- Email: [email protected]
The magnetocrystalline anisotropy in YTi(Fe,Co)11 single crystals
Journal of Alloys and Compounds 283 (1999) 45–48
L
The magnetocrystalline anisotropy in YTi(Fe,Co) 11 single crystals a, a a a b b I.S. Tereshina *, S.A. Nikitin , I.V. Telegina , V.V. Zubenko , Yu.G. Pastushenkov , K.P. Skokov a
Physics Department of Moscow State University, Moscow, Russia b Physics Department of Tver State University, Tver, Russia
Received 19 August 1998; received in revised form 1 October 1998
Abstract The effect of the substitution of Co for Fe on the magnetic anisotropy in YTiFe 112x Cox (0#x#5) single crystals has been studied by using torque and vibrating-sample magnetometry techniques. The K1 and K2 values were calculated. The Co ion gives a resultant anisotropy which is opposite to that of Fe. A competition is expected to take place in the composition range 5,x,6. The obtained results indicate that the itinerant magnetism of cobalt sublattice produces an effect of magnetic anisotropy as a result of electronic structure change in these alloys. 1999 Elsevier Science S.A. All rights reserved. Keywords: Intermetallic compounds; Single crystal; Magnetic anisotropy constant
1. Introduction Intermetallic compounds of RFe 11 Ti (R5rare earth) with tetragonal ThMn 12 -type structure have attracted much attention because of their interesting magnetic properties [1–3] and their potential application as permanent magnets. It is well known, that the origin of the high coercive force in the rare-earth magnets basically is due to the strong easy-axis magnetocrystalline anisotropy (MCA). The MCA of the RFe 11 Ti compounds can be described as a sum of contributions from the rare earth and 3dtransition metal sublattices. The rare-earth contribution can be analyzed in a single-ion model rather well. The iron contribution is more complex mainly because of the itinerant nature of the 3d electrons. Another complication is the number of crystallographical inequivalent sites, which is larger for the 3d sites (Fe occupy the 8i, 8j, 8f sites in the ThMn 12 -type structure) compared to the rareearth sites (R fills only the 2a site). To understand the magnetic properties of the RFe 11 Ti compounds it is important to examine the properties of the 3d sublattice magnetism. In the YFe 11 Ti compound, where the rareearth-like Y atom has a closed shell and therefore makes
*Corresponding author. [email protected].
Fax:
17-095-932-8820;
e-mail:
no contribution to the anisotropy, the measured uniaxial anisotropy constant K1 is about (1.960.1)310 7 erg / cm 3 at T54.2 K [4] and is due entirely to the Fe atoms. A standard approach to improving iron-based permanent magnet material is to add small amounts of cobalt. The substitution effects of Co in RFe 11 Ti have been investigated in several laboratories [5–9] and the Curie temperature and saturation magnetization were found to increase with substitution of Co for Fe. However, reliable values of the anisotropy constants have not been obtained because, for such information, single crystals are necessary. This paper reports the basic magnetic properties of the YTi(Fe,Co) 11 compounds measured on single crystals. The concentration and temperature dependencies of the anisotropy constant are discussed.
2. Experimental Samples of YTi(Fe 112x Co x ) (x50,1,2,3,4,5) have been prepared by induction melting 99.95% pure primary materials in argon atmosphere. The experimental details employed in this study are essentially the same as those described in our previous papers [4,10]. Unfortunately, for high Co concentrations (x.5) the samples were multiphase
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00884-6
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I.S. Tereshina et al. / Journal of Alloys and Compounds 283 (1999) 45 – 48
and single crystals were not found in the cast alloys thus obtained.
3. Results and discussion
3.1. Structural properties The X-ray diffraction patterns of the YTiFe 112x Co x alloys showed that for the entire composition range (x50– 5) the alloys were isostructural. They crystallize in the body-centered tetragonal ThMn 12 -type crystal structure. Fig. 1 shows the concentration dependence of the lattice parameters a and c determined at room temperature. Our results for the compounds YTiFe 112x Co x agree well with the data obtained earlier [8,11]. The measured lattice parameters were found to decrease monotonically with increasing cobalt concentration. The decrease of the lattice parameters occurs because the metallic radius of cobalt ˚ is smaller than that of iron (r Fe 51.26 A). ˚ (r Co 51.25 A)
3.2. Magnetic properties The values of the saturation magnetization sS (emu / g) and MS (m B / f.u.), average moment of every magnetic atom m3d (m B / f.u.) at T577 K and Curie temperature are collected in Table 1. The average moment per Fe atom for YFe 11 Ti is approximately 1.69 m B . This is smaller than the
Table 1 Magnetic characteristics of YTiFe 112x Co x single crystals at T577 K Composition x
sS emu / g
MS m B / f.u.
m3d m B / f.u.
TC K
0 1 2 3 4 5
138 143 145 146 140 135
18.565 19.316 19.667 19.883 19.143 18.534
1.687 1.756 1.787 1.807 1.74 1.684
538 645 740 828 895 943
value of 2.2 m B in Fe metal. It can therefore be inferred that the 3d magnetism in these compounds will exhibit a complex character and that the magnetic properties cannot be explained in a localized magnetism picture alone. Increasing the cobalt content leads initially to an increase in saturation magnetization with a maximum at x53. The occurrence of a maximum in MS is not only characteristic of Fe–Co binary alloys but occurs also in R 2 (Fe,Co) 17 and R 2 (Fe,Co) 14 B systems. This can, as for the Slater–Pauling curve, be explained in terms of the rigid band model. The Curie temperatures T C of the YTiFe 112x Co x compounds were determined from the thermomagnetic analysis at low external fields (1.5 kOe) in the temperature range 295–1000 K using a standard vibrating-sample magnetometry (VSM). As shown in Table 1, the T C increases drastically when Fe is replaced by Co. As discussed earlier [10], the decrease of interatomic distances and a preferential substitution of Co for Fe (8j and 8f at x#5) [11] in regions where there is negative exchange may account for a strengthening of the overall exchange interaction and thus for an enhancement of T C . It is well known, in the case of tetragonal crystals, where the anisotropy within the basal plane can be neglected, i.e. K3 is approximately zero, the free energy is given by: Fa 5 K1 sin 2u 1 K2 sin 4u The temperature dependence of magnetic anisotropy constants K1 and K2 in the range 77–1000 K have been determined for single crystals using torque and vibratingsample magnetometry techniques. The easy magnetization direction of YTiFe 112x Co single crystals was found to coincide with the c axis for x#5 in the whole investigated temperature range. For a stable configuration in which the easy magnetization axis is parallel to the tetragonal c-axis, the anisotropy field is: HA 5 2(K1 1 2K2 ) /MS
Fig. 1. The composition dependence of the lattice parameters a and c determined at room temperature.
The HA values measured at different temperatures are listed in Table 2. In order to take into account the large variation of T C values caused by Co substitution, the anisotropy field HA and anisotropy constant K1 are reported both versus a reduced temperature T /T C and a fixed
I.S. Tereshina et al. / Journal of Alloys and Compounds 283 (1999) 45 – 48
47
Table 2 Values of the anisotropy field HA for different temperatures Compound
HA (T5300 K) kOe
HA (T577 K) kOe
HA (T /T C 50.25) kOe
YFe 11 Ti YFe 10 CoTi YFe 9 Co 2 Ti YFe 8 Co 3 Ti YFe 7 Co 4 Ti YFe 6 Co 5 Ti
21 22 20 17 13 6
42 43 40 31 21 11
41 38 33 21 15 11
temperature. The concentration dependence of the anisotropy constant K1 and K2 at T5300 K and T /T C 50.25 are plotted in Fig. 2. A weak maximum in the anisotropy is observed as a function of the Co concentration at x51 at T577 K and T5300 K for the YTiFe 112x Co x single crystals. At x.1 K1 rapidly decreases with the increasing Co concentration. From Table 1 and Fig. 2 it can also be deduced that after the substitution of about 50% Co for Fe the anisotropy constant K1 changes its sign giving rise to a change in the easy magnetisation direction from axis to cone or plane. K2 increases with increasing Co content. As it is shown in Fig. 2 the concentration behavior of K1 depends on the temperature at which the comparison is made. At T /T C 50.25 K1 decreases monotonically with increasing temperature. This indicates that Co gives an average negative contribution to the anisotropy constant K1 . Fig. 3 shows the dependence of the anisotropy constant K1 on the reduced temperature for YTiFe 112x Co x for various Co concentrations. The variation of the anisotropy versus T /T C is linear in the intermediate temperature range. The analysis of the temperature dependence of the anisotropy shows that the slope of K1 decreases with
Fig. 3. Dependence of the anisotropy constant on the reduced temperature for YTi(Fe,Co) 11 for various Co contents.
increasing Co concentration and at x55 the values of K1 change very modestly with increasing temperature. A spin–reorientation transition (SRT) at x.5 can be predicted for increasing Co concentration as a consequence of the competition between the Fe and Co anisotropies. This SRT was observed for the YTiFe 112x Cox compounds for 5,x,6 [7]. A neutron-diffraction study has been carried out on YTi(Fe 0,5 Co 0,5 ) 11 [11]. A strong preferential occupation on 8f and 8j sites for Co atoms was observed, which may affect the magnetic anisotropies of the YTi(Fe,Co) 11 ¨ compounds. Besides, 57 Fe Mossbauer spectra of YTi(Fe,Co) 11 alloys at T5300 K have been studied by Li et al. [12]. They found, that both hyperfine fields for the three sites and the average field kHhf l increase at first, and then decrease with increasing Co concentration. The maximum of kHhf l and the average moments of Fe atoms, mFe is 27.7 T and 1.91 mB , respectively, at x50.6 for YTi(Fe 12x Co x ) 11 . It is interesting, that the average moments of the Co atoms, mCo 51.3 m B , are almost constant over the whole investigated composition range (x50–0.8) [12]. The hyperfine fields acting on the iron and cobalt nuclei in the rare-earth-3d transition metal compounds are known to be anisotropic [13]. The change in the hyperfine field arises from the change in local environment around the Fe sites in the ThMn 12 structure, when Fe is replaced by Co in these compounds. Consequently, the individual site anisotropy of the Fe ions may change too, leading to the macroscopic behavior that we have observed.
4. Conclusion
Fig. 2. Composition dependencies of the anisotropy constant 1: K1 (x) and 2: K2 (x) for YTi(Fe,Co) 11 at T5300 K and 3: K1 at T /T C 50.25.
The main conclusion of our work is that the average anisotropy of the f and j sites, when occupied by Co, is negative. In addition, the 3d electrons in RTi(Fe,Co) 11 have essentially a delocalized character, hence it is reason-
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I.S. Tereshina et al. / Journal of Alloys and Compounds 283 (1999) 45 – 48
able to use band theory of magnetism to account for their magnetic properties: namely, both magnetization values and MCA.
Acknowledgements We are indebted to T.I. Ivanova for helpful discussions. The work has been supported by the Federal Program on Support of Leading Scientific Schools N96-15-96429 and RFBR Grant N96-02-18-271.
References [1] B.P. Hu, H.S. Li, J.P. Gavigan, J.M.D. Coey, J. Phys.: Condens. Matter 1 (1989) 755. ¨ [2] X.C. Kou, T.S. Zhao, R. Grossinger, H.R. Kirchmayr, X. Li, F.R. de Boer, Phys. Rev. B 47 (1993) 3231.
¨ [3] K.Yu. Guslienko, X.C. Kou, R. Grossinger, J. Magn. Magn. Mater. 150 (1995) 383. [4] S.A. Nikitin, I.S. Tereshina, V.N. Verbetsky, A.A. Salamova, Phys. Solid State (in Russian) 40(2) (1998) 285. [5] V.K. Sihna, S.F. Cheng, W.E. Wallace, S.G. Sankar, JMMM 81 (1989) 227. [6] M. Solzi, R.H. Xue, L. Pareti, JMMM 88 (1990) 44. [7] S.F. Cheng, V.K. Sinha, B.M. Ma, S.G. Sankar, W.E. Wallace, J. Appl. Phys. 69(8) (1991) 5605. [8] Y.-c. Yang, S. Hong, Z. Zhen-yong, L. Tong, G. Jian-liang, Solid State Commun. 68(2) (1988) 175. [9] C.V. Thang, N.P. Thuy, N.M. Hong, T.D. Hien, N.S. Almodova, R. ¨ Grossinger, J. Magn. Magn. Mater. 140–144 (1995) 1017. [10] I.S. Tereshina, S.A. Nikitin, T.I. Ivanova, K.P. Skokov J. Alloys Comp. (1998) in press. [11] Y.-C. Yang, H. Sun, L.-S. Kong, J.-L. Yang, Y.-F. Ding, B.-S. Zhang, C.-T. Ye, L. Jin, J. Appl. Phys. 67(9) (1990) 4632. [12] Z.W. Li, X.Z. Zhou, A.H. Morrish, J. Appl. Phys. 69(8) (1991) 5602. [13] R.L. Steever, Phys. Rev. B 19 (1979) 2704.
L
The magnetocrystalline anisotropy in YTi(Fe,Co) 11 single crystals a, a a a b b I.S. Tereshina *, S.A. Nikitin , I.V. Telegina , V.V. Zubenko , Yu.G. Pastushenkov , K.P. Skokov a
Physics Department of Moscow State University, Moscow, Russia b Physics Department of Tver State University, Tver, Russia
Received 19 August 1998; received in revised form 1 October 1998
Abstract The effect of the substitution of Co for Fe on the magnetic anisotropy in YTiFe 112x Cox (0#x#5) single crystals has been studied by using torque and vibrating-sample magnetometry techniques. The K1 and K2 values were calculated. The Co ion gives a resultant anisotropy which is opposite to that of Fe. A competition is expected to take place in the composition range 5,x,6. The obtained results indicate that the itinerant magnetism of cobalt sublattice produces an effect of magnetic anisotropy as a result of electronic structure change in these alloys. 1999 Elsevier Science S.A. All rights reserved. Keywords: Intermetallic compounds; Single crystal; Magnetic anisotropy constant
1. Introduction Intermetallic compounds of RFe 11 Ti (R5rare earth) with tetragonal ThMn 12 -type structure have attracted much attention because of their interesting magnetic properties [1–3] and their potential application as permanent magnets. It is well known, that the origin of the high coercive force in the rare-earth magnets basically is due to the strong easy-axis magnetocrystalline anisotropy (MCA). The MCA of the RFe 11 Ti compounds can be described as a sum of contributions from the rare earth and 3dtransition metal sublattices. The rare-earth contribution can be analyzed in a single-ion model rather well. The iron contribution is more complex mainly because of the itinerant nature of the 3d electrons. Another complication is the number of crystallographical inequivalent sites, which is larger for the 3d sites (Fe occupy the 8i, 8j, 8f sites in the ThMn 12 -type structure) compared to the rareearth sites (R fills only the 2a site). To understand the magnetic properties of the RFe 11 Ti compounds it is important to examine the properties of the 3d sublattice magnetism. In the YFe 11 Ti compound, where the rareearth-like Y atom has a closed shell and therefore makes
*Corresponding author. [email protected].
Fax:
17-095-932-8820;
e-mail:
no contribution to the anisotropy, the measured uniaxial anisotropy constant K1 is about (1.960.1)310 7 erg / cm 3 at T54.2 K [4] and is due entirely to the Fe atoms. A standard approach to improving iron-based permanent magnet material is to add small amounts of cobalt. The substitution effects of Co in RFe 11 Ti have been investigated in several laboratories [5–9] and the Curie temperature and saturation magnetization were found to increase with substitution of Co for Fe. However, reliable values of the anisotropy constants have not been obtained because, for such information, single crystals are necessary. This paper reports the basic magnetic properties of the YTi(Fe,Co) 11 compounds measured on single crystals. The concentration and temperature dependencies of the anisotropy constant are discussed.
2. Experimental Samples of YTi(Fe 112x Co x ) (x50,1,2,3,4,5) have been prepared by induction melting 99.95% pure primary materials in argon atmosphere. The experimental details employed in this study are essentially the same as those described in our previous papers [4,10]. Unfortunately, for high Co concentrations (x.5) the samples were multiphase
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00884-6
46
I.S. Tereshina et al. / Journal of Alloys and Compounds 283 (1999) 45 – 48
and single crystals were not found in the cast alloys thus obtained.
3. Results and discussion
3.1. Structural properties The X-ray diffraction patterns of the YTiFe 112x Co x alloys showed that for the entire composition range (x50– 5) the alloys were isostructural. They crystallize in the body-centered tetragonal ThMn 12 -type crystal structure. Fig. 1 shows the concentration dependence of the lattice parameters a and c determined at room temperature. Our results for the compounds YTiFe 112x Co x agree well with the data obtained earlier [8,11]. The measured lattice parameters were found to decrease monotonically with increasing cobalt concentration. The decrease of the lattice parameters occurs because the metallic radius of cobalt ˚ is smaller than that of iron (r Fe 51.26 A). ˚ (r Co 51.25 A)
3.2. Magnetic properties The values of the saturation magnetization sS (emu / g) and MS (m B / f.u.), average moment of every magnetic atom m3d (m B / f.u.) at T577 K and Curie temperature are collected in Table 1. The average moment per Fe atom for YFe 11 Ti is approximately 1.69 m B . This is smaller than the
Table 1 Magnetic characteristics of YTiFe 112x Co x single crystals at T577 K Composition x
sS emu / g
MS m B / f.u.
m3d m B / f.u.
TC K
0 1 2 3 4 5
138 143 145 146 140 135
18.565 19.316 19.667 19.883 19.143 18.534
1.687 1.756 1.787 1.807 1.74 1.684
538 645 740 828 895 943
value of 2.2 m B in Fe metal. It can therefore be inferred that the 3d magnetism in these compounds will exhibit a complex character and that the magnetic properties cannot be explained in a localized magnetism picture alone. Increasing the cobalt content leads initially to an increase in saturation magnetization with a maximum at x53. The occurrence of a maximum in MS is not only characteristic of Fe–Co binary alloys but occurs also in R 2 (Fe,Co) 17 and R 2 (Fe,Co) 14 B systems. This can, as for the Slater–Pauling curve, be explained in terms of the rigid band model. The Curie temperatures T C of the YTiFe 112x Co x compounds were determined from the thermomagnetic analysis at low external fields (1.5 kOe) in the temperature range 295–1000 K using a standard vibrating-sample magnetometry (VSM). As shown in Table 1, the T C increases drastically when Fe is replaced by Co. As discussed earlier [10], the decrease of interatomic distances and a preferential substitution of Co for Fe (8j and 8f at x#5) [11] in regions where there is negative exchange may account for a strengthening of the overall exchange interaction and thus for an enhancement of T C . It is well known, in the case of tetragonal crystals, where the anisotropy within the basal plane can be neglected, i.e. K3 is approximately zero, the free energy is given by: Fa 5 K1 sin 2u 1 K2 sin 4u The temperature dependence of magnetic anisotropy constants K1 and K2 in the range 77–1000 K have been determined for single crystals using torque and vibratingsample magnetometry techniques. The easy magnetization direction of YTiFe 112x Co single crystals was found to coincide with the c axis for x#5 in the whole investigated temperature range. For a stable configuration in which the easy magnetization axis is parallel to the tetragonal c-axis, the anisotropy field is: HA 5 2(K1 1 2K2 ) /MS
Fig. 1. The composition dependence of the lattice parameters a and c determined at room temperature.
The HA values measured at different temperatures are listed in Table 2. In order to take into account the large variation of T C values caused by Co substitution, the anisotropy field HA and anisotropy constant K1 are reported both versus a reduced temperature T /T C and a fixed
I.S. Tereshina et al. / Journal of Alloys and Compounds 283 (1999) 45 – 48
47
Table 2 Values of the anisotropy field HA for different temperatures Compound
HA (T5300 K) kOe
HA (T577 K) kOe
HA (T /T C 50.25) kOe
YFe 11 Ti YFe 10 CoTi YFe 9 Co 2 Ti YFe 8 Co 3 Ti YFe 7 Co 4 Ti YFe 6 Co 5 Ti
21 22 20 17 13 6
42 43 40 31 21 11
41 38 33 21 15 11
temperature. The concentration dependence of the anisotropy constant K1 and K2 at T5300 K and T /T C 50.25 are plotted in Fig. 2. A weak maximum in the anisotropy is observed as a function of the Co concentration at x51 at T577 K and T5300 K for the YTiFe 112x Co x single crystals. At x.1 K1 rapidly decreases with the increasing Co concentration. From Table 1 and Fig. 2 it can also be deduced that after the substitution of about 50% Co for Fe the anisotropy constant K1 changes its sign giving rise to a change in the easy magnetisation direction from axis to cone or plane. K2 increases with increasing Co content. As it is shown in Fig. 2 the concentration behavior of K1 depends on the temperature at which the comparison is made. At T /T C 50.25 K1 decreases monotonically with increasing temperature. This indicates that Co gives an average negative contribution to the anisotropy constant K1 . Fig. 3 shows the dependence of the anisotropy constant K1 on the reduced temperature for YTiFe 112x Co x for various Co concentrations. The variation of the anisotropy versus T /T C is linear in the intermediate temperature range. The analysis of the temperature dependence of the anisotropy shows that the slope of K1 decreases with
Fig. 3. Dependence of the anisotropy constant on the reduced temperature for YTi(Fe,Co) 11 for various Co contents.
increasing Co concentration and at x55 the values of K1 change very modestly with increasing temperature. A spin–reorientation transition (SRT) at x.5 can be predicted for increasing Co concentration as a consequence of the competition between the Fe and Co anisotropies. This SRT was observed for the YTiFe 112x Cox compounds for 5,x,6 [7]. A neutron-diffraction study has been carried out on YTi(Fe 0,5 Co 0,5 ) 11 [11]. A strong preferential occupation on 8f and 8j sites for Co atoms was observed, which may affect the magnetic anisotropies of the YTi(Fe,Co) 11 ¨ compounds. Besides, 57 Fe Mossbauer spectra of YTi(Fe,Co) 11 alloys at T5300 K have been studied by Li et al. [12]. They found, that both hyperfine fields for the three sites and the average field kHhf l increase at first, and then decrease with increasing Co concentration. The maximum of kHhf l and the average moments of Fe atoms, mFe is 27.7 T and 1.91 mB , respectively, at x50.6 for YTi(Fe 12x Co x ) 11 . It is interesting, that the average moments of the Co atoms, mCo 51.3 m B , are almost constant over the whole investigated composition range (x50–0.8) [12]. The hyperfine fields acting on the iron and cobalt nuclei in the rare-earth-3d transition metal compounds are known to be anisotropic [13]. The change in the hyperfine field arises from the change in local environment around the Fe sites in the ThMn 12 structure, when Fe is replaced by Co in these compounds. Consequently, the individual site anisotropy of the Fe ions may change too, leading to the macroscopic behavior that we have observed.
4. Conclusion
Fig. 2. Composition dependencies of the anisotropy constant 1: K1 (x) and 2: K2 (x) for YTi(Fe,Co) 11 at T5300 K and 3: K1 at T /T C 50.25.
The main conclusion of our work is that the average anisotropy of the f and j sites, when occupied by Co, is negative. In addition, the 3d electrons in RTi(Fe,Co) 11 have essentially a delocalized character, hence it is reason-
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I.S. Tereshina et al. / Journal of Alloys and Compounds 283 (1999) 45 – 48
able to use band theory of magnetism to account for their magnetic properties: namely, both magnetization values and MCA.
Acknowledgements We are indebted to T.I. Ivanova for helpful discussions. The work has been supported by the Federal Program on Support of Leading Scientific Schools N96-15-96429 and RFBR Grant N96-02-18-271.
References [1] B.P. Hu, H.S. Li, J.P. Gavigan, J.M.D. Coey, J. Phys.: Condens. Matter 1 (1989) 755. ¨ [2] X.C. Kou, T.S. Zhao, R. Grossinger, H.R. Kirchmayr, X. Li, F.R. de Boer, Phys. Rev. B 47 (1993) 3231.
¨ [3] K.Yu. Guslienko, X.C. Kou, R. Grossinger, J. Magn. Magn. Mater. 150 (1995) 383. [4] S.A. Nikitin, I.S. Tereshina, V.N. Verbetsky, A.A. Salamova, Phys. Solid State (in Russian) 40(2) (1998) 285. [5] V.K. Sihna, S.F. Cheng, W.E. Wallace, S.G. Sankar, JMMM 81 (1989) 227. [6] M. Solzi, R.H. Xue, L. Pareti, JMMM 88 (1990) 44. [7] S.F. Cheng, V.K. Sinha, B.M. Ma, S.G. Sankar, W.E. Wallace, J. Appl. Phys. 69(8) (1991) 5605. [8] Y.-c. Yang, S. Hong, Z. Zhen-yong, L. Tong, G. Jian-liang, Solid State Commun. 68(2) (1988) 175. [9] C.V. Thang, N.P. Thuy, N.M. Hong, T.D. Hien, N.S. Almodova, R. ¨ Grossinger, J. Magn. Magn. Mater. 140–144 (1995) 1017. [10] I.S. Tereshina, S.A. Nikitin, T.I. Ivanova, K.P. Skokov J. Alloys Comp. (1998) in press. [11] Y.-C. Yang, H. Sun, L.-S. Kong, J.-L. Yang, Y.-F. Ding, B.-S. Zhang, C.-T. Ye, L. Jin, J. Appl. Phys. 67(9) (1990) 4632. [12] Z.W. Li, X.Z. Zhou, A.H. Morrish, J. Appl. Phys. 69(8) (1991) 5602. [13] R.L. Steever, Phys. Rev. B 19 (1979) 2704.