IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013 4281

Synthesis and Magnetic Properties of Non-Stoichiometric HexaferriteLijun Jia, Huaiwu Zhang, Lei Xu, Fiming Bai, and Baoyuan Liu

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technologyof China, Chengdu, 610054, China

A several of non-stoichiometric hexaferrites with composition of have beensynthesized. Their structural andmagnetic properties have been characterized. It is found that the increase of amount is beneficialto the formation of Z-type phase and the reduction of sintering temperature. Excess ions distributed in the large interspace alonga-axis orientation enhance the grain growth and densification due to the activation of lattice, which in turn first lead to an increaseand then a decrease of saturation magnetization. Meanwhile, owing to the compact and uniform microstructures, and high sinteringdensity, relatively high initial permeability was obtained in the samples with proper amount while magnetic loss no significantchange. On the contrary, when , the Z-type crystal structure of becomes unstable because of deficiency. Theimpurity phase was detected in these samples. Saturation magnetization decreases and coercive force increase with deficiency.Consequently, the initial permeability decreases.

Index Terms—Hexaferrites, magnetic properties, microstructure.

I. INTRODUCTION

T HE development of mobile communication technologyhas fueled the trend of antenna used in personal mobile

terminal to high frequency and miniaturization. type hex-aferrite materials have been proposed as inductive loading inbuilt-in antenna due to their higher initial permeability, ferro-magnetic resonance in the GHz region, and proper dielectricconstant, all of which can be tailored by various doping strate-gies [1], [2]. To reduce the magnetic and dielectric losses ofhexaferrites, the sintering process usually has been at lower sin-tering temperatures below 1200 . But the densification andinitial permeability of these samples are not ideal. Zhao et al. re-ported that the excess barium cations can improve the magneticproperties of M-type hexaferrites [3]. For this, the effects of ex-cess and deficiency of ions on the densification, phasecomposition, microstructures and magnetic properties ofhexaferrites have been studied respectively in this paper.

II. EXPERIMENTAL PROCEDURE

A. Sample Preparation

A several of non-stoichiometric hexaferrites with com-position of ( , 0.15, 0.1,0.05, 0, 0.05, 0.10, 0.15, 0.20) were synthesized by a solid-

state reaction method. High purity , , and ,were used as raw materials. Firstly, the mixture of raw mate-rials was planetary ball-mixed for 6 h, and calcined in the airat 1150 for 2 h. Then the powders were ground in the plan-etary ball mill for 12 h to obtain a fine grain size of about 0.8. In addition, 8 wt% polyvinyl alcohol was added to press

into toroidal and disc-shaped samples. Finally, the green bodies

Manuscript received November 12, 2012; revised January 07, 2013; acceptedJanuary 29, 2013. Date of current version July 15, 2013. Corresponding author:L. Jia (e-mail: jlj@uestc.edu.cn).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2245314

Fig. 1. XRD spectra of with different values: (a); (b) ; (c) ; (d) ; (e) .

were sintered at 1200 for 3 h in the air and then cooled in thefurnace.

B. Characterization

The microstructures of the sintered samples were studied byscanning electron microscopy (SEM: JOEL JSM-6490LV) andx-ray diffraction (XRD: Philips X’Pert) with Cu Ka radiation.The magnetic properties were measured by a vibrating samplemagnetometer (IWATSHBHV525). The permeability spectrumwas measured by an HP4291B impedance analyzer. The bulkdensity was determined by the Archimedes method.

III. RESULTS AND DISCUSSION

The XRD patterns of non-stoichiometric ferrite samples areshown in Fig. 1. It can be seen that all the samples are mainlyof Z-type hexaferrite phase.The hexagonal crystal lattice in the samples with excess

ions appears to be distorted possibly because the larger cation

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4282 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

Fig. 2. Dependence of lattice parameters on amount.

Fig. 3. SEM images of the samples with different amount: (a); (b) ; (c) ; (d) .

size of enter into ferrite lattice, as seen in Fig. 2. Further-more, it is notable that the lattice parameter a monotonicallyincreases while the lattice parameter c keeps almost constant asthe x increases in the range of 0 to 0.10.The unit cell of Z-type hexagonal ferrite consists of 44 atomic

layers which pile up to the c-axis. This structure may be de-scribed as a stack of [4]. Generally, the cations aredistributed among various octahedral, tetrahedral, and pseudo-tetrahedral sites by close packing of oxygen anions and bariumcations. If excess ions with large radius enter into thecationic sites, the lattice parameters will increase with x by thespace constraints of crystal structure [5]. So, excess ionsprobably enter into the large interspace along a-axis orientationnear spinel block. This explains well the dependence of latticeparameters on amount.TheXRD analysis also indicates that the proper amount

is beneficial to the formation of Z-type hexaferrite phase. When, the intensity of peak (1016) weakened. And the co-

existence of a small amount of Y-phase with Z-phase was de-tected. Obviously, the Z-type crystal structure ofbecomes unstable because of deficiency. Since ionsare mainly on the outside of or layers, the parameters de-crease slightly as amount decreases.

Fig. 4. Dependence of Ms and Hc on amount.

Fig. 5. Dependence of sintering density and initial permeability onamount.

Fig. 3 shows the SEM images of the samples with differentamount. It can be seen that the average grain size increases

with amount. Namely, excess ions distributed inthe large interspace along a-axis orientation enhance the graingrowth and densification due to the activation of lattice.Fig. 4 gives the change of saturation magnetization (Ms) and

coercive force (Hc) with different amounts. Upon furtherincreasing , Ms reaches a peak value, and then it decreaseswhen amount reaches 0.15 due to the existence of impu-rity phase. When , the grain growth of Z-type hexafer-rite phase is restrained due to existence of secondary phase. Onthe other hand, the saturation magnetization of impurity phase(Y-phase) is lower than that of pure Z-phase markedly. As a re-sult, Ms decreases and Hc increases with deficiency. It isknown that the initial permeability is proportional to Ms. Con-sequently, the initial permeability decreases.The bulk density of Z-type barium ferrite increases with the

increase of amount. It can be seen that excess ionsenhance the grain growth and the sintering process, which inturn lead to an obvious increase of the bulk density and initialpermeability (as shown in Fig. 5). The compact and uniform mi-crostructures, and high sintering density, relatively high initial

JIA et al.: SYNTHESIS AND MAGNETIC PROPERTIES OF NON-STOICHIOMETRIC HEXAFERRITE 4283

permeability was obtained in the samples with excess ionswhile magnetic loss no significant change.

IV. CONCLUSION

Non-stoichiometric hexaferrites have been synthesizedby conventional ceramic process. Our results show that theproper excess of ions is beneficial to improve the initialpermeability while the magnetic loss no significant change. Thedistribution of excess ions in the large interspace alonga-axis orientation can enhance the bulk density and the initialpermeability by increasing the grain growth of hexaferrite andthe diffusion of ions in the sintering process due to the acti-vation of lattice, which is accompanied by the distribution ofexcess ions in the ferrites. On the contrary, when ,Z-type crystal structure of becomes unstablebecause of deficiency and the impurity phase has beenfound. As a result, Ms decreases and Hc increases withdeficiency. Consequently, the initial permeability decreases.

ACKNOWLEDGMENT

This work was supported in part by the National Fund ofChina under Grant 50872017, the Foundation for InnovativeResearch Groups of NSFC under Grant 60721001, the In-ternational S&T Cooperation Program of China under Grant2007DFR10250, 2012DFR10730 and the National Basic Re-search Program of China under Grant 2007CB31407.

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