mag-netic force and soft magnetic moments rotate alongwith the hard phase. The difference in the demagne-tization behaviour is the interplay of three types ofspin interactions in the composite particles, i.e. betweensoft/soft phase, hard/hard phase and hard/soft phases.The interfacial interaction between the hard and softphases should be dominant for exchange couple sys-tems. Also, the grain size of the soft phase should besmaller than the domain wall width of the hard phase[10,24]. The hard ferrite possesses a high magnetocrys-talline anisotropy energy compared to the soft ferrite.

In the samples prepared by physical mixing andone-pot method, exchange coupling interaction betweenhard (SrFe12O19) and soft (Zn0.4Co0.2Ni0.4Fe2O4) fer-rites can be helpful to both align the magnetization andarrange magnetic moments which are parallel to eachother. This effect can improve the magnetic propertiesand get higher saturation magnetization [7]. The mag-netic parameters of the composite particles includingsaturation magnetization Ms, remanent magnetizationMr, coercivity Hc and Mr/Ms ratio obtained from hys-teresis loops are shown in Table 3. The high Ms of one-pot composite particles is a consequence of interfacialcoupling arising from the alignment of more magneticmoments [27]. If there was no exchange coupling be-tween the two phases, the saturation magnetization ofthe composite would be:

Ms = Ms,hard fhard + Ms,so f t fso f t (2)

where, Ms,hard and Ms,so f t are the saturation magnetiza-tion of hard and soft phase and fhard and fso f t are theweight fraction of the hard and soft phase [16]. Due tothe fact that Ms of all composite particles are higherthan the calculated Ms from equation 2, consequently,in all composite particles prepared via one-pot sol-gelauto combustion and physical mixing methods, there isan exchange coupling between two phases.

Coercivities of all the composite particles are lowerthan those of the pure hard phase or soft phase for one-pot synthesized composite particles (Fig. 7). This meansthat by increasing the reverse field, the spinel domainwalls move towards the interface between the soft andhard phases and exceed into the hard phase and causemagnetization reverse to that phase. So, the coerciv-ity of the samples decreases in comparison to the hardphase region. At high values of the soft phase (60 and80 wt.%), the exchange force on the spinel is reducedand the interaction among soft moments becomes im-portant. This reduces the coercivity of composite [28].

Optimum exchange coupling is related to the grainsize of the composite particles [2]. Maximum exchangecoupling was reached when the critical length of soft

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(a) (b)

(c) (d)

Figure 7. Hysteresis loops of (SrFe12O19)1-x(Zn0.4Co0.2Ni0.4Fe2O4)x composite particles prepared by one-pot sol-gelauto-combustion route and physical mixing method with the various amount of soft phase in the composite

Table 3. Ms and Hc values of the (SrFe12O19)1-x(Zn0.4Co0.2Ni0.4Fe2O4)x composite particles prepared by one-pot andphysical mixing method

SampleOne-pot Physical mixing Theoretical Ms

Hc Ms MrMr/Ms

Hc Ms MrMr/Ms

without exchange[Oe] [emu/g] [emu/g] [Oe] [emu/g] [emu/g] coupling

Pure soft 64 67 7.5 0.11 – – – – –80% soft 50 84 7.2 0.08 60 70 7.2 0.10 6360% soft 43 71 3.7 0.05 576 69 20 0.28 5940% soft 54 91 5.5 0.06 90 72 5 0.07 5420% soft 350 60 9.6 0.16 708 58 17 0.29 50Pure hard 5818 46 26 0.57 – – – – –

phase was equal or less than twice of the hard phase do-main wall width [10]. The ratio Mr/Ms is reported in Ta-ble. 2 to determine the type of intergrain exchanges [29].For Mr/Ms = 0.5 no interaction between particles causecoherent rotations. On the other hand, for Mr/Ms < 0.5magnetostatic interactions cause multi-domain walls ex-istence. Finally, for Mr/Ms > 0.5 exchange coupling ispresent between hard and soft phases. Our results showthat the values of Mr/Ms of the composite particles arelower than 0.5, therefore, the particles interact magne-tostatically. Thus, the prepared composite particles are

good candidates for producing exchange-spring mag-nets.

IV. Conclusions

Different weight percentages of soft/hard phase(SrFe12O19)x(Zn0.4Co0.2Ni0.4Fe2O4)1-x composite par-ticles were prepared with one-pot sol-gel auto-combustion and physical mixing. The FTIR spectra andthe XRD patterns confirmed the formation of the desiredhard and soft phases. The FESEM micrographs showed

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that the hard phase featured a hexagonal platelet-likemorphology, while the soft phase was spherical withfaceted edges.

The hard/soft ferrite composite particles exhibitedgood exchange coupling between hard and soft ferritephases in the samples prepared by physical mixing andone-pot methods and possessed high saturation magne-tization. The coercivity of the samples is reduced by in-creasing soft phase amount, which is related to the dom-inance of dipolar interaction in soft phase over exchangeinteraction. Our results show that the values of Mr/Ms

of the composite particles are lower than 0.5, therefore;the particles interact magnetostatically. Thus, the pre-pared composite particles are good candidates for pro-ducing exchange-spring magnets.

Acknowledgement: The authors acknowledge the sup-port of Department of Chemistry, Science and ResearchBranch, Islamic Azad University of Tehran; and aregreatly thankful to Shadi Ghezelbash and Sahel Mes-daghi for collaborating in this work.

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