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Frequency dispersion of complex permeability of Y-type hexagonal ferrites
Materials Letters 58 (2004) 1602 – 1606 www.elsevier.com/locate/matlet
Frequency dispersion of complex permeability of Y-type hexagonal ferrites Yang Bai *, Ji Zhou, Zhilun Gui, Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 25 August 2003; accepted 18 September 2003
Abstract The frequency characteristics of Y-type hexagonal ferrites were studied. The samples were prepared by the conventional ceramics method. X-ray diffraction was used to characterize the phase formation. The microstructure was observed via scanning electron microscopy, and magnetic properties were investigated using vibrating samples magnetometer and impedance analyzer. Experimental results show that composition and processing will determine the magnetization mechanism of Y-type hexagonal ferrites in very high frequency and ultra high frequency. Both domain wall motion and magnetization rotation exist in magnetization processing. There is a double resonance peak with obvious dispersion character for low Co substitution. Co substitution will increase the magnetic anisotropy distinctly. As Co content rises, the increasing magnetic anisotropy leads to the movement of resonance peak towards higher frequency and disappearance of magnetic domain wall resonance; initial permeability decreases at the same time. The rise of sintering temperature will enhance magnetic domain wall resonance remarkably and move resonance peak towards lower frequency. D 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Gg Keywords: Y-type hexagonal ferrite; Magnetization mechanism; Frequency characteristic; Complex permeability; Dispersion
1. Introduction Discovered at Philips in the 1950s [1,2], hexagonal ferrites constitute an interesting subfamily of ferrites. Their own planar magnetic structure endows them special magnetic properties. They have been used in a variety of applications and attracted much attention over the years. M-type hexaferrite is the most popular hard magnetic material in the industry; W-type hexagonal ferrite is a type of crucial gyromagnetic materials; and Z-type and Y-type hexagonal ferrites are excellent soft magnetic materials in very high frequency (VHF) and ultra high frequency (UHF) [3– 10]. Currently, the development of information and communication technology drives a great demand for chip soft magnetic components [including multilayer chip inductors (MLCIs) and multilayer chip beads (MLCBs)] in VHF and UHF. Y-type hexagonal ferrites with planar magnetic anisot-
ropy have much higher cutoff frequency than spinel ferrites and exhibit excellent magnetic properties in VHF and UHF [3]. These ferrites are anticipated to meet the requirement of soft magnetic materials for chip components [3]. The frequency dispersion of the complex permeability of magnetic materials is extremely important for the design and application of inductive components. There are few reports on the magnetic properties of Y-type hexagonal ferrites in VHF and UHF. Our work focuses on magnetization mechanism and frequency dispersion of Y-type hexagonal ferrites in VHF and UHF. We selected the material system with stoichimetric compositions of Ba2Zn1.2 2xCo2xCu0.8Fe12O22, where the substitution of strong planar magnetic Co2 + ion has a good effect for tuning the magnetic properties; as well, a proper Cu substitution lowers the sintering temperature.
2. Experimental * Corresponding author. Tel.: +86-10-62784579; fax: +86-1062771160. E-mail address: [email protected] (Y. Bai). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.09.049
Y-type hexagonal ferrite powders were prepared by the solid-state reaction method. According to the stoichiomet-
Y. Bai et al. / Materials Letters 58 (2004) 1602–1606
ric compositions of Ba2Zn1.2 2xCo2xCu0.8Fe12O22 (0 V x V 0.1), the analytical-grade raw materials, BaCO3, Co2O3, ZnO, CuO ,and Fe2O3, were weighted. After mixing in a ball mill for 24 h, the powders were calcined at 1050 jC for 4 h in the air, and ground again for 24 h. The resulting powders were pressed in a stainless steel die under a pressure of 7 MPa, with 5 wt.% polyvinyl alcohol as lubricant. After dry pressing, the toroidal samples (20 mm outside diameter, 10 mm inside diameter, and about 3 mm thickness) were sintered in the temperature range of 1000 – 1150 jC for 4 h in air and cooled in the furnace. The phase structure was characterized via a Rigaku X-ray diffractometer equipped with CuKa radiation (k = 1.5405), and the microstructure of fracture surface was observed by scanning electron microscopy (SEM). The vibrating samples magnetometer (VSM), LakeShore VSM 7307, was used to measure the saturation magnetization (Ms), remanent magnetization (Mr), and coercive force (Hc) through the magnetic hysteresis loop. A Hewlett Packard HP4291B impedance analyzer was used to measure complex permeability from 1 MHz to 1 GHz.
3. Results and discussion 3.1. Phase identification The samples were calcined at 1050 jC and identified via X-ray diffractometer. After indexing the patterns, pure Y-type hexagonal ferrite phase was corroborated. The results indicate that there is a well-defined Y-type hexagonal ferrite crystalline phase for each composition in the samples. Fig. 1 shows a typical X-ray diffraction (XRD) output for Y-type hexagonal ferrite compared with the standard XRD spectra.
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3.2. Microstructure and densities The SEM results presented in Fig. 2a and b show grain morphology. The grains grow larger and become more compactly stacked with the rise of sintering temperature. Hence, grain boundaries and porosities decrease with sintering temperature. Determined by the Archimedes method, the densities of the samples are plotted in Table 1. The density increases with the sintering temperature, in agreement with the SEM results. 3.3. Magnetic properties of sintered samples Table 2 lists the main magnetic parameters characterizing the samples. As Co content increases, Hc and Mr increase, whereas Ms rises at first and decreases after reaching a maximum. Nonmagnetic Zn2 + ions preferentially occupy tetrahedral sites (A sites) where magnetic sublattices lie in antiparallel orientation to the whole lattice. Zn2 + ions will not change the spin direction of the magnetic sublattice, but simply reduce the magnetic moment. As a result, the net magnetic moment of the crystal increases. Substitution of nonmagnetic ion will also result in a reduction of the Curie temperature. At room temperature, the saturation magnetization will decrease slightly for the samples with high Zn content due to the effect of thermal agitation. Substitution of strong planar magnetic Co2 + ion will increase the magnetic anisotropy remarkably. Hence, coercive force increases with Co content. Fig. 3 shows complex permeability spectra of Y-type ferrites with different Co substitutions that were sintered at 1100 jC. Experimental results show that permeability lV decreases and the resonance frequency increases with Co content. There is a double resonance peak with the dispersion character for low Co substitution. The resonance peak
˚ ). Fig. 1. The typical XRD pattern output of Y-type hexagonal ferrite (CuKa, k = 1.5405 A
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Y. Bai et al. / Materials Letters 58 (2004) 1602–1606 Table 2 The saturation magnetization (Ms), remanent magnetization (Mr), and coercive force (Hc) of the samples for various Co contents (Ts = 1050 jC) x
Ms (emu/g)
Mr (emu/g)
Hc (G)
0 0.025 0.05 0.075 0.1
34.50 34.86 36.91 36.31 34.77
4.05 4.31 4.80 4.78 5.21
85.95 77.01 84.23 90.25 96.77
motion and magnetization rotation. The magnetization rotation used to be considered as the predominant mechanism of hexagonal ferrites because their strong planar magnetic anisotropy will block the domain wall motion. Our results show that both domain wall motion and magnetization rotation exist in magnetization processing. The samples with high Zn content have rather low planar magnetic anisotropy. For these samples, the domain wall motion is a competitive mechanism with magnetization
Fig. 2. SEM photos of the fracture surface of the sintered samples for Ba2Zn1.2 2xCo2xCu0.8Fe12O22. (a) x = 0, Ts = 1000 jC; (b) x = 0, Ts = 1100 jC.
at low frequency originates from the magnetic domain wall resonance, whereas the one at high frequency originates from nature resonance. There are two mechanisms that contribute to the permeability of magnetic materials, namely, domain wall Table 1 Density of the samples sintered at different temperatures x
Density (g/cm3) 1000 jC
1050 jC
1100 jC
1150 jC
0 0.025 0.05 0.075 0.1
4.53 4.59 4.68 4.56 4.72
5.12 5.21 5.19 5.25 5.23
5.34 5.31 5.35 5.31 5.35
5.25 5.27 5.27 5.27 5.29
Fig. 3. Frequency dependence of (a) real and (b) imaginary parts of permeability (lV and lW) for the samples of Ba2Zn1.2 2xCo2xCu0.8Fe12O22 sintered at 1100 jC.
Y. Bai et al. / Materials Letters 58 (2004) 1602–1606
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motion. As a result, magnetic domain walls can move more easily with the magnetic field. The remarkable rise of permeability lV in low frequency can be attributed to magnetic domain wall motion. At the same time, resonance peak moves towards low frequency with sintering temperature.
4. Conclusions The important conclusions of our work can be summarized as follows: 1. Complex Y-type hexagonal ferrites containing Co, Zn, and Cu can form well-defined hexagonal structure. After being calcined at 1050 jC, pure Y-type phase can be obtained for each composition studied. 2. Co substitution can increase the magnetic anisotropy distinctly. The permeability decreases and the resonance frequency increases with Co content. There is a double resonance peak with the dispersion character for low Co substitution. The dominant magnetic mechanism changes from domain wall motion to magnetization rotation with increasing magnetic anisotropy. Magnetic domain wall resonance disappears with increasing Co content. 3. As the sintering temperature increases, the magnetic domain wall resonance is enhanced remarkably, which results in the increase of the initial permeability in low frequency and the movement of the resonance peak towards lower frequency. Fig. 4. (a) Real and (b) imaginary parts of permeability (lV and lW) for the samples of Ba2Zn1.2 2xCo2xCu0.8Fe12O22 sintered at various temperatures (x = 0).
rotation. The contribution of domain wall motion to the complex permeability is obvious in low-frequency range. In high-frequency range, domain wall motion cannot keep up with the frequency variation. A domain wall resonance appears before nature resonance. There is a double peak with the dispersion character; the one in low frequency can be attributed to the domain wall resonance. The magnetic anisotropy increases remarkably with Co substitution. Magnetic domain wall resonance disappears increasingly; magnetization rotation, instead of the domain wall motion, becomes the main contribution to permeability gradually with the rise of magnetic anisotropy. At the same time, resonance peak moves to high frequency with increasing Co content. Fig. 4 shows the complex permeability of the samples sintered at various temperatures. As the sintering temperature rises, grains grow larger and stack more compactly. Then the grain boundaries and porosities decrease, thus reducing a crucial obstacle to magnetic domain wall
This selected series of materials has both high real and imaginary parts of complex permeability in VHF and UHF. These excellent magnetic properties show promising future in EMI application in VHF and UHF. It is anticipated to satisfy the needs of MLCBs. Further development of lowsintering-temperature ferrites will enable them to be cofired with less expensive contact materials in multilayer chip devices easily.
Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 program no. 2001AA320502). The authors would like to thank Prof. Zhenxing Yue for fruitful discussions.
References [1] J. Smit, H.B.J. Wijn, Ferrites, Cleaver-Hume Press, 1959. [2] J. Smit, Magnetic Properties of Materials, McGraw-Hill, New York, 1971. [3] Y. Bai, J. Zhou, Z. Gui, L. Li, Journal of Magnetism and Magnetic Materials 246 (2002) 140 – 144.
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Y. Bai et al. / Materials Letters 58 (2004) 1602–1606
[4] H. Zhang, J. Zhou, Journal of European Ceramic Society, (2001) 149 – 153. [5] S.G. Lee, S.J. Kwon, Journal of Magnetism and Magnetic Materials 153 (1996) 279 – 284. [6] G. Albanese, A. Deriu, F. Licci, S. Rinaldi, IEEE Transactions on Magnetics Mag-14 (1978) 710 – 712. [7] A. Deriu, S. Licci, F. Rinaldi, T. Besagni, Journal of Magnetism and Magnetic Materials 22 (1981) 257 – 262.
[8] Y.H. Chang, C.C. Wang, T.S. Chin, F.S. Yen, Journal of Magnetism and Magnetic Materials 72 (1988) 343 – 348. [9] A.M.A. El Ata, M.A. Ahmed, M.A. El Hiti, M.K. El Nimr, Journal of Materials Science Letters 18 (1999) 563 – 567. [10] M.A. El Hiti, A.M.A. Abo El Ata, Journal of Magnetism and Magnetic Materials 195 (1999) 667 – 678.
Frequency dispersion of complex permeability of Y-type hexagonal ferrites Yang Bai *, Ji Zhou, Zhilun Gui, Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 25 August 2003; accepted 18 September 2003
Abstract The frequency characteristics of Y-type hexagonal ferrites were studied. The samples were prepared by the conventional ceramics method. X-ray diffraction was used to characterize the phase formation. The microstructure was observed via scanning electron microscopy, and magnetic properties were investigated using vibrating samples magnetometer and impedance analyzer. Experimental results show that composition and processing will determine the magnetization mechanism of Y-type hexagonal ferrites in very high frequency and ultra high frequency. Both domain wall motion and magnetization rotation exist in magnetization processing. There is a double resonance peak with obvious dispersion character for low Co substitution. Co substitution will increase the magnetic anisotropy distinctly. As Co content rises, the increasing magnetic anisotropy leads to the movement of resonance peak towards higher frequency and disappearance of magnetic domain wall resonance; initial permeability decreases at the same time. The rise of sintering temperature will enhance magnetic domain wall resonance remarkably and move resonance peak towards lower frequency. D 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Gg Keywords: Y-type hexagonal ferrite; Magnetization mechanism; Frequency characteristic; Complex permeability; Dispersion
1. Introduction Discovered at Philips in the 1950s [1,2], hexagonal ferrites constitute an interesting subfamily of ferrites. Their own planar magnetic structure endows them special magnetic properties. They have been used in a variety of applications and attracted much attention over the years. M-type hexaferrite is the most popular hard magnetic material in the industry; W-type hexagonal ferrite is a type of crucial gyromagnetic materials; and Z-type and Y-type hexagonal ferrites are excellent soft magnetic materials in very high frequency (VHF) and ultra high frequency (UHF) [3– 10]. Currently, the development of information and communication technology drives a great demand for chip soft magnetic components [including multilayer chip inductors (MLCIs) and multilayer chip beads (MLCBs)] in VHF and UHF. Y-type hexagonal ferrites with planar magnetic anisot-
ropy have much higher cutoff frequency than spinel ferrites and exhibit excellent magnetic properties in VHF and UHF [3]. These ferrites are anticipated to meet the requirement of soft magnetic materials for chip components [3]. The frequency dispersion of the complex permeability of magnetic materials is extremely important for the design and application of inductive components. There are few reports on the magnetic properties of Y-type hexagonal ferrites in VHF and UHF. Our work focuses on magnetization mechanism and frequency dispersion of Y-type hexagonal ferrites in VHF and UHF. We selected the material system with stoichimetric compositions of Ba2Zn1.2 2xCo2xCu0.8Fe12O22, where the substitution of strong planar magnetic Co2 + ion has a good effect for tuning the magnetic properties; as well, a proper Cu substitution lowers the sintering temperature.
2. Experimental * Corresponding author. Tel.: +86-10-62784579; fax: +86-1062771160. E-mail address: [email protected] (Y. Bai). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.09.049
Y-type hexagonal ferrite powders were prepared by the solid-state reaction method. According to the stoichiomet-
Y. Bai et al. / Materials Letters 58 (2004) 1602–1606
ric compositions of Ba2Zn1.2 2xCo2xCu0.8Fe12O22 (0 V x V 0.1), the analytical-grade raw materials, BaCO3, Co2O3, ZnO, CuO ,and Fe2O3, were weighted. After mixing in a ball mill for 24 h, the powders were calcined at 1050 jC for 4 h in the air, and ground again for 24 h. The resulting powders were pressed in a stainless steel die under a pressure of 7 MPa, with 5 wt.% polyvinyl alcohol as lubricant. After dry pressing, the toroidal samples (20 mm outside diameter, 10 mm inside diameter, and about 3 mm thickness) were sintered in the temperature range of 1000 – 1150 jC for 4 h in air and cooled in the furnace. The phase structure was characterized via a Rigaku X-ray diffractometer equipped with CuKa radiation (k = 1.5405), and the microstructure of fracture surface was observed by scanning electron microscopy (SEM). The vibrating samples magnetometer (VSM), LakeShore VSM 7307, was used to measure the saturation magnetization (Ms), remanent magnetization (Mr), and coercive force (Hc) through the magnetic hysteresis loop. A Hewlett Packard HP4291B impedance analyzer was used to measure complex permeability from 1 MHz to 1 GHz.
3. Results and discussion 3.1. Phase identification The samples were calcined at 1050 jC and identified via X-ray diffractometer. After indexing the patterns, pure Y-type hexagonal ferrite phase was corroborated. The results indicate that there is a well-defined Y-type hexagonal ferrite crystalline phase for each composition in the samples. Fig. 1 shows a typical X-ray diffraction (XRD) output for Y-type hexagonal ferrite compared with the standard XRD spectra.
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3.2. Microstructure and densities The SEM results presented in Fig. 2a and b show grain morphology. The grains grow larger and become more compactly stacked with the rise of sintering temperature. Hence, grain boundaries and porosities decrease with sintering temperature. Determined by the Archimedes method, the densities of the samples are plotted in Table 1. The density increases with the sintering temperature, in agreement with the SEM results. 3.3. Magnetic properties of sintered samples Table 2 lists the main magnetic parameters characterizing the samples. As Co content increases, Hc and Mr increase, whereas Ms rises at first and decreases after reaching a maximum. Nonmagnetic Zn2 + ions preferentially occupy tetrahedral sites (A sites) where magnetic sublattices lie in antiparallel orientation to the whole lattice. Zn2 + ions will not change the spin direction of the magnetic sublattice, but simply reduce the magnetic moment. As a result, the net magnetic moment of the crystal increases. Substitution of nonmagnetic ion will also result in a reduction of the Curie temperature. At room temperature, the saturation magnetization will decrease slightly for the samples with high Zn content due to the effect of thermal agitation. Substitution of strong planar magnetic Co2 + ion will increase the magnetic anisotropy remarkably. Hence, coercive force increases with Co content. Fig. 3 shows complex permeability spectra of Y-type ferrites with different Co substitutions that were sintered at 1100 jC. Experimental results show that permeability lV decreases and the resonance frequency increases with Co content. There is a double resonance peak with the dispersion character for low Co substitution. The resonance peak
˚ ). Fig. 1. The typical XRD pattern output of Y-type hexagonal ferrite (CuKa, k = 1.5405 A
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Y. Bai et al. / Materials Letters 58 (2004) 1602–1606 Table 2 The saturation magnetization (Ms), remanent magnetization (Mr), and coercive force (Hc) of the samples for various Co contents (Ts = 1050 jC) x
Ms (emu/g)
Mr (emu/g)
Hc (G)
0 0.025 0.05 0.075 0.1
34.50 34.86 36.91 36.31 34.77
4.05 4.31 4.80 4.78 5.21
85.95 77.01 84.23 90.25 96.77
motion and magnetization rotation. The magnetization rotation used to be considered as the predominant mechanism of hexagonal ferrites because their strong planar magnetic anisotropy will block the domain wall motion. Our results show that both domain wall motion and magnetization rotation exist in magnetization processing. The samples with high Zn content have rather low planar magnetic anisotropy. For these samples, the domain wall motion is a competitive mechanism with magnetization
Fig. 2. SEM photos of the fracture surface of the sintered samples for Ba2Zn1.2 2xCo2xCu0.8Fe12O22. (a) x = 0, Ts = 1000 jC; (b) x = 0, Ts = 1100 jC.
at low frequency originates from the magnetic domain wall resonance, whereas the one at high frequency originates from nature resonance. There are two mechanisms that contribute to the permeability of magnetic materials, namely, domain wall Table 1 Density of the samples sintered at different temperatures x
Density (g/cm3) 1000 jC
1050 jC
1100 jC
1150 jC
0 0.025 0.05 0.075 0.1
4.53 4.59 4.68 4.56 4.72
5.12 5.21 5.19 5.25 5.23
5.34 5.31 5.35 5.31 5.35
5.25 5.27 5.27 5.27 5.29
Fig. 3. Frequency dependence of (a) real and (b) imaginary parts of permeability (lV and lW) for the samples of Ba2Zn1.2 2xCo2xCu0.8Fe12O22 sintered at 1100 jC.
Y. Bai et al. / Materials Letters 58 (2004) 1602–1606
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motion. As a result, magnetic domain walls can move more easily with the magnetic field. The remarkable rise of permeability lV in low frequency can be attributed to magnetic domain wall motion. At the same time, resonance peak moves towards low frequency with sintering temperature.
4. Conclusions The important conclusions of our work can be summarized as follows: 1. Complex Y-type hexagonal ferrites containing Co, Zn, and Cu can form well-defined hexagonal structure. After being calcined at 1050 jC, pure Y-type phase can be obtained for each composition studied. 2. Co substitution can increase the magnetic anisotropy distinctly. The permeability decreases and the resonance frequency increases with Co content. There is a double resonance peak with the dispersion character for low Co substitution. The dominant magnetic mechanism changes from domain wall motion to magnetization rotation with increasing magnetic anisotropy. Magnetic domain wall resonance disappears with increasing Co content. 3. As the sintering temperature increases, the magnetic domain wall resonance is enhanced remarkably, which results in the increase of the initial permeability in low frequency and the movement of the resonance peak towards lower frequency. Fig. 4. (a) Real and (b) imaginary parts of permeability (lV and lW) for the samples of Ba2Zn1.2 2xCo2xCu0.8Fe12O22 sintered at various temperatures (x = 0).
rotation. The contribution of domain wall motion to the complex permeability is obvious in low-frequency range. In high-frequency range, domain wall motion cannot keep up with the frequency variation. A domain wall resonance appears before nature resonance. There is a double peak with the dispersion character; the one in low frequency can be attributed to the domain wall resonance. The magnetic anisotropy increases remarkably with Co substitution. Magnetic domain wall resonance disappears increasingly; magnetization rotation, instead of the domain wall motion, becomes the main contribution to permeability gradually with the rise of magnetic anisotropy. At the same time, resonance peak moves to high frequency with increasing Co content. Fig. 4 shows the complex permeability of the samples sintered at various temperatures. As the sintering temperature rises, grains grow larger and stack more compactly. Then the grain boundaries and porosities decrease, thus reducing a crucial obstacle to magnetic domain wall
This selected series of materials has both high real and imaginary parts of complex permeability in VHF and UHF. These excellent magnetic properties show promising future in EMI application in VHF and UHF. It is anticipated to satisfy the needs of MLCBs. Further development of lowsintering-temperature ferrites will enable them to be cofired with less expensive contact materials in multilayer chip devices easily.
Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 program no. 2001AA320502). The authors would like to thank Prof. Zhenxing Yue for fruitful discussions.
References [1] J. Smit, H.B.J. Wijn, Ferrites, Cleaver-Hume Press, 1959. [2] J. Smit, Magnetic Properties of Materials, McGraw-Hill, New York, 1971. [3] Y. Bai, J. Zhou, Z. Gui, L. Li, Journal of Magnetism and Magnetic Materials 246 (2002) 140 – 144.
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Y. Bai et al. / Materials Letters 58 (2004) 1602–1606
[4] H. Zhang, J. Zhou, Journal of European Ceramic Society, (2001) 149 – 153. [5] S.G. Lee, S.J. Kwon, Journal of Magnetism and Magnetic Materials 153 (1996) 279 – 284. [6] G. Albanese, A. Deriu, F. Licci, S. Rinaldi, IEEE Transactions on Magnetics Mag-14 (1978) 710 – 712. [7] A. Deriu, S. Licci, F. Rinaldi, T. Besagni, Journal of Magnetism and Magnetic Materials 22 (1981) 257 – 262.
[8] Y.H. Chang, C.C. Wang, T.S. Chin, F.S. Yen, Journal of Magnetism and Magnetic Materials 72 (1988) 343 – 348. [9] A.M.A. El Ata, M.A. Ahmed, M.A. El Hiti, M.K. El Nimr, Journal of Materials Science Letters 18 (1999) 563 – 567. [10] M.A. El Hiti, A.M.A. Abo El Ata, Journal of Magnetism and Magnetic Materials 195 (1999) 667 – 678.