- Email: [email protected]
Magnetostriction enhancement in Laves compounds (Sm,Yb)Fe2
Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
Magnetostriction enhancement in Laves compounds (Sm,Yb)Fe2 Zhi-jun Guoa,*, Z.D. Zhanga, S.C. Busbridgeb, B.W. Wanga, X.G. Zhaoa, D.Y. Genga a
International Center for Materials Physics, Chinese Academy of Sciences, Institute of Metal Research, Academia Sinica, 72 Wenhua Road, Shenyang 110015, People’s Republic of China b School of Engineering, University of Brighton, BN2 4GJ, UK Received 1 November 2000
Abstract The magnetic and magnetostrictive properties of compounds (Sm1xYbx)Fe2 (04x40:3) have been studied. When x50:15, the compounds crystallize in the cubic structure of a C15 MgCu2-type Laves phase. For x > 0:15 a small amount of (Sm,Yb)Fe3 compound with a PuNi3-type structure appears and its amount increases with increasing Yb content. The magnetic ordering of 1 : 2 compounds is slightly strengthened by the substitution of Yb for Sm. Lattice parameters for Laves phase compounds keep almost unchanged for the whole range investigated, indicating the deviation from Vegard’s law linear variation with composition. A magnetostriction enhancement of about 15% when x ¼ 0:05% both in spontaneous and polycrystalline magnetostrictions in Sm0.95Yb0.05Fe2 has been observed, which can be ascribed to the anisotropy compensation effect. # 2001 Elsevier Science B.V. All rights reserved. PACS: 75.80.+q; 81.40.z Keywords: Magnetostriction; Rare earths; Laves compounds
Magnetostriction, i.e. so-called Joule’s effect, is one of the intrinsic properties of magnetic materials [1]. The magnetostrictive materials have found their industrial applications as energy transforming devices, e.g. transducers, sensors and actuators [2]. Much attention has been paid to rare earths or their compounds with transition metals because of their giant intrinsic magnetostriction [3]. Low magnetic fields are required for the practical applications in industries. Since rare *Corresponding author. Fax: +86-24-2389-1320. E-mail address: [email protected] (Z.-j. Guo).
earth transition metal compounds of 1 : 2 type, e.g. TbFe2, SmFe2, have high anisotropies at room temperature, a much higher field is necessary for large magnetostriction. Clark et al. proposed [3] that the compounds with opposite anisotropy can reduce their anisotropic energy while maintaining high magnetostriction. Therefore, the so-called pseudobinary compound, R11xR2xFe2 (R1,R2=rare earths), was invented. In particular, (Tb0.7Dy0.3)Fe2 (Terfenol-D) has become a popular function material in practical applications. Relatively speaking, little work has been done on SmFe2 although it has at room temperature the
0304-8853/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 0 6 0 - 9
192
Z.-j. Guo et al. / Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
highest negative magnetostriction. According to the single-ion model, YbFe2 (l1 1 1 ¼ 3600 ppm) has even larger magnetostriction than SmFe2 (l1 1 1 ¼ 3200 ppm) [3]. In addition, YbFe2 has an opposite anisotropy to that of SmFe2. Therefore, Sm1xYbxFe2 is an ideal anisotropy compensation system. However, little information is available on this system since YbFe2 can only be synthesized at high pressure [3,4]. In this paper, we reported the structure, magnetic properties and magnetostriction enhancement effect of Sm1xYbxFe2. All samples of Sm1xYbxFe2 for x ¼ 0, 0.05, 0.1, 0.15, 0.2 and 0.3 were prepared by arc melting appropriate metals in a magneto-controlled arc furnace under high purity argon atmosphere. This procedure has been repeated several times for the homogeneity of samples. The purity of Sm, Yb and Fe is 99.9%, Co 99.8%. The ingots were homogenized under high purity argon atmosphere at 8208C for 50 h. X-ray diffraction (XRD) patterns were recorded at room temperature with CuKa radiation in a D/max-gA diffractometer with a pyrolytic monochromator (Fig. 1). The lattice parameters were determined and the absolute error of the XRD measurement was less than 0.0002 nm. Curie temperatures Tc were determined by means of AC initial susceptibility.
Fig. 1. X-ray diffraction patterns of Sm1xYbxFe2 annealed at 8208C for 50 h.
The samples in powder form were step scanned with CuKa radiation by the X-ray diffractometer at a high Bragg angle 2y radiation ranging from 708 to 748 in order to study the cubic (4 4 0) line. The spontaneous magnetostriction l1 1 1 of the compounds was calculated by l1 1 1 ¼ Da;
ð1Þ
where Da is the deviation of the angle between neighboring edges of the distorted cube from p=2 [5]. Magnetostrictions lk and l? of the samples with a size of 4 8 8 mm3 parallel and perpendicular to the applied field up to 796 kA/m were measured at room temperature by using standard strain gauage techniques. The saturation magnetostriction ls was obtained by the relation ls ¼ 23ðljj þ l? Þ. XRD data indicate that the homogenized alloys of Sm1xYbxFe2 are essentially the single laves phase (Sm,Yb)Fe2 (space group Fd3 m) with C15 MgCu2-type cubic structure when x40:15. For x > 0:15, a non-cubic phase (Sm,Yb)Fe3 with PuNi3 structure appears and its amount increases progressively with increasing Yb content up to 0.3. This is due to that YbFe2 can only be formed in high pressure [4], in a good agreement with that of Clark et al. [3]. The composition dependence of lattice parameter a of (Sm,Yb)Fe2 compounds in Sm1xYbxFe2 is shown in Fig. 2. It can be seen that the lattice parameter a decreases gradually up to x ¼ 0:15 and then keeps almost unchanged for x > 0:15. The composition dependence of a calculated from Vegard’s law is also shown as the dotted line in Fig. 2 in assumption that Yb is in the trivalent state [4]. It is obvious that a is much deviated from Vegard’s law although the matrix is essentially single Laves phase (Sm,Yb)Fe2 when x40:15. For 0:155x40:3, a remains almost unchanged because the added Yb enters into the PuNi3-type phase (Sm,Yb)Fe3. It seems reasonable to conclude that Yb exhibits the valence fluctuation (VF) effect and lies in the intermediate state [6]. Curie temperature Tc of the Laves phase (Sm,Yb)Fe2 compounds in Sm1xYbxFe2 increases with substitution of Yb for Sm up to x ¼ 0:15 and remains almost constant for 0:155x40:3
Z.-j. Guo et al. / Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
Fig. 2. Composition dependence of Curie temperature Tc and lattice parameter a for Sm1xYbxFe2. Open circle * denotes Curie temperature interpolated from that of SmFe2 and YbFe2 [4].
due to that the additional Yb may enter into the (Sm,Yb)Fe2 phase. According to Meyer et al. [4], Yb in the high-pressure induced phase YbFe2 is in the trivalent state and the Curie temperature of YbFe2 is 542 K, which is much lower than that of SmFe2 (676 K). Thus the interaction of Yb–Fe is much smaller than that of Sm–Fe. It is reasonably expected that the Tc of Sm1xYbxFe2 would decrease with increasing Yb content if Yb were in the trivalent state. The expected composition dependence of Tc0 of Sm1x(Yb3+)xFe2 is also shown by the dotted line in Fig. 2. This slight increase in Tc might be caused by the valence fluctuation effect of Yb ions, similar to the anomalies observed in composition dependence of lattice constant. In general, for RT2 (R=rare earth, T=transition metal) compounds, the T-3d state and R-5d state hybridize strongly to form bonding and antibonding bands, which can be described on the basis of itinerant electron model (IEM) [7]. According to the rigid band model (RBM), it is assumed that the delocalized 4f electron can change the d-electron concentration nd , keeping invariable the shape of the density of state NðEF Þ [8]. Due to Buschow et al. [9], Tc2 ¼ TF2 ðINðEF Þ 1Þ is valid. Here TF2 represents the degeneracy temperature, I denotes the effective Coulomb repulsion between the itinerant electrons, and NðEF Þ is the itinerant electron density
193
Fig. 3. X-ray diffraction spectra of (4 4 0) line of Sm1xYbxFe2.
of states at the Fermi level [9]. The Fermi level (EF ) is situated between the minimum and the maximum in the density of states curve. In this case, the increase of nd will lead to the increase of EF , thus TF or Tc , as that a small amount of Mn/Al substitution for Co can also enhance the magnetically ordering temperature of RCo2 [10]. Therefore, it is possible that the increase of Curie temperature of (Sm,Yb)Fe2 compounds is caused by the delocalized 4f electron due to the VF. It has been well known that the anisotropic magnetrostriction leads to crystal-structure distortions of magnetic materials. In particular, it is easy to calculate the spontaneous magnetostriction l1 1 1 for the rhombohedrally distorted material, e.g. SmFe2, from Eq. (1). The XRD spectra after deduction of Ka2 with a standard method for the compounds are shown in Fig. 3, in which the splitting of the (4 4 0) line is clearly observed for all the compounds. According to the splitting of (4 4 0) line, the composition dependence of the spontaneous magnetostriction l1 1 1 calculated from Eq. (1) is shown in Fig. 4(a). The field dependence of the magnetostriction l ¼ ðlk l? Þ is shown in Fig. 5. It is obvious that the saturation of the magnetostriction is not achieved up to the maximum field of 796 kA/m available. But the saturation magnetostriction ls can be obtained by means of the approaching saturation relation l ¼ ls ð1 A=HÞ when H ! 1, where A is a
194
Z.-j. Guo et al. / Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
Fig. 4. Composition dependence of (a) the spontaneous magnetostriction l1 1 1 and (b) the ratio of saturation magnetostrictions for SmFe2 to Sm1xYbxFe2.
discussed above, SmFe2 has an opposite anisotropy to that of YbFe2, and they form an anisotropy compensation system. It is anticipated that Yb substitution for Sm should decrease the anisotropy of SmFe2 and thus increase the magnetostriction. Therefore, it is not surprising that the magnetostriction can be enhanced in Sm1xYbxFe2 when Yb content is in the range of 04x40:05. With further substitution of Yb for Sm, both the magnetostriction l1 1 1 and ls are reduced and ls decreases more quickly than l1 1 1 (see Fig. 4). This decrease of the magnetostriction when x50:05 is in a good agreement with Clark’s result [3], which might be caused by the lower magnetostriction of YbFe2 than that of SmFe2 at room temperature. In conclusion, it is found that a small amount of Yb substitution for Sm can increase the magnetostriction in Sm1xYbxFe2 due to the anisotropy reduction. The unusual composition dependence of the lattice parameters, magnetic ordering temperatures have been recorded and might be ascribed to the valence fluctuation effect of Yb ions. This work has been supported by the Projects No. 59725103 and 59871054 of the National Natural Sciences Foundation of China and by the Science and Technology Commission of Shenyang and Liaoning. References
Fig. 5. Magnetic field dependence of magnetostriction l of Sm1xYbxFe2.
constant. The composition dependence of the saturation magnetostriction ls for the present samples with respect to that of SmFe2 is shown in Fig. 4(b). Clearly, the curves in Figs. 4(a) and (b) demonstrate a similar variation, exhibiting a maximum when Yb content was 0.05. It is reported that a small amount of Pr replacement with Tb can increase the spontaneous and polycrystalline magnetostriction of TbFe2 at low field due to the reduction of anisotropy [11]. As
[1] J.R. Cullen, Scr. Metall. Mater. 32 (1995) 1849. [2] D.C. Jiles, J.B. Thoelke, J. Magn. Magn. Mater. 134 (1994) 143. [3] A.E. Clark, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Vol. 1, North-Holland, Amsterdam, 1980, p. 531. [4] C. Meyer, F. Hartmann-Boutron, Y. Gros, Y. Berthier, J.L. Burvoz, J. Phys. 42 (1981) 605. [5] R.Z. Levitin, A.S. Markosyan, J. Magn. Magn. Mater. 84 (1990) 247. [6] D.T. Adroja, S.K. Malik, J. Magn. Magn. Mater. 100 (1991) 126. . M.S.S. Brooks, B. Johansson, [7] O. Eriksson, L. Nordstrom, Phys. Rev. Lett. 60 (1988) 2523. [8] A.R. Piercy, K.N.R. Taylor, J. Phys. C 1 (1968) 1112. [9] K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179. [10] R. Ballou, A.S. Markosyan, I.S. Dubenko, R.Z. Levitin, J. Magn. Magn. Mater. 110 (1992) 209. [11] B.W. Wang, L.C. Cheng, K.Y. He, J. Appl. Phys. 80 (1996) 6903.
Magnetostriction enhancement in Laves compounds (Sm,Yb)Fe2 Zhi-jun Guoa,*, Z.D. Zhanga, S.C. Busbridgeb, B.W. Wanga, X.G. Zhaoa, D.Y. Genga a
International Center for Materials Physics, Chinese Academy of Sciences, Institute of Metal Research, Academia Sinica, 72 Wenhua Road, Shenyang 110015, People’s Republic of China b School of Engineering, University of Brighton, BN2 4GJ, UK Received 1 November 2000
Abstract The magnetic and magnetostrictive properties of compounds (Sm1xYbx)Fe2 (04x40:3) have been studied. When x50:15, the compounds crystallize in the cubic structure of a C15 MgCu2-type Laves phase. For x > 0:15 a small amount of (Sm,Yb)Fe3 compound with a PuNi3-type structure appears and its amount increases with increasing Yb content. The magnetic ordering of 1 : 2 compounds is slightly strengthened by the substitution of Yb for Sm. Lattice parameters for Laves phase compounds keep almost unchanged for the whole range investigated, indicating the deviation from Vegard’s law linear variation with composition. A magnetostriction enhancement of about 15% when x ¼ 0:05% both in spontaneous and polycrystalline magnetostrictions in Sm0.95Yb0.05Fe2 has been observed, which can be ascribed to the anisotropy compensation effect. # 2001 Elsevier Science B.V. All rights reserved. PACS: 75.80.+q; 81.40.z Keywords: Magnetostriction; Rare earths; Laves compounds
Magnetostriction, i.e. so-called Joule’s effect, is one of the intrinsic properties of magnetic materials [1]. The magnetostrictive materials have found their industrial applications as energy transforming devices, e.g. transducers, sensors and actuators [2]. Much attention has been paid to rare earths or their compounds with transition metals because of their giant intrinsic magnetostriction [3]. Low magnetic fields are required for the practical applications in industries. Since rare *Corresponding author. Fax: +86-24-2389-1320. E-mail address: [email protected] (Z.-j. Guo).
earth transition metal compounds of 1 : 2 type, e.g. TbFe2, SmFe2, have high anisotropies at room temperature, a much higher field is necessary for large magnetostriction. Clark et al. proposed [3] that the compounds with opposite anisotropy can reduce their anisotropic energy while maintaining high magnetostriction. Therefore, the so-called pseudobinary compound, R11xR2xFe2 (R1,R2=rare earths), was invented. In particular, (Tb0.7Dy0.3)Fe2 (Terfenol-D) has become a popular function material in practical applications. Relatively speaking, little work has been done on SmFe2 although it has at room temperature the
0304-8853/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 0 6 0 - 9
192
Z.-j. Guo et al. / Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
highest negative magnetostriction. According to the single-ion model, YbFe2 (l1 1 1 ¼ 3600 ppm) has even larger magnetostriction than SmFe2 (l1 1 1 ¼ 3200 ppm) [3]. In addition, YbFe2 has an opposite anisotropy to that of SmFe2. Therefore, Sm1xYbxFe2 is an ideal anisotropy compensation system. However, little information is available on this system since YbFe2 can only be synthesized at high pressure [3,4]. In this paper, we reported the structure, magnetic properties and magnetostriction enhancement effect of Sm1xYbxFe2. All samples of Sm1xYbxFe2 for x ¼ 0, 0.05, 0.1, 0.15, 0.2 and 0.3 were prepared by arc melting appropriate metals in a magneto-controlled arc furnace under high purity argon atmosphere. This procedure has been repeated several times for the homogeneity of samples. The purity of Sm, Yb and Fe is 99.9%, Co 99.8%. The ingots were homogenized under high purity argon atmosphere at 8208C for 50 h. X-ray diffraction (XRD) patterns were recorded at room temperature with CuKa radiation in a D/max-gA diffractometer with a pyrolytic monochromator (Fig. 1). The lattice parameters were determined and the absolute error of the XRD measurement was less than 0.0002 nm. Curie temperatures Tc were determined by means of AC initial susceptibility.
Fig. 1. X-ray diffraction patterns of Sm1xYbxFe2 annealed at 8208C for 50 h.
The samples in powder form were step scanned with CuKa radiation by the X-ray diffractometer at a high Bragg angle 2y radiation ranging from 708 to 748 in order to study the cubic (4 4 0) line. The spontaneous magnetostriction l1 1 1 of the compounds was calculated by l1 1 1 ¼ Da;
ð1Þ
where Da is the deviation of the angle between neighboring edges of the distorted cube from p=2 [5]. Magnetostrictions lk and l? of the samples with a size of 4 8 8 mm3 parallel and perpendicular to the applied field up to 796 kA/m were measured at room temperature by using standard strain gauage techniques. The saturation magnetostriction ls was obtained by the relation ls ¼ 23ðljj þ l? Þ. XRD data indicate that the homogenized alloys of Sm1xYbxFe2 are essentially the single laves phase (Sm,Yb)Fe2 (space group Fd3 m) with C15 MgCu2-type cubic structure when x40:15. For x > 0:15, a non-cubic phase (Sm,Yb)Fe3 with PuNi3 structure appears and its amount increases progressively with increasing Yb content up to 0.3. This is due to that YbFe2 can only be formed in high pressure [4], in a good agreement with that of Clark et al. [3]. The composition dependence of lattice parameter a of (Sm,Yb)Fe2 compounds in Sm1xYbxFe2 is shown in Fig. 2. It can be seen that the lattice parameter a decreases gradually up to x ¼ 0:15 and then keeps almost unchanged for x > 0:15. The composition dependence of a calculated from Vegard’s law is also shown as the dotted line in Fig. 2 in assumption that Yb is in the trivalent state [4]. It is obvious that a is much deviated from Vegard’s law although the matrix is essentially single Laves phase (Sm,Yb)Fe2 when x40:15. For 0:155x40:3, a remains almost unchanged because the added Yb enters into the PuNi3-type phase (Sm,Yb)Fe3. It seems reasonable to conclude that Yb exhibits the valence fluctuation (VF) effect and lies in the intermediate state [6]. Curie temperature Tc of the Laves phase (Sm,Yb)Fe2 compounds in Sm1xYbxFe2 increases with substitution of Yb for Sm up to x ¼ 0:15 and remains almost constant for 0:155x40:3
Z.-j. Guo et al. / Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
Fig. 2. Composition dependence of Curie temperature Tc and lattice parameter a for Sm1xYbxFe2. Open circle * denotes Curie temperature interpolated from that of SmFe2 and YbFe2 [4].
due to that the additional Yb may enter into the (Sm,Yb)Fe2 phase. According to Meyer et al. [4], Yb in the high-pressure induced phase YbFe2 is in the trivalent state and the Curie temperature of YbFe2 is 542 K, which is much lower than that of SmFe2 (676 K). Thus the interaction of Yb–Fe is much smaller than that of Sm–Fe. It is reasonably expected that the Tc of Sm1xYbxFe2 would decrease with increasing Yb content if Yb were in the trivalent state. The expected composition dependence of Tc0 of Sm1x(Yb3+)xFe2 is also shown by the dotted line in Fig. 2. This slight increase in Tc might be caused by the valence fluctuation effect of Yb ions, similar to the anomalies observed in composition dependence of lattice constant. In general, for RT2 (R=rare earth, T=transition metal) compounds, the T-3d state and R-5d state hybridize strongly to form bonding and antibonding bands, which can be described on the basis of itinerant electron model (IEM) [7]. According to the rigid band model (RBM), it is assumed that the delocalized 4f electron can change the d-electron concentration nd , keeping invariable the shape of the density of state NðEF Þ [8]. Due to Buschow et al. [9], Tc2 ¼ TF2 ðINðEF Þ 1Þ is valid. Here TF2 represents the degeneracy temperature, I denotes the effective Coulomb repulsion between the itinerant electrons, and NðEF Þ is the itinerant electron density
193
Fig. 3. X-ray diffraction spectra of (4 4 0) line of Sm1xYbxFe2.
of states at the Fermi level [9]. The Fermi level (EF ) is situated between the minimum and the maximum in the density of states curve. In this case, the increase of nd will lead to the increase of EF , thus TF or Tc , as that a small amount of Mn/Al substitution for Co can also enhance the magnetically ordering temperature of RCo2 [10]. Therefore, it is possible that the increase of Curie temperature of (Sm,Yb)Fe2 compounds is caused by the delocalized 4f electron due to the VF. It has been well known that the anisotropic magnetrostriction leads to crystal-structure distortions of magnetic materials. In particular, it is easy to calculate the spontaneous magnetostriction l1 1 1 for the rhombohedrally distorted material, e.g. SmFe2, from Eq. (1). The XRD spectra after deduction of Ka2 with a standard method for the compounds are shown in Fig. 3, in which the splitting of the (4 4 0) line is clearly observed for all the compounds. According to the splitting of (4 4 0) line, the composition dependence of the spontaneous magnetostriction l1 1 1 calculated from Eq. (1) is shown in Fig. 4(a). The field dependence of the magnetostriction l ¼ ðlk l? Þ is shown in Fig. 5. It is obvious that the saturation of the magnetostriction is not achieved up to the maximum field of 796 kA/m available. But the saturation magnetostriction ls can be obtained by means of the approaching saturation relation l ¼ ls ð1 A=HÞ when H ! 1, where A is a
194
Z.-j. Guo et al. / Journal of Magnetism and Magnetic Materials 231 (2001) 191–194
Fig. 4. Composition dependence of (a) the spontaneous magnetostriction l1 1 1 and (b) the ratio of saturation magnetostrictions for SmFe2 to Sm1xYbxFe2.
discussed above, SmFe2 has an opposite anisotropy to that of YbFe2, and they form an anisotropy compensation system. It is anticipated that Yb substitution for Sm should decrease the anisotropy of SmFe2 and thus increase the magnetostriction. Therefore, it is not surprising that the magnetostriction can be enhanced in Sm1xYbxFe2 when Yb content is in the range of 04x40:05. With further substitution of Yb for Sm, both the magnetostriction l1 1 1 and ls are reduced and ls decreases more quickly than l1 1 1 (see Fig. 4). This decrease of the magnetostriction when x50:05 is in a good agreement with Clark’s result [3], which might be caused by the lower magnetostriction of YbFe2 than that of SmFe2 at room temperature. In conclusion, it is found that a small amount of Yb substitution for Sm can increase the magnetostriction in Sm1xYbxFe2 due to the anisotropy reduction. The unusual composition dependence of the lattice parameters, magnetic ordering temperatures have been recorded and might be ascribed to the valence fluctuation effect of Yb ions. This work has been supported by the Projects No. 59725103 and 59871054 of the National Natural Sciences Foundation of China and by the Science and Technology Commission of Shenyang and Liaoning. References
Fig. 5. Magnetic field dependence of magnetostriction l of Sm1xYbxFe2.
constant. The composition dependence of the saturation magnetostriction ls for the present samples with respect to that of SmFe2 is shown in Fig. 4(b). Clearly, the curves in Figs. 4(a) and (b) demonstrate a similar variation, exhibiting a maximum when Yb content was 0.05. It is reported that a small amount of Pr replacement with Tb can increase the spontaneous and polycrystalline magnetostriction of TbFe2 at low field due to the reduction of anisotropy [11]. As
[1] J.R. Cullen, Scr. Metall. Mater. 32 (1995) 1849. [2] D.C. Jiles, J.B. Thoelke, J. Magn. Magn. Mater. 134 (1994) 143. [3] A.E. Clark, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Vol. 1, North-Holland, Amsterdam, 1980, p. 531. [4] C. Meyer, F. Hartmann-Boutron, Y. Gros, Y. Berthier, J.L. Burvoz, J. Phys. 42 (1981) 605. [5] R.Z. Levitin, A.S. Markosyan, J. Magn. Magn. Mater. 84 (1990) 247. [6] D.T. Adroja, S.K. Malik, J. Magn. Magn. Mater. 100 (1991) 126. . M.S.S. Brooks, B. Johansson, [7] O. Eriksson, L. Nordstrom, Phys. Rev. Lett. 60 (1988) 2523. [8] A.R. Piercy, K.N.R. Taylor, J. Phys. C 1 (1968) 1112. [9] K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179. [10] R. Ballou, A.S. Markosyan, I.S. Dubenko, R.Z. Levitin, J. Magn. Magn. Mater. 110 (1992) 209. [11] B.W. Wang, L.C. Cheng, K.Y. He, J. Appl. Phys. 80 (1996) 6903.