Studies of static and high-frequency magnetic properties for M-type ferrite BaFe 12À2x Co x Zr x O 19

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 92, NUMBER 7 1 OCTOBER 2002 Studies of static and high-frequency magnetic properties for M-type ferrite BaFe 12À2x Co x Zr x O 19 Z. W. Li a) and Linfeng Chen Temasek Laboratories, The National University of Singapore, 10 Kent Ridge Crescent, Singapore , Singapore C. K. Ong Centre for Superconducting and Magnetic Materials and Department of Physics, The National University of Singapore, 10 Kent Ridge Crescent, Singapore , Singapore Received 9 April 2002; accepted for publication 17 July 2002 Static and high-frequency magnetic properties have been studied for BaFe 12 2x Co x Zr x O 19 with x The x-ray diffraction and M H curves of aligned samples show that Co Zr substitution can modify the anisotropy of BaFe 12 2x Co x Zr x O 19. BaFe 12 2x Co x Zr x O 19 with x 0.6 has an easy c-axis anisotropy and the anisotropy fields rapidly decrease with Co Zr substitutions. The sample of x 1.2 has an easy c-plane anisotropy with H a 5 koe. The saturation magnetizations of BaFe 12 2x Co x Zr x O 19 are almost constant at x 0.4 and decrease with higher substitutions. The coercivity decreases rapidly from 2400 Oe for the sample of x 0 to about 10 Oe for the sample of x 0.8. The natural resonance frequencies are observed at 4.5 and 5.0 GHz for the samples of x 0.8 and x 1.2, respectively. Based on microwave measurement and theoretical estimations on reflectivity, BaFe 12 2x Co x Zr x O 19 may be a good candidate for electromagnetic compatibility and other practical applications at high frequency American Institute of Physics. DOI: / I. INTRODUCTION As the running speeds of electronic circuits are coming into microwave frequency range, the problems of electromagnetic radiation and electromagnetic interference are becoming more and more serious in the development of highspeed electronic circuits. Special electromagnetic materials are often used in high-speed electronic circuits to reduce the electromagnetic radiations, decrease the noise level of signals, and ensure the electromagnetic compatibility. Such electromagnetic materials generally fall into two categories: dielectric composites with conductive fillers and magnetic composites. As far as thickness and working frequency bandwidth are concerned, magnetic composites have obvious advantages. The magnetic fillers often used in such composites are ferrite materials, such as spinel ferrites 1,2 and hexaferrites. As compared to the usual spinel ferrites, the hexaferrites with a planar magnetic anisotropy are of great interest for use as electromagnetic energy dissipation materials in GHz range. Many works about Co 2 Z ferrite, Ba 3 Co 2 Fe 24 O 41, have been reported. 3 8 The ferrite is a soft material with a planar anisotropy; it has a relatively high resonant frequency and high permeability. On the other hand, in the hexaferrite family, M-type ferrite BaFe 12 O 19 has a high saturation magnetization and high Curie temperature as compared to the Co 2 Z ferrite. Unfortunately, its strong uniaxial anisotropy leads to low permeability and too high resonant frequency f GHz. 9 However, these drawbacks will be improved if we are able to decrease the anisotropy field or even modify a Electronic mail: tsllizw@nus.edu.sg the anisotropy from uniaxial to planar. Kreisel et al. 10 have confirmed that BaFe 12 2x Co x Ti x O 19 has a planar anisotropy when Co Ti substitutions are larger than 1.1. Brando et al. 11 and Cho et al. 12 indicated that Co Ru and Co Ir substitutions can lead to a planar anisotropy at substitution x 0.5. In this article, substituted Co Zr M-type ferrites, BaFe 12 2x Co x Zr x O 19 with x 0 1.2, have been prepared. The static and high-frequency magnetic properties have been measured. The predicted absorption and reflection loss demonstrates that BaFe 12 2x Co x Zr x O 19 may be a good candidate for electromagnetic materials with low reflectivity at microwave frequency. II. EXPERIMENT Samples of BaFe 12 2x Co x Zr x O 19 with x 0, 0.2, 0.4, 0.6, 0.8, and 1.2 were synthesized using conventional ceramic techniques. A mixture of BaCO 3,Fe 2 O 3,Co 3 O 4 and ZrO 2 in the ratio required for the BaM ferrite was calcined at 1300 C for 3 h. The calcined samples were then crushed and ball milled. Finally, the powders were shaped and sintered in O 2 at 1200 C for 6 h. Aligned samples were prepared by mixing the fine powders of BaFe 12 2x Co x Zr x O 19 with epoxy resin and then placing the mixture in an applied field of 4 8 koe for x-ray diffraction and magnetic measurement. The composite samples were prepared by mixing the fine powders of BaFe 12 2x Co x Zr x O 19 with epoxy resin; the ratio of the powders to the resin is 35% by volume. Then, they were fabricated into cylinders with an outer diameter of 6.9 mm, inner diameter of 3 mm, and length of 8 mm for microwave measurement /2002/92(7)/3902/6/$ American Institute of Physics

2 J. Appl. Phys., Vol. 92, No. 7, 1 October 2002 Li, Chen, and Ong 3903 The x-ray diffraction XRD of BaFe 12 2x Co x Zr x O 19 was performed using a Philips diffractometer with Cu K radiation. The magnetization curves and M H loops were measured in applied fields of 0 80 koe and between 30 and 30 koe, respectively, at room temperature using VSM made by Oxford Instrument. The real and imaginary parts of permeability in the frequency range of 1 14 GHz were measured using transmission/reflection method based on the algorithms developed by Nicolson and Ross, 13 and by Weir. 14 The measurement fixture is a set of 7-mm coaxial airline with length mm. The microwave measurement is conducted on an HP8722D Vector Network Analyzer with TRL calibration. Saturation magnetization M s, high-field susceptibility p and anisotropy field H a were deduced from a numerical analysis of the magnetization curves based on the law of approach to saturation; 15 M H M s 1 A H B H 2... p H, 1 where H H 0 H d, H 0 is the applied field and H d is the demagnetizing field of the samples. The A/H term is related to the existence of inhomogeneities in the microcrystal and theoretically should vanish at high fields, 16,17 and the B/H 2 term is related to the magnetocrystalline anisotropy. 15 III. RESULTS AND DISCUSSION A. X-ray diffraction Some typical x-ray diffraction patterns are shown in Fig. 1 for nonaligned and aligned samples. The x-ray diffraction shows that all samples are single phase with hexagonal structure. The structural parameters are listed in Table I. The cell volumes increase with Co Zr substitutions. As compared to the sample of x 0, the sample of x 1.2 has a cell expansion of about 0.5%. The expansion is attributed to larger ion radii for Co and Zr that are and nm, respectively, as compared to the ion radius of nm for Fe. 18 With increasing x, the lattice parameter a almost is constant nm, the parameter c increases from nm to nm, and the ratio of c/a increases from to This implies that the Co and Zr ions may preferentially occupy some sites in the five different crystallographic sites of the M-type ferrite. For the aligned samples of x 0.8, the intensity of the (00l) reflections increases dramatically, whereas the intensity of the other reflections almost vanishes. This means that these samples have an easy c-axis anisotropy. In contrast, the greatly increased intensity of the (hk0) reflections shows that the sample of x 1.2 has an easy c-plane anisotropy. The aligned sample cannot be obtained for x 0.8, even if a small alignment field of 2 koe was applied. Therefore, the type of magnetic anisotropy cannot be identified. However, we believe that the anisotropy field will be very small, regardless of the easy c axis or the c-plane anisotropy for the sample. FIG. 1. Some typical x-ray patterns for BaFe 12 2x Co x Zr x O 19 with a the nonaligned sample of x 0 and the aligned samples of b x 0 and c x 1.2. B. Magnetic properties, H c, M s, and p Some typical magnetization curves and M H loops are shown in Fig. 2 for BaFe 12 2x Co x Zr x O 19. The dependence of the coercivity H c on the Co Zr substitution is shown in Fig. 3. The coercivity decreases rapidly from 2400 Oe for the sample of x 0 to about 10 Oe for the sample of x 0.8, and then slightly increases for the sample of x 1.2. Similar results have also been reported for the Co Ru substituted MBa ferrite by Cho and Kim. 12 However, only linear decrease in coercivity was found in Co Ti and Co Sn substituted BaM ferrites with substitution x 0.9 and x 1.2, respectively. 19,20 Saturation magnetization M s and high-field susceptibility p can be derived from the law of approach to saturation Eq. 1. It is observed that the dependence of M(H) onh is linear in high fields from 55 to 80 koe. Hence, the second and third terms in the bracket of Eq. 1 can be neglected. The values of M s and p can be obtained based on a linear least-squares method in the fields of koe. The saturation magnetizations M s, as a function of Co Zr substitutions, are plotted in Fig. 3. The saturation magnetization appears to be constant for samples of x 0.4 TABLE I. Lattice parameters a and c, ratio of c/a and cell volume V for BaFe 12 2x Co x Zr x O 19. x a nm c nm c/a V (nm 3 )

3 3904 J. Appl. Phys., Vol. 92, No. 7, 1 October 2002 Li, Chen, and Ong FIG. 4. High-field susceptibilities p of BaFe 12 2x Co x Zr x O 19. FIG. 2. Some typical magnetization curves and M H loops for BaFe 12 2x Co x Zr x O 19 with nonaligned sample. and decreases for x 0.6. This is related to the distribution of ions on the five Fe sites. It is known that Fe 3 ions with up-spin are distributed on the 2a, 12k, and 2b sites, and ions with down-spin are located on the 4 f VI and 4 f IV sites. The occupancy of the Co Zr ions on the 4 f VI and 4 f IV sites led to an increase in the net magnetization, and the occupancy on the 2a, 12k, and 2b sites gave rise to a decrease in the net magnetization. Due to the competition between these two effects, the magnetization of BaFe 12 2x Co x Zr x O 19 remains constant for x 0.4, and decreases for higher substitutions. The magnetization decreases more rapidly for BaFe 12 2x Co x Zr x O 19 than for the Co Ti and Co Sn substituted BaM ferrites. The magnetizations reduced by 23%, for x 0 tox 0.8, for the Co Zr substituted BaM ferrite, and only by 6.3% for the Co Sn substituted BaM ferites. 20 In addition, the magnetization has a maximum; it increases for x and then decreases for the Co Ti substituted BaM ferrite. 19 Mossbauer spectra indicated that the Co Zr ions preferentially occupy the 12k and 2b sites, 21 but the Co Ti and Co Sn ions preferentially occupy the 4 f VI site, besides the 2b and 12k sites. 19,20 The preferential occupancy of the Co Sn and Co Ti ions on the 4 f VI led to larger decrease in the negative magnetization. Consequently, an increase or slow decrease in magnetization is observed. For BaFe 12 2x Co x Zr x O 19, the relatively fast decrease in magnetization has its origin in the small occupancy of Co Zr ions on the 4 f VI and 4 f IV sites. It is obvious from Fig. 2 that for samples of x 0.8, the magnetization does not reach saturation even in a large applied field of 80 koe. The high-field susceptibilities p,as shown in Fig. 4, indicate that the values of p increase slightly in the range of x However, the susceptibilities increase rapidly for x 0.6; the value of p is almost tripled for the sample of x 1.2, as compared to the sample of x 0.6. Similar results were also observed in the substituted Co Ti and Zn Zr BaM ferrites with high substitutions. 22,23 This implies that a noncollinear magnetic structure spin canting occurs in the sample of x 0.8. The canting magnetic structure is another cause that leads to a low magnetization in high Co Zr substitutions. C. Anisotropy field H a Two methods were used to determine the anisotropy field H a of BaFe 12 2x Co x Zr x O 19. One method is based on the law of approach to saturation. The B/H 2 term in Eq. 1 is related to the magnetocrystalline anisotropy. Based on the domain rotation model and the assumption of K 1 K 2, B can be expressed as B 1 15 H a 2. 2 FIG. 3. Saturation magnetizations M s and coercivities H c of BaFe 12 2x Co x Zr x O 19. It was found that there is a good linear relationship between M(H) and 1/H 2 in a range of H 8 20 koe for samples of x 0.6. Therefore, the A/H and p terms in Eq. 1 can be neglected and the value of B can be derived from the slope of straight line M(H) M s (1 B/H 2 ) against 1/H 2. Furthermore, the anisotropy fields can be obtained from Eq. 2 ; the results are shown by the square markers in Fig. 5 b.

4 J. Appl. Phys., Vol. 92, No. 7, 1 October 2002 Li, Chen, and Ong 3905 FIG. 5. Anisotropy fields H a of BaFe 12 2x Co x Zr x O 19. The squares are obtained from the law of approach to saturation and the circles from the intersection of magnetization curves parallel and perpendicular to the alignment direction. However, this method cannot be used for samples of x 0.8, because the condition of K 1 K 2 is not satisfied. The other method is based on the magnetization curves parallel and perpendicular to the alignment direction for an aligned sample. The two M H curves have been measured for the aligned BaFe 12 2x Co x Zr x O 19 with x 0, 0.2, 0.4, 0.6, and 1.2. As an example, the curves are shown in Fig. 5 a for the aligned sample of x 0.4. The field corresponding to the intersection of the two curves is considered as the anisotropy field H a. The results are also shown by the circular markers in Fig. 5 b. The values of H a obtained from two methods are close. Samples of x 0.6 have an easy c-axis anisotropy. With increasing Co Zr substitution, a linear decrease in H a is observed. The values of H a decrease from 17.2 koe for the sample of x 0 to 8.2 koe for the sample of x 0.6. Higher Co Zr substitution can modify the anisotropy from uniaxial to planar; the sample of x 1.2 has an easy c-plane anisotropy with an anisotropy field of 5 koe. It has been known that Co 2 W and Co 2 Z ferrites have a planar anisotropy and other divalent ions, such as Zn 2,Fe 2 and Ni 2, lead to an easy c-axis anisotropy for the two type ferrites Albanese et al. 28 indicated that Co 2 Z ferrites gave a negative contribution to K 1 and a positive contribution to K 2. Also, Co Ti and Co Ir substitution can modify the anisotropy from uniaxial to planar for M-type ferrites. 10,29 Therefore, it seems reasonable to assume that the Co ions contribute preferentially to planar anisotropy. On the other hand, Fe ions tend to contribute to c-axis anisotropy. Due to competition of the two contributions, the anisotropy is modified from uniaxial to planar for BaFe 12 2x Co x Zr x O 19. As shown in Fig. 5 b, the dependence of the anisotropy fields H a on the Co Zr substitutions is considered to be linear, to a good approximation, for x By extrapolating from x 0.6, the value of H a is equal to zero at x 1.0. Therefore, it is reasonable to deduce that BaFe 12 2x Co x Zr x O 19 has an easy c-axis anisotropy below x 1.0, and an easy c-plane anisotropy above x 1.0. The deduction is consistent with the results of x-ray diffraction for the aligned samples. We note that Co Ti substitution can also modify the anisotropy from uniaxial to planar for x However, Co Sn substituted BaM ferrite keeps an easy c-axis anisotropy for x 0 tox 1.4; the magnetocrystaline anisotropy constant K 1 is about 1.93 cm 1 /ion for x 0 and 0.72 cm 1 /ion for x It appears that the difference between the Co Zr/Co Ti and the Co Sn substitutions is related to the ion occupancy on the 2b site. It is known that the high magnetocrystalline anisotropy of BaM ferrite has its primary origin in Fe 3 ions on the trigonal bipyramidal site, i.e., the 2b site, 30 which has large asymmetry. Therefore, the preference of nonmagnetic ions on the 2b site can lead to a decrease in the anisotropy field. Mossbauer spectra showed that Co Zr, Co Ti, and Co Sn ions preferentially occupy the 2b site Perhaps, there are more occupancy on the 2b site for the nonmagnetic Zr and Ti ions, as compared to the Sn ions. Thus, the anisotropy field reduces more rapidly for the Co Zr and Co Ti substitutions than for the Co Sn substitution. The value of H c is closely related to anisotropy fields. The rapid decrease in coercivity can be attributed to the reduction of anisotropy fields for samples of x 0.8. Further Co Zr substitution gives rise to a slight increase in coercivity (H c 82 Oe for the sample of x 1.2). This may be due to a change in the anisotropy from uniaxial to planar for the sample of x 1.2. D. Magnetic properties at microwave frequencies The electromagnetic properties of the composites filled with 35% by volume BaM ferrite powders are characterized from 1 to 14 GHz. The dielectric properties of the samples do not have obvious changes at the frequency range. The real part of the complex permittivity is about 6.5 and the imaginary part is about 0.5 for all the samples. The real and imaginary parts of the relative complex permeability, and, are shown in Figs. 6 a and 6 b, respectively. It should be indicated that the measurement has some uncertainties at frequencies around 2.5n GHz (n 1, 2, 3, etc., especially at 7.5 GHz, where the sample length is equal to integer multiples of half wavelength. For samples of x 0 and x 0.4, the real part of the relative permeability changes little and the imaginary part is close to zero. The magnetic resonance behavior was not found within the measurement range. For samples of x 0.8 and x 1.2, the natural resonance frequencies are observed at 4.5 and 5.0 GHz, respectively, corresponding to the maximum in - f curves. As we know, the natural resonance frequencies, f 0, can be expressed as f 0 H a, 3 f 0 H H 1/2, 4

5 3906 J. Appl. Phys., Vol. 92, No. 7, 1 October 2002 Li, Chen, and Ong FIG. 7. Theoretical estimations on the reflectivity for the composite samples of BaFe 12 2x Co x Zr x O 19. FIG. 6. Complex permeability spectrums for the composite samples of BaFe 12 2x Co x Zr x O 19, a the real part and b the imaginary part of relative complex permeabilities. for easy c axis and easy c-plane anisotropies, respectively. 31 H and H are the anisotropy fields along c axis and in c plane, respectively. 2.8 MHz/Oe is the gyromagnetic ratio. In Sec. III C, we have obtained that the anisotropy fields are 17.2 and 10.0 koe for the samples of x 0 and x 0.4. Based on Eq. 3, the calculated resonance frequencies are about 48.2 and 28.0 GHz, which is way beyond the measurement range. Pullar et al. 9 have reported that the natural resonance frequency is about 42.5 GHz for BaFe 12 O 19. With increasing Co Zr substitutions, the anisotropy fields decrease rapidly and the anisotropy is even modified from uniaxial to planar. Consequently, the corresponding resonance frequencies should be reduced and the resonant characteristics are observed for samples of x 0.8 and x 1.2. Figure 7 shows the effects of magnetic resonance to reflectivity, which is a theoretical estimation based on the electromagnetic properties of our composite samples. In the calculations, we assume that the samples are backed by a metal and the thickness of composites is 5 mm. The dielectric constant used in the calculation is 6.5 and the imaginary part of the relative permittivity is 0.5. The relative complex permeability is based on the values shown in Fig. 6. The magnetic resonance leads to an obviously lower reflectivity for samples of x 0.8 and x 1.2. This can be understood from two aspects. First, for a single layer of isotropic material, its thickness should be equal to a quarter wavelength at the frequency of interest. Due to the magnetic resonance, the real part of the relative permeability decreases with frequency. The quarter wavelength requirement could be fulfilled in a frequency range around the magnetic resonance. Therefore, we can obtain low reflectivity over a broader frequency range. Second, more electromagnetic energy is dissipated due to the magnetic resonance. As a result, the lower reflectivity is achieved. Therefore, magnetic resonance is useful to the development of electromagnetic materials with broad bandwidth and low reflectivity. IV. CONCLUSIONS 1 All samples of BaFe 12 2x Co x Zr x O 19 with x are single phase with hexagonal structure. The lattice parameter a remains constant and the parameter c increases with the Co Zr substitutions. 2 The saturation magnetizations of BaFe 12 2x Co x Zr x O 19 are almost constant at x 0.4 and decrease with higher substitutions. The coercivity decreases rapidly from 2400 Oe for the sample of x 0 to about 10 Oe for the sample of x BaFe 12 2x Co x Zr x O 19 has a c-axis anisotropy for Co Zr substitution x 0.6. The anisotropy fields decrease linearly from 17.2 koe for the sample of x 0 to 10.0 koe for the sample of x 0.6. Higher Co Zr substitution can modify the anisotropy from uniaxial to planar. The sample of x 1.2 has the c-plane anisotropy with H a 5 koe. 4 Co Zr substitution can modify the natural resonance frequencies f 0 ; the frequencies are observed at 4.5 and 5.0 GHz for BaFe 12 2x Co x Zr x O 19 with x 0.8 and x 1.2, respectively. Based on microwave measurement and theoretical estimations on the reflectivity, BaFe 12 2x Co x Zr x O 19 is a potentially good candidate for electromagnetic materials with low reflectivity at microwave frequency. 1 D. Y. Kim, Y. C. Chung, T. W. Kang, and H. C. Kim, IEEE Trans. Magn. 32, T. Tsutaoka, T. Kasagi, T. Nakamura, and K. Hatakeyama, J. Phys. IV 7, C H. J. Kwon, J. Y. Shin, and J. H. Oh, J. Appl. Phys. 75, D. Autissier, A. Podembski, and C. Jacquiod, J. Phys. IV 7, C M. Matsumoto and Y. Miyata, J. Appl. Phys. 79, H. Zhang, J. Zhou, Z. Yeu, P. Wu, Z. Gui, and L. Li, Mater. Lett. 43, ; X. Wang, T. Ren, L. Li, Z. Gui, S. Su, Z. Yue, and J. Zhou, J. Magn. Magn. Mater. 234, ; H. Zhang, L. Li, J. Zhou, Z. Yue, Z. Ma, and Z. Gui, J. Eur. Ceram. Soc. 21,

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