Investigation on physical characteristics of novel Z-type Ba₃Co_2(o.8-x)Cu_o.4oZn₂xFe₂₄0₄i hexaferrite

 

Hongguo Zhang*, Ji Zhou, Yongli Wang, Longtu Li, Zhenxing Yue,

Xiaohui Wang, Zhilun Gui

 

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Abstract

The influence of Zn incorporation into Cu-modified Co₂Z (Ba₃Co₂Fe₂4O41) ferrite on physical characteristics of low-sintered materials, which have stoichiometric compositions of Ba₃Co₂(₀.8 - x)Cu₀.4Zn_2xFe₂₄O41 (x V 0.50), is investigated. The expe­rimental results show that Zn has little effect on the morphology of Cu-modified ferrites. The average 10-15 Am of lathy grain size for Ba₃Co₂(₀.8 - x)Cu0. ₄Zn_2xFe₂₄O41 is just like that of Ba₃Co₂₍₁ -_x)Cu₀.4Fe₂₄O4i. In the range of solid solubility of Zn, the lattice parameters (a and c) of ferrites with Z-type hexagonal phase structure remain stable, compared with the sole Cu-modified ferrite. The variation of key magnetic properties, such as saturation magnetization, remnant magnetization, coercive force, magnetic hysteresis loop and initial permeability, magnetic loss, dielectric constant and Curie temperature of the ferrites as well as the variation mechanism are also addressed in this paper.

Keywords: Z-type hexaferrite; Zn incorporation; Morphology; Magnetic properties

1. Introduction

Ferrites are the main materials to produce multi­layer chip inductors (MLCI) that are the most impor­tant passive surfaces mounting devices (SMD) in modern electronic industry [1]. However, the tradi­tional ferrites are far from meeting the requirements of high performance, miniaturization size, light weight and low cost. Recently, there has been a growing in-

terest in Co2Z ferrite (Ba₃Co2Fe24O41), which is con­sidered an ideal candidate for MLCI, because Co2Z has high performance (high initial permeability, high quality factor, high resonant frequency and low loss tangent) in hyper-frequencies of 300-1000 MHz [2,3]. Especially, its high cut-off frequencies up to the 3-GHz region, compared with the 300-MHz ceiling encoun­tered with the spinel ferrites, bring it into the hyper-frequency region useful for chip inductors or beads.

Traditionally, the formation of Ba₃Co2Fe24O41 in­volves high temperature firing at more than 1270 jC. However, MLCI fabrication requires ferrites that are sintered at arelatively low temperature of 850-900 jC. From the crystallographic point of view, Co2Z is among the most complex compound in the family of

ferrites with planar hexagonal structure. The hexagonal cell parameters, planar a and axial c, are 0.59 and 5.23 nm, respectively [4]. The complexity of the structure mainly results from large Ba^{2 +}. Since the ionic radius of Ba^{2 +} is comparable to that of O² , these ions prefer the oxygen positions rather than the interstitial sites. Metal ions (Fe^{3 +}, Fe^{2 +}, Co^{2 +}, Cu^{2 +} and Zn^{2 +}), how­ever, are located in non-equivalent interstitial sites. The presence of ‘^‘strongly anisotropic^’’ Co^{2 +} ions leads to a higher complexity of the magnetic properties of the material [5].

However, Cu²⁺ have been successfully incorpo­rated in Ba₃Co2Fe24O41 and the sintering temperature of Ba₃Co1.₆Cu0.4Fe24O41 lowered to 1050-1130 jC [6]. In order to improve the initial permeability of the Cu-modified Co2Z, Zn was also considered to incor­porate, just like Ni-Zn-Cu ferrites. By this way, one kind of novel compound Ba₃Co2(1 _ _x _ y)Cu2yZn2xFe O41, with low melting point and high performance should be expected to achieve. To facilitate the study, the optimal Cu content was fixed (y = 0.20) in this pa­per. In terms of soft magnetic and planar anisotropy, the amount of Zn (2x) is less than 0.80, according to Jonker et al.'s research work [2].

2. Experimental procedure

The stoichiometric compositions of Ba₃Co2(0.8 -_y) Cu0.4Zn2_xFe24O41 were prepared by solid-state reac­tion method. The raw materials, BaCO₃, Co₃O4, CuO, ZnO and Fe2O₃, of high purity, were mixed in a ball mill for 4 h. The mixed powders were calcined at 1000-1050 jC/6 h in air. The resulting powders were pressed in a stainless-steel die under a pressure of 40000-60000 N/m² with 5 wt.% PVA as lubricant. All pellet and toroidal samples were sintered at 1100— 1150 jC/4 h in air and cooled in the furnace. The pressed pellets (10-mm diameter, 0.5-1.5-mm thick­ness) were used to measure the electrical properties and toroidal samples (20-mm outside diameter, 10-mm inside diameter, about 3-mm thickness) were used to obtain the initial permeability.

SEM was used to observe the micro structure of the fracture surface of the samples. The phase characteri­zation was analyzed using X-ray diffractometry (XRD). Data collection was carried out using FeKa radiation at 35 kV and 25 mA.

DC resistivity and dielectric constant of pellet sam­ples (Ag-Pd contact coated onto both sides of pellet samples) were measured at room temperature using HP4140 and HP4194A analyzers, respectively. HP4194A impedance analyzer from 1 to 1000 MHz was also used to determine the variation of initial per­meability of different toroidal samples. The magnetic parameters of the corresponding ВазСо2(о.8-у)Сио.4 Zn_2xFe24O41 powders were recorded on a LDJ9600 vibrating sample magnetometer.

3. Results and discussion

3.1.  Phase formation and cell parameters

As can be seen in Fig. 1, XRD characterization shows typical Z-type hexagonal phase structure of the low-sintered ferrites having Ba₃Co2(0.8 - x)Cu0.4Zn_2x Fe24O41 stoichiometry, sometimes accompanied by some traces of Y-type phase (Ba2Co2Fe12O22 or Ba2Zn2Fe12O22). With the increase of dwelling time, the probability of Y-type phase formation increases. The variation of crystalline cell parameters calculated by XRD program is plotted in Fig. 2. Compared with the sole Cu-modified ferrite, the cell parameters basi­cally remain stable in planar parameter (a f 0.582 nm) and axial parameter (c f 5.236 nm) in the range of Zn solubility (x V 0.40). However, due to the amount and incorporation of different modifying elements, the tem­perature range of Z-type hexagonal phase becomes more and more narrow.

3.2.  Grain morphology

Unlike Cu-modified ferrites, a compact and homo­geneous micro structure of Ba₃Co2(o.8-x)Cuo.4Zn_2x Fe24O41 can only be obtained in the narrow temperature range of 1100-1130 jC (Fig. 3). This may be mainly attributed to the effect of Zn incorporation, resulting in more severe conditions of the formation of the new solid solution compound, which has more complicated element distribution and mass transport process. At low sintering temperature, some difficulties are encoun­tered to form Z-type phase, but higher sintering temper­ature easily results in abnormal grain growth and intergranular pores, which deteriorate the properties, as seen in Fig. 3e. With the increase of Zn content, the lathy grains have no great change in morphology. However, with the increase of dwelling time, the grains obviously grow in hexagonal plane and gradually change from lathy to platelet in morphology (Fig. 3d). Compare to pure Co2Z, the 10-15 Am of average grain size for new compound indicates that grain growth is effectively suppressed by the existence of liquid Cu. Furthermore, the melting point of CuO is 1026 jC, i.e. lower than the firing temperature, and appropriate segregation and volatilization of Cu may occur, which not only improves grain morphology and distribution of inner pores [7], but also inhibits the generation of Fe²⁺ (not detected by wet chemical analysis).

3.3. Magnetic characterization

Fig. 4 shows the frequency dependence of the initial permeability and quality factor for Zn-modified sam­ples. It can be seen that the initial permeability of about 10.5 is obviously higher than 9.0 of sole Cu-modified ferrites, and the quality factor very near to that of Cu-modified ferrites. However, the cut-off frequency is also much higher than that of pure Co2Z [8], especially in hyper-frequencies, and is not lower than that of Cu-modified ferrites.

Fig. 5 shows that the shape of the magnetic hyste­resis loop of different modified samples of Zn-modi-fied low-sintered ferrite (x = 0.20, 1120 jC/4 h) is

typical of a soft magnetic material, just like that of sole Cu-modified ferrites. Furthermore, the value of saturation magnetization (r_s) is also higher and the magnetic loss (the area of hysteresis loops) is lower than those of Cu-modified ferrites. Table 1 lists other magnetic parameters of Zn-modified low-sintered fer­rite.

3.4. Discussion

Just like spinel ferrites, the magnetism of polycrys-talline ferrites mainly results from the combination of spin rotation and domain walls motion and is deter­mined by the grain size, densification, saturation mag­netization, internal stress, etc. However, suppressed grain growth for low-sintered ferrites, which com­monly takes place in isotropic ferrites, has great effect on the initial permeability. Furthermore, the increase of magnetization should be the main reason for the initial permeability increase. This can be implied from the fact that grain growth proceeds in two-dimensional manner, thus preferring in the plane perpendicular to the c-axis because the total number of Co^{2 +} ions is higher than that of Zn^{2 +} + Cu^{2 +} (Fig. 3). As regard to the process of atom transportation and variation of crystalline structure, it has not been perfectly known by now [9].

In Z-type ferrites, metal ions (Fe^{3 +}, Co^{2 +}, Cu^{2 +} and Zn²+) are located in non-equivalent interstitial sites: octahedral and tetrahedral. Especially, Co^{2 +} belongs to the strongly magnetocrystalline anisotropic ele­ment, and its partial replacement by Zn^{2 +} and Cu^{2 +} leads to more complex magnetic properties [5]. The ionic radius, coordination, magnetic moment and align­ment of metal ions in Z-type ferrite are reported in Table 2.

When Zn^{2 +} ions enter the lattice interstitial, Fe-Fe strong interactions that dominate in saturation magnet­ization are partially replaced by Zn-Fe antiferromag-netic interactions. Due to the isotropic arrangement of Fe-Fe strong interactions, the Zn-Fe antiferromag-netic interactions will counteract some of Fe-Fe strong interactions in a certain range of solubility, and consequently, the anisotropy of the ferrites in­creases, then the magnetization increases. This phe­nomenon is in accordance with Jonker et al.'s report [2]. Moreover, Zn^{2 +} specially prefers A sites and this will modify the original distribution of the Fe^{3 +} or Cu²⁺ on two interstatial sites (A site: octahedral int­erstice; B site: tetrahedral interstice). Due to electronic configuration, Zn^{2 +} will distort the lattice or crystal­line field, and then generate an internal stress. As a result, the magnetization decreases.

It can also be seen in Table 1 that T_c greatly decreases with the increase of Zn in Ba₃Co2(0.8 - _x) Cu0.4Zn2xFe24O41. As stated in Ref. [2], T_c is deter­mined by the strong magnetic interactions among various ions. Due to the incorporation of Zn^{2 +}, the number of strong magnetocrystalline anisotropic Co^{2 +} progressively decreases, the anisotropy of Z-type fer-rite becomes weak and the number of Fe-Fe or Co-Fe strong magnetic interactions decreases, and all these can result in the decrease of enduring ability of thermal vibration for the ferrites. Consequently, T_c decreases. As regard to other magnetic parameters, such as rem­nant magnetization and coercive force, their variations are also listed in Table 1.

The DC resistivity arrives at more than 3.20 X 10⁷ П cm and dielectric constant is measured to be about 23 at 400 MHz (Table 1). As to the variation of DC resis­tivity and dielectric constant, it is mainly correlated to the variation of the number of different Fe ions in the two interstatial sites. Zn^{2 +} ions specially prefer A site, and this will increase the hopping probability between Fe^{3 +} and Fe^{2 +} ions, and then the DC resistivity decrea­ses and dielectric constant increases [10,11].

4. Conclusions

The present study shows that the incorporation of Zn into Ba₃Co1. 6Cu0.4Fe24O41 decreases the tempera-

ture range of the Z-type phase solid solution between 1100 and 1130 jC and has no obvious effect on the microstructure, as well as on cell parameters. How­ever, the dwelling time is a key factor that affects grain growth.

The incorporation of Zn improves the magnetic properties of Ba₃Co2(1 -^СигуРег^^ ferrites as fol­lows: initial permeability of about 10.5, quality factor of average 30, cut-off frequency above 1 GHz , resis­tivity of about 3.20 X 10⁷ V cm and dielectric constant of about 23.

This kind of Zn-modified Z-type ferrites which offer a good compromise of magnetic properties can be synthesized at temperature of 1100-1130 jC and should be expected to meet the requirements for low temperature sintering of MLCI with the aid of proper additives, such as Bi2O3, V2O5 and so on.

Acknowledgements

This work obtained the financial support from the High Technology and Development Project of the P.R. China (Grant No. 715-Z33-006-0050), National Nat­ural Science Foundation of P.R. China (Grant: 59995523), and the Ceramic Technology Center, Mo­torola, USA.

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