Preparation and magnetic characterization of the ferroxplana

ferrites Ba₃Co2-_xZn_xFe24O₄i

 

Xiaohui Wang*, Longtu Li, Zhenxing Yue, Shuiyuan Su, Zhilun Gui, Ji Zhou

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,

Tsinghua University, Beijing, 100084, China

Abstract

Ferroxplana ferrites Ba₃Co2__xZn_xFe24O₄₁ (X — 0 2 1_:0) were prepared by a citrate precursor method. X-ray diffraction analysis indicated that single Z-type phase was obtained at a relatively low temperature between 11501C and 12001C and no appreciable secondary phase was found. The substitution of Co with Zn caused an increase in the saturation magnetization and a decrease of the Curie temperature. The influences of Zn-substitution on the micro structure and high-frequency properties of the ceramics were investigated. The materials exhibited excellent high-frequency properties.

Keywords: Ferroxplana ferrite; High-frequency property; Zn-substitution; Citrate precursor method; Z-type phase

1. Introduction

Recently, more attention has been paid to the planar hexagonal ferrites including Ba3Co₂Fe₂4O41 (Co₂Z), Ba₂Co₂Fe₁₂O₂₂ (Co₂Y) and so on [1,2], which has a much higher permeability, high thermal stability (high T_c) and ferromagnetic resonance up to the GHz region compared to the 300 MHz ceiling encountered with the spinel ferrite [3]. These properties make the ferroxplana ferrites potentially ideal for use in high-frequency com­munications [4], especially as excellent soft mag­netic materials in producing chip inductors which are one of the most important surface mounting devices (SMDS) [5]. However, the phase formation

and sintering temperature of Z-type ferrite is higher than 13001C [6] on using a conventional ceramic route. How to decrease the Z-type phase formation and sintering temperature and how to get a larger initial permeability and low magnetic loss remains a problem till now.

Among various chemical powder synthesis techniques, sol-gel method was proposed and has been proven to be effective in reducing the sintering temperature and achieving good proper­ties, meanwhile it is also simple and effective in precisely controlling the composition homogeneity of the complex oxides. Using this method, ultrafine powders of Co₂Z with planar structure were obtained at about 11501C [7]. Nevertheless, there have been few studies regarding the structure and magnetic properties of Zn-substituted Co₂Z. Con­sidering the similar ion radii of Zn and Co, partial substitution of Co with Zn ion in Ba₃Co₂Fe₂₄O₄₁

hexaferrite is expected to enhance the magnetic properties, because the addition of Zn ion could improve molecular magnetic moment resulting in an increase in saturation magnetization [8].

Therefore, in an attempt either to promote the formation of Z-type ferrite at a lower temperature or to improve the magnetic properties, Zn-substituted Co₂Z hexaferrites synthesized by a novel sol-gel-citrate precursor method were in­vestigated for the first time. The influences of the substitution of Co with Zn on the phase forma­tion, micro structure as well as magnetic properties have been reported in this paper.

2. Experimental details

The polycrystalline powders of Ba₃Co₂__x-Zn_xFe₂₄O₄₁ have been prepared by a novel sol-gel method—citrate precursor method [7], where X = 0.00, 0.20, 0.40, 0.60, 0.80, and 1.0, respec­tively. All raw materials used were of AR grades, including iron citrate, cobalt acetate, barium acetate, zinc acetate and citric acid. Iron citrate was dissolved in an appropriate amount of citric acid aqueous at 701C before mixing with barium, cobalt and zinc acetate in stoichiometric quantities to give the required Ba₃Co₂__xZn_xFe₂₄O4i compo­sition. Then some ammonia solution was added into the above solution to adjust the pH value between 6 and 8. After that the transparent homogeneous sol was dried at 1351C for 6h, resulting in a dried gel-citrate precursor. After heat-treating the gel at temperatures between 11001C and 12501C for 6 h, Zn substituted Co₂Z powders in Z-type phase were obtained. The resulting powders were milled, dried and pressed. The pressed pellets (j = 10 mm, t= 1mm) and toroidal samples (j_outer = 20 mm, j_inner = 10 mm, t = 3 mm) were then sintered in air at various temperatures.

X-ray powder diffraction patterns of the pow­ders were recorded in the range of 2y = 20 2 801 with a scanning speed of 21/min on a Rigaku diffractometer using Cu K_a radiation. The morphologies of the powders as well as the microstructures of sintered samples were carried out on HITACHI S-450 SEM. Specific saturation

magnetization and coercive field strength of the powders were measured with a LDJ 9600 vibrat-ing-sample magnetometer. The complex perme­ability spectra of the ceramics were determined using a Hewlett-Packard HP4291B RF impedance analyzer from 1 MHz to 1.8 GHz at room tem­perature. The resistivity measurements were per­formed on a HP 4140B using silver contacts. The X-ray photoelectron spectroscopy (XPS) was carried on a Per kin Elmer PHI 5300 ESCA/610 SAM using a spherical capacitance analyzer (SCA). The elemental composition of the samples was measured on a Philips PW2400 sequential X-ray spectrometer fitted with a rhodium target end window X-ray tube.

3. Results and discussion

3.1. XRD analysis of powders

Because the ferroxplana compounds have such complex chemical composition, a deviation in either the stoichiometries or oxidation states of the components can have an adverse effect on their magnetic properties. The advantage of using the citrate precursor method is to optimize the composition homogeneity, which is particularly important for doping element. The citrate precursors were decomposed at 6001C for 4h first in order to remove organic matters, and then heat-treated at different temperature from 11001C to 12501C for 6h in air. As described in Ref. [7], Z-type phase did not seem easy to be formed directly by simple oxides, but gradually transited from simple oxides or spinels to a BaM and Co₂Y mixture, and finally to a pure Z-type phase. The relationships between the components of this system are notoriously complex and the precise mechanism is unknown for the formation of Z-type phase, although it has been suggested that it proceeds via a topotactic reaction between interfaces of the phases involved [9].

Fig. 1. XRD patterns of Ba₃Co2-^Zn^Fe24O4i  (x = 0_:2 and x = 1_:0) powders heat-treated at 12001C for 6h.

From the XRD data of the samples heat-treated at various temperatures, it is found that the formation temperatures of Z-type phase for Zn-substituted    Co₂Z    were    between    11501C and 12001C, and pure Z-type phases of Ba₃Co₂__xZn_xFe₂₄O₄₁ were obtained at 12001C. Fig. 1 shows the XRD patterns for Ba_3-Co₂__xZn_xFe₂₄O₄₁ (x = 0_:2 and 1.0) samples heat-treated at 12001C. The peaks indicated the formation of the pure Z-type phase according to JCPDS file 19-97, and no appreciable secondary phase was found. Unlike the Cu-substituted Co₂Z, whose Z-type phase formation temperature de­creases with increasing Cu content [10], the formation temperature of Z-type phase for all the Ba₃Co2__xZn_xFe24O₄₁ samples seemed insensi­tive to Zn content. It is also found that the stability domains of Z-type phase for all the Ba_3-Co₂__xZn_xFe₂₄O₄₁ samples are rather broad (1200-13001C), and there was no evidence of the Z-type phase decomposing to give W-type phase at temperatures up to 13001C.

3.2. XPS and XRF results

The XPS analysis of the Ba₃Co₂__xZn_xFe₂₄O₄₁ samples showed that the oxidation state of the surface iron to be Fe (III) with a binding energy of 709.6 eV for the main Fe 2p peak, and XRF element analysis for the oxides BaO, Fe₂O₃, ZnO and CoO confirmed their composition to be Ba₃Co₂__xZn_xFe₂₄O₄₁ as expected.

3.3.   Powder morphology

The morphologies of Ba₃Co₂__xZn_xFe₂₄O₄₁ powders were observed using SEM. All the samples show similar perfect hexagonal platelet grains. Fig. 2 gives the SEM micrographs of Zn-substituted Co₂Z powders. It is interesting that, at the same heat-treatment temperature of 12001C, the platelet grains of Zn-substituted Co₂Z powder were bigger than that of undoped one and seemed to grow up with the increase in Zn content. The average hexagonal grains were 3.1, 4.0 and 4.8 mm across the hexagonal plane for X = 0_:2_; 0.4 and 0.6, respectively.

3.4.   Magnetization measurement

Magnetization curves at room temperature of the powders were measured by VSM with a maximum field up to 20KOe. The specific satura­tion magnetization and coercive field strength of the powders are given in Table 1. The result indicates that all the samples were single Z-type phase without any second phase, showing typical characteristics of soft magnetic materials. When the specific saturation magnetization (s_s) is plotted against Zn content (X), it is obvious that the substitution of Co by Zn results in an increase of s_s (Fig. 3). This increase is due to the distribution of Zn among the tetrahedral sites of the structure as a non-magnetic atom instead of Co, which enhances molecular magnetic moment. The Curie temperature, determined by thermo-magnetic measurement, turned out to vary linearly with the composition from 683 K for undoped Co₂Z to 658 K for X = 1_:0 (as seen in Fig. 3). This means that Zn-substituted Co₂Z material still has a high thermal stability, despite a little drop of T_c compared to undoped Co₂Z.

3.5.   Frequency behavior of Ba₃Co₂-_xZn_xFe24O4j ceramics

The Ba₃Co₂__xZn_xFe₂₄O₄i hexaferrite prepared by the citrate precursor could be sintered to high density at a temperature of 12001C, about 1001C lower than pure Co₂Z prepared by a classic ceramic method   [11].   Fig. 4   shows   the   microstructure of Ba₃Co₂__xZn_xFe₂4O₄₁ (X = 0_; 1.0) hexaferrite ceramic sintered at 12001C for 4h. There were few pores on either surface or fracture of the sintered specimens. The ceramics show homogeneous hexagonal platelet grains (with average size of 8.0 and 10.0 mm across hexagonal plane for X = 0 and X = 1_:0_; respectively). The densities for the Ba₃Co2__xZn_xFe24O₄₁ ceramics sintered at 12001C for 6h reached maximum values of 5.15g/cm³ determined by the Archimedes method (more than 96% of theoretical density). Due to the higher density, electrical resistivity of the ceramic samples was measured to be above 1.7 x 10⁹Ocm.

The frequency variations of magnetic perme­ability m have been measured from 1 MHz to 1.8 GHz for the ceramics sintered at 12001C for 4h. As shown in Fig. 5, the Zn-substituted Co₂Z ceramics exhibit excellent magnetic behavior than undoped Co₂Z ceramic. With increasing Zn content, initial permeability has raised, respec­tively, from 9.5 for undoped Co₂Z to 14.8 for X = 1_:0_: Normally, the magnetizing mechanism of soft ferrite results from spin domain rotation and domain walls motion. The domain walls motion may be affected by the grain size and density, and enhanced by the increase of the grain size. From the microstructures for the ceramic samples of the composition x= 1_:0 (as shown in Fig. 4c), it is obvious that grain size grows up with Zn-substitution. Hence, the grain growth is the main reason which contributes to the increase in the initial permeability by enhancing the domain walls motion. On the other hand, this increase could also be partially attributed to the effect of the enhanced specific saturation magnetization due to the addition of Zinc as discussed above.

4. Conclusions

(1)  Ferroxpalna ferrite Ba3Co2__xZn_xFe24O4i has been   successfully   synthesized  by   a  citrate precursor method at a relatively low tempera­ture of 1150-12001C.

(2)  The substitution of Co with Zn ion in Co₂Z could increase the specific saturation magne­tization and consequently improve the mag­netic permeability of the ceramics.

(3)  Ba₃Co2__xZn_xFe24O₄₁    hexaferrite    ceramics with high density could be  achieved  after sintering at 12001C, showing good magnetic properties with high initial permeability of

14.8 (X = 1_:0) and high resistivity. This makes it a great potential as a soft magnetic material for high-frequency chip inductors.

Acknowledgements

This work was supported by the National Natural Science Foundation of P.R. China under Grant No. 59995523, and the High Technology and Development Project of P.R. China under Grant No. 2001AA325020

References

[1] J.J.  Went,  G.W.   Rathenau,  E.W.  Gorter,  G.W.  Van Oosterhaut, Philos. Tech. Rev. 13 (1952) 194.

[2] H. Jonker, H.P. Wijn, P.B. Braun, Philos. Tech. Rev. 18 (1956) 145.

[3] M. Fujimoto, J. Am. Ceram. Soc. 77 (1994) 2873.

[4] C.   Heck,  Magnetic  Materials  and  Their  Application, Butterworths, London, 1974, pp. 511-517.

[5] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 285.

[6] S. Nicolopoulos, M. Vallet-Regi, J.M. Gonzalez-calbet, Mater. Res. Bull. 25 (1990) 845.

[7] X. Wang, L. Li et al., Mater. Lett., 2002, in press.

[8] J. Smit, H.P.J. Wijin. Ferrites, Philips, Eindhoven, 1959 (Chapter XIII-XVI).

[9] S.I. Kuznetsova, E.P. Naiden, Stepanova, Inorg. Mater. 24 (1988) 856.

[10] X. Wang, T. Ling Ren, L. Li, et al., J. Magn. Magn. Mater. 234 (2001) 255.

[11] P. Allegri, D. Autissier, T. Taffary, Key Eng. Mater. 132-136 (1997) 1424.