JOURNAL OF APPLIED PHYSICS 107, 09A507 共2010兲
Study on magnetic properties of low temperature sintering M-Barium hexaferrites Yingli Liu,a兲 Yuanxun Li, Huaiwu Zhang, DaMing Chen, and Qiye Wen State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
共Presented 19 January 2010; received 6 November 2009; accepted 31 December 2009; published online 19 April 2010兲 The effects of Zn2+ and Ti4+ substitutions on the microstructure and properties of low temperature sintered M-type barium hexaferrites Ba共ZnTi兲xFe12−2xO19 have been studied in order to adapt the development of low temperature cofired ferrites technology and produce circulators with a multilayer process. It is found that part of Zn2+ ions can enter into 2b sublattice and the saturation magnetization of the samples decrease when x increases. The additive of 3–5 wt % Bi2O3 · B2O3 · SiO2 glass lowers the sintering temperature to about 900 ° C, which is ideal for cofiring with silver paste. Scanning electron microscope and x-ray diffraction analysis show that the samples have excellent crystalline grains with a uniform size about 1.0 m. A high density of 4.85 g / cm3 is obtained in the samples sintered at 900 ° C with 5 wt % glass additive. Magnetic measurements show that the saturation magnetization reaches 63.5 emu/g 共about 308 kA/m兲 at 900 ° C and increases as the sintering temperature arises. © 2010 American Institute of Physics. 关doi:10.1063/1.3338846兴 I. INTRODUCTION
It is well known that low temperature cofired ceramics and low temperature cofired ferrites 共LTCF兲 technologies are based on different kinds of low temperature 共⬍950 ° C兲 sintered ceramics and ferrites. For ferrites, the typical materials are NiCuZn and MgCuZn spinel ferrites, which are applied in multilayer chip inductors, beads, and Electro-Magnetic Interference components due to the good soft magnetic performance in high frequency.1–3 Aiming at increasing the working frequency, low temperature sintered Z-type Ba ferrites 共Co2Z兲 have also been studied and are desired to find the same applications such as spinel in radio frequency.4–6 In recent years, a lot of research is focusing on surface mounting devices circulators/isolators fabricated by LTCF process.7–9 Low temperature sintered M-Ba 共M-barium兲 hexaferrite is an excellent candidate for LTCF-technologybased devices due to its high intrinsic magnetic anisotropy field, proper saturation magnetization and dielectric constant, and relative lower dielectric loss, all of which can be adjusted by ion substitution. In this paper, we have studied the effects of different additives and process of powder preparation on the sintering temperature, microstructure, and magnetic properties of M-Ba ferrite. II. EXPERIMENTS
M-barium hexaferrite powder with compositions of Ba共ZnTi兲xFe12−2xO19 共x = 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2兲 is synthesized by traditional ceramic process. High purity BaCO3, Fe2O3, ZnO, and TiO2 are used as raw materials. First, the mixture of raw materials is ball-milled for 8 h and a兲
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calcinated at 1200 ° C in air for 2 h. The powder is then grounded in the ball mill for 6 h and high energy ball-milled for 1 h to obtain a fine grain size of 0.6– 0.8 m. 8 wt % polyvinyl alcohol is added to press into toroidal and rodshaped samples. The green sample is sintered at 1150 ° C to verify the sintering behavior of fine powder without glass additive. Then various amounts of Bi2O3 · B2O3 · SiO2 glass additive 共1 – 5 wt %兲 are added to Ba共ZnTi兲Fe12O19 using the same process above. The pressed green samples are sintered at various temperatures from 880 to 940 ° C and held for 2 h. The microstructures of sintered samples are studied by scanning electron microscope 共SEM, Quanta 200, Philips, Netherlands兲 and x-ray diffraction 共XRD, Bede TM 2000兲 with a Cu K␣ radiation. The basic magnetic properties are measured by vibrating sample magnetometer 共BHV 525, IWATSH, Japan兲 and HP4291B impedance/materials analyzer. Moreover, the bulk densities are measured by Archimedes method. III. RESULTS AND DISCUSSION
The XRD results of 1150 ° C sintered samples with different Zn2+ and Ti4+ substitutions are shown in Fig. 1. It can be seen that the peaks of M phase become shaper and stronger with increasing substitution amount, which indicates that Zn2+ and Ti4+ ions are beneficial to forming M-type barium ferrite 共M-Ba兲 phase in a relative lower temperature. Figure 2 illustrates the tendency of saturation magnetiand Curie temperature 共TC兲 of zation 共M S兲 Ba共ZnTi兲xFe12−2xO19 versus the amount of Zn2+ and Ti4+ substitution x. It can be found that both M S and TC decrease sharply with increasing x. In previous work, the nonmagnetic ion Ti4+ is considered to enter octahedral sublattices 12k共R-S, up兲, 2a共S, up兲, 4f2共R, down兲, and the nonmagnetic
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FIG. 1. Ba共ZnTi兲xFe12−2xO19 XRD curve vs x.
ion Zn2+ has the strong tendency to occupy the tetrahedral sublattices 4f1共S, down兲 to substitute Fe3+ ions. However, here we note that it is also possible that Zn2+ ions enter into 2b sublattice 共hexafedral, R, up兲 to substitute Fe3+, which results in the decrease in the difference between spin up and spin down magnetic moments. This can explain the decrease in M S with x. In the same time, the decrease in Curie temperature with increasing amount of Zn2+ and Ti4+ ions is related to the weakening of the superexchange interactions between Fe3+ ions.10 Figure 3 shows the SEM images of the sample sintered at 1150 ° C without any glass additive and the sample sintered at 900 ° C with 5 wt % glass additive, respectively. It can be seen from Fig. 3共a兲 that the growth of grain is not complete and the densification degree is not sufficient to lower the sintering temperature to 900 ° C. From Fig. 3共b兲, the grain size increases to 1 m with the density of the sample reaching 4.85 g / cm3, which can be attributed to the effects of glass. However, grain growth will be hindered by the glass additive with a relative large weight percents because the excessive glass additive can act as a flux wrapping
around the grains of sample and prevent the M-Ba ferrite interparticles from diffusing and transporting. Figure 4 gives the changing law of M S with different amounts of glass additive. It can be found that with increasing the amount of glass, M s increases first, and then M S reaches a peak value. Finally it decreases dramatically when the amount reaches 5 wt %. Apparently, the reason is that the proper amount of additive has the benefit of increasing the density due to liquid phase sintering mechanism, which will lead to the enhancement of the magnetic moment per unit volume. With further increasing x, the nonmagnetic additive will degrade the basic magnetic properties of low temperature sintered hexaferrites. The average bulk shrinkage of the samples is about 11%. There is a large difference in shrinkage between the ferrite and Ag electrode paste 共⬃15%兲. The possible reason is that the densification of the sample in not sufficient since small grain size leads to more boundaries and porosities in the samples. Finally, the density of the sample can be further improved with increasing sintering temperature, as confirmed in Fig. 5. IV. CONCLUSIONS
Single phase M-Ba hexaferrites are successfully synthesized by traditional ceramic process. Our results show that
FIG. 2. 共Color online兲 The dependence of M s and Tc on the substitutions amount.
FIG. 3. SEM image of 共a兲 1150 ° C sintered sample without glass additive and 共b兲 900 ° C sintered sample with 5 wt % glass additive.
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FIG. 4. M s vs amount of glass additive.
Zn2+ and Ti4+ substitutions have the advantage of improving the magnetic performance and adjusting the saturation magnetization of M-Ba. It is suggested that Zn2+ ions may enter into 2b sublattice to substitute Fe3+ ions and cause the decrease in M s. Fine particles of M-Ba mixed with 5 wt % glass additive can realize low temperature sintering 共900 ° C兲 with the basic material properties of M S = 308 kA/ m, HC = 18 kA/ m, and 4.85 g / cm3 density, all of which make the material potential for applications such as self-bias multilayer chip circulators/isolators and microwave passive integrated. Restricted by the measurement conditions, microwave properties such as ferromagnetic resonance line width, dielectric properties, etc. have not been studied yet, which need further research in future works. ACKNOWLEDGMENTS
This work was supported by the Foundation for Innovative Research Groups of the NSFC under Grant No.
FIG. 5. Density vs amount of glass additive.
60721001, the Youth Fund of Sichuan Province under Grant No. 08ZQ026-013, and the New Century Talent Project 2007 of Education Ministry. 1
D. N. Bhosale, V. Y. Patilb, K. S. Rane, R. R. Mahajan, P. P. Bakare, and S. R. Sawant, Thermochim. Acta 316, 159 共1998兲. 2 Y.-R. Wang and S.-F. Wang, Int. J. Inorg. Mater. 3, 1189 共2001兲. 3 S. Wang, H. Zhou, and L. Luo, Mater. Res. Bull. 38, 1367 共2003兲. 4 S. H. Gee, Y. K. Hong, I. T. Nam, C. Weatherspoon, A. Lyle, and J. C. Sur, IEEE Trans. Magn. 42, 2843 共2006兲. 5 H. Zhang, J. Zhou, Y. Wang, L. Li, Z. Yue, and Z. Gui, IEEE Trans. Magn. 38, 1797 共2002兲. 6 X. Wang, L. Li, S. Shu, Z. Yue, Z. Ma, and J. Zhou, Mater. Lett. 55, 8 共2002兲. 7 W. Che, X. J. Ji, and E. K. N. Yung, Int. J. RF Microwave Comput.-Aided Eng. 18, 8 共2007兲. 8 X. Zuo, H. How, S. Somu, and C. Vittoria, IEEE Trans. Magn. 39, 3160 共2003兲. 9 P. Shi, H. How, X. Zuo, S. D. Yoon, and S. A. Oliver, IEEE Trans. Magn. 37, 2389 共2001兲. 10 Q. Feng and L. Jen, IEEE Trans. Magn. 38, 1391 共2002兲.
