- Email: [email protected]
High field magnetization process of (Sm, Nd)2Fe17Ny compounds
Physica B 177 (1992) North-Holland
238-242
High field magnetization compounds M.J.
Yu”,
G.F.
Zhoub
N. Tang”, and
“Magnetism Laboratory, %‘an der Waals-Zeeman The Netherlands
Y.L.
F.R.
Liu”,
process of (Sm, Nd),Fe 17Ny
0.
Tegus”,
Y. Lu”, J.P.
Kuanga,
F.M.
Yang”,
X. Lib,
de Boerb
Institute of Physics, Academia Sinica, P. 0. Box 603, Beijing 100080, China Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam,
The crystal structure and high-field magnetization process of (Sm,_,Nd,)zFe,,N, compounds (x = 0.0, 0.1,. , 1.0, 2 < y < 3) have been investigated. X-ray diffraction patterns of random and magnetically aligned powder samples were obtained. The magnetizations were measured in steady fields up to 7 T at temperatures ranging from 1.5 K to room temperature and in quasi-static fields up to 35T at 4.2 K and 77 K. All (Sm,_,Nd,),Fe,,N, compounds were found to crystallize in the rhombohedral ThJn,, structure. As x increases, the Curie temperature decreases. The anisotropy fields and easy magnetization direction were investigated from 1.5 K to room temperature by means of high-field magnetization measurements and AC-susceptibility measurements, combined with X-ray diffraction on random and magnetically aligned powder samples. The anisotropy field decreases with increasing x and approaches a minimum value at about x = 0.6, then increases again. A tentative spin phase diagram for the (Sml_rNdr)zFe,,Nr series is presented. At room temperature, the easy magnetization direction remains along the c-axis up to x = 0.6.
1. Introduction Sm,Fe,,N, and Nd,Fe,,N, both crystallize in the rhombohedral Th,Zn,, structure and have similar Curie temperatures [l-3]. However, the magnetocrystalline anisotropies of the two compounds are totally different. In Sm2Fe,,Nv, the easy magnetization direction (EMD) is along the c-axis from zero temperature up to the Curie temperature, while Nd,Fe,,N, has basal-plane anisotropy [l, 4-61. Therefore, a spin reorientation must occur in the (Sm,_,Nd,),Fe,,N, series. A study of the high-field magnetization process can be expected to contribute to a better understanding of the magnetocrystalline anisotropy in this system. Moreover, investigation of this series may also be helpful for applicational purposes, because in nature Nd is much more abundant, and therefore cheaper, than Sm. In the present investigation, we have focused our attention on the crystal structure and magnetic properties of the (Sm,_,Nd,),Fe,,N, series, es0921-4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
pecially on magnetization and magnetocrystalline anisotropy, in order to investigate the possibility of substitution of some Nd for Sm in Sm,Fe,,N, in the application of this material as a starting material in the permanent-magnet production.
2. Experimental
methods
host compounds were All (Sm,_,Nd,),Fe,, prepared by arc melting starting elements with purity of 99% or higher, followed by annealing in vacuum at 1150°C for 4 h. The ingots were pulverized to an average particle size of 25 Frn and the powder samples obtained were heated in purified N, under a pressure of about 1 atm at 550°C for 4 h to form the nitride compounds (Sm,_.Nd,),Fe,,N,. The value of y was determined to be between 2 and 3 by weighing. X-ray diffraction was employed to determine the structure of the nitrides. X-ray diffraction patterns of magnetically aligned powder samples
B.V. All rights
reserved
M.J.
Yu et al. I High field magnetization process of (Sm, Nd)?Fe,,N,
obtained at room temperature were used to determine the EMD of the nitrides. The Curie temperatures T, were derived from M-T curves measured in a Faraday balance. Magnetization curves were measured with an extracting-sample magnetometer in applied fields up to 7 T at temperatures between 4.2 K and room temperature. The high-field magnetization in applied fields up to 35 T was measured at 4.2 and 77 K in the High Field Installation at the University of Amsterdam [7]. The magnetic isotherms were recorded with the external field applied either parallel or perpendicular to the alignment direction of cylindrical samples which were prepared by aligning powder particles at room temperature in a magnetic field of 1 T applied parallel and perpendicular to the cylinder axis and by fixing their direction with epoxy resin. The anisotropy fields B, were derived from the intersection point of the two curves, measured with the field parallel and perpendicular to the alignment direction. The spin-reorientation temperatures T,, were determined from AC-susceptibility measurements below room temperature and from u-T curves between room temperature and the Curie temperature.
3. Results and discussion X-ray diffraction patterns showed that, like their host compounds, all (Sm,_,Nd,),Fe,,N, compounds crystallize in the rhombohedral Th,Zn,, structure. In all compounds, a small amount of a-Fe was detected. The X-ray diffraction patterns of (Sm,,,Nd,.,),Fe,, and are shown in fig. 1. Com(Sm,.,NdO.,),Fe,,N, pared with the host, the peak positions of the nitride exhibit a clear shift toward lower angles, indicating the lattice expansion upon nitrogenation. As the nitrogen atoms enter the host lattice interstitially [6], the crystal structure is retained and only lattice expansion takes place upon nitrogenation. The lattice parameters a and c of the (Sm,_,Nd,),Fe,,N, compounds change very little with x. The average a and c values for the nitrides are 8.77 and 12.65 A, respectively.
compounds
30"
40’
239
0”
28 Fig.
1. X-ray
diffractions
patterns
of (a) (Sm,,Nd,,6)zFel,
and (b) (Sm, ,Nd,, &Fe,,N,.
After nitrogenation the average cell volume expansion AVW amounts to 6.6%. Figure 2 shows the temperature dependence of the AC-susceptibility x for several compositions between 4.2 K and room temperature. Indications for spin reorientations were found in the form of maxima or jumps in the x(T) curves. For x = 0.4 and 0.6 clear maxima are observed at about 120 and 240 K, respectively. No transition was found for the compound with x = 0.7. For the cases x = 0.8 and 0.9, the x(T) curves exhibit anomalies again, this time in the form of jumps. The estimated T,, values for x = 0.8 and 0.9 are 170 and 210 K, respectively. In the tentative spin phase diagram in fig. 3, the T,, values deduced from the AC-susceptibility data are represented by the open circles. The values for x = 0.4 and 0.8 are in good agreement with the T,, values obtained from u(T) measurements. The values for 0.8 and 0.9 should certainly be associated with another spin reorientation. Therefore, tentatively, we suggest, as indicated in fig. 3, that the transition in the compounds with x = 0.4 and 0.6 corresponds to a reorientation from a uniaxial to a cone spin structure, whereas the transitions in the compounds with x = 0.8 and 0.9 are from a cone to a planar spin structure.
M.J.
240
Yu et al. I High field magnetization process of (Sm. Nd)?Fe,,N,,
ISml-xNd,)
0
2Fe17Ny
L
0
300
200
100 T
[Kl
Fig. 2. Temperature dependence of the AC susceptibility (in arbitrary units) of (Sm,-,Nd,),Fe,,N, compounds with x = 0.4, 0.6, 0.7, 0.8 and 0.9.
which sensitively depends on the distance between Fe-Fe atoms. The strong increase of the Curie temperature may partly be explained in terms of the lattice expansion of the nitrides which leads to an increase in the average nearest neighbor Fe-Fe distance and, therefore, to a decrease of the contribution of the negative FeFe interactions. In fig. 4, the high-field magnetization curves of the (Sm,PXNd,),Fe,,N, compounds are shown, measured at 4.2 K with the external magnetic fields up to 35 T applied either parallel or perpendicular to the alignment direction of the samples. Values for the saturation magnetization a, were derived from the easy direction magnetization curves by means of w versus l/B plots. The us values have been corrected for the contribution of the (Y-Fe impurity phase to the magnetization, which could be deduced from high temperature magnetization measurements. The concentration dependence of the saturation magnetization at 4.2 K is shown in fig. 5. us increases
300
The Curie temperatures of the (Sm,_,Nd,),Fe,,N, compounds, also indicated in fig. 3, are as much as 340 K higher than those of their hosts and change only very little with X. The Curie temperature is dominated by the strength of the exchange coupling between Fe-Fe moments
compounds
250
I
-
,
I
I
(Sml-xNdx)
I
2Fe17Ny
-
800 2 -
600
+ 400 01
0
200 n 0.2
0.4
Fig. 3. Spin phase
diagram
“0.0
0.6
0.8
X
of (Sm,+,Nd,)ZFe,,N,.
1.0
10
20
a [Tl
30
40
Fig. 4. Magnetization curves at 4.2 K for magnetically aligned powder samples of (Sm,+,Nd,),Fe,,N, compounds for two directions of the external field, parallel (0) and perpendicular (LJ) to the alignment direction.
M.J.
Yu et al. I High field magnetization process of (Sm, Nd),Fe,,N,
40
Eml-xNdxl
t
Eiml-xNd,) T
0 0.0
0.2
0.4
241
compounds
2Fe17Ny
2Fe17Ny = 4.2
0.6
K
0.0
1.0
5.0
X
Fig. 5. Concentration dependence of the saturation magnetization of (Sm,_,Nd,)?Fe,,N, compounds at 4.2 K.
rapidly from 162.5 Am*/kg for x = 0 to 193.0 Am*/kg as x increases from 0 to 0.6. For x larger than 0.6, a, maintains constant. The observed concentration dependence of us may be connected with preferential substitution of Nd for Sm. The substituted Nd atoms may preferentially substitute for Sm atoms at sites where the Sm moment is antiferromagnetically coupled with the Fe moments, which obviously leads to an increase in a,. For x larger than 0.6, the Sm atoms with negative exchange coupling with Fe moments have completely been substituted, so that as remains unchanged due to the small difference between Sm and Nd moments. From the magnetization curves in fig. 4 it can be seen that Sm,Fe,,N, has the largest anisotropy field B,, amounting to 32 T. With increasing Nd content B, decreases until x = 0.6. For x = 0.7 the two magnetization curves measured with the field parallel and perpendicular to the alignment direction coincide, showing that the anisotropy has disappeared. For x > 0.7, the planar anisotropy increases. Figure 6 shows the concentration dependence of B,. X-ray diffraction patterns of aligned powder samples (fig. 7) gave good evidence for the change of anisotropy with x at room temperature. It can be seen in fig. 7 that in the (Sm,_,Nd,),Fe,,N, series at room temperature the uniaxial anisotropy retains up to x = 0.6. When x reaches 0.7, the compound be-
0.2
0.4
0.6
0.8
1.0
X
Fig. 6. Anisotropy at 4.2 K.
field of (Sm,_,Nd,),Fe,,N,
compounds
Fig. 7. Room temperature X-ray diffraction patterns of magnetically aligned (Sm,_,Nd,),Fe,,NY powder samples with x = 0.6, 0.7 and 0.8.
comes nearly isotropic. For x = 0.8, cone anisotropy appears. The observation that at room temperature the easy magnetization direction remains along the c axis up to x = 0.7 is consistent with the spin phase diagram in fig. 3.
242
M.J.’ Yu et al. I High field magnetization process of (Sm, Nd)?Fe,,N,.
4. Conclusions
Substitution of Nd for Sm in Sm,Fe,,N, leads to a moderate decrease of T, and to an increase of c>. However, it has a large effect on the magnetocrystalline anisotropy. In the (Sm, _X)2Fe,,Ny compounds, low temperature and large x lead to a phase transition from uniaxial anisotropy to planar anisotropy. At room temperature, the phase-transition boundary is at approximately x = 0.7. These results suggest that it is possible to replace some Sm with Nd in Sm,Fe,,N, for practical application of these compounds in permanent-magnet production.
compounds
been carried out within the scientific exchan: program between China and The Netherlanc
References [II J.M.D. (1990)
PI K.H.J.
[31
[41 [51
Acknowledgements
bl
We are grateful to J.F. Liu, S.Q. Ji, W.G. Lin and J.R. Zhang for their valuable suggestions and kind help. The present investigation has
[71
Coey L251.
and Hong
Sun, J. Magn.
Magn.
Mater.
Buschow, R. Coehoorn, D.B. de Mooij, K. Waard and T.H. Jacobs, J. Magn. Magn. Mater. (1990) L35. J.M.D. Coey, Hong Sun and Yoshichilca Otani, Proc. t Internat. Symp. on Magnetic Anisotropy and Coerciv in Rare-Earth-Transition Metal Alloys, S1.3, Pittsburg USA (1990). Bo-ping Hu, Hong-shuo Li, Hong Sun and J.M.D. Cot J. Phys. Condens. Mat. 3 (1991) 3983. M. Katter, J. Wecker and L. Schultz, J. Magn. Mag Mater. 92 (1990) L14. R.M. Ibberson, 0. Moze, T.H. Jacobs and K.H.J. Bt chow, J. Phys. 3 (1991) 1219. R. Gersdorf, F.R. de Boer, J.C. Wolfrat, F.A. Mull and L.W. Roeland, in: High Field Magnetism, ed. I Date (North-Holland, Amsterdam, 1983) p. 277.
238-242
High field magnetization compounds M.J.
Yu”,
G.F.
Zhoub
N. Tang”, and
“Magnetism Laboratory, %‘an der Waals-Zeeman The Netherlands
Y.L.
F.R.
Liu”,
process of (Sm, Nd),Fe 17Ny
0.
Tegus”,
Y. Lu”, J.P.
Kuanga,
F.M.
Yang”,
X. Lib,
de Boerb
Institute of Physics, Academia Sinica, P. 0. Box 603, Beijing 100080, China Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam,
The crystal structure and high-field magnetization process of (Sm,_,Nd,)zFe,,N, compounds (x = 0.0, 0.1,. , 1.0, 2 < y < 3) have been investigated. X-ray diffraction patterns of random and magnetically aligned powder samples were obtained. The magnetizations were measured in steady fields up to 7 T at temperatures ranging from 1.5 K to room temperature and in quasi-static fields up to 35T at 4.2 K and 77 K. All (Sm,_,Nd,),Fe,,N, compounds were found to crystallize in the rhombohedral ThJn,, structure. As x increases, the Curie temperature decreases. The anisotropy fields and easy magnetization direction were investigated from 1.5 K to room temperature by means of high-field magnetization measurements and AC-susceptibility measurements, combined with X-ray diffraction on random and magnetically aligned powder samples. The anisotropy field decreases with increasing x and approaches a minimum value at about x = 0.6, then increases again. A tentative spin phase diagram for the (Sml_rNdr)zFe,,Nr series is presented. At room temperature, the easy magnetization direction remains along the c-axis up to x = 0.6.
1. Introduction Sm,Fe,,N, and Nd,Fe,,N, both crystallize in the rhombohedral Th,Zn,, structure and have similar Curie temperatures [l-3]. However, the magnetocrystalline anisotropies of the two compounds are totally different. In Sm2Fe,,Nv, the easy magnetization direction (EMD) is along the c-axis from zero temperature up to the Curie temperature, while Nd,Fe,,N, has basal-plane anisotropy [l, 4-61. Therefore, a spin reorientation must occur in the (Sm,_,Nd,),Fe,,N, series. A study of the high-field magnetization process can be expected to contribute to a better understanding of the magnetocrystalline anisotropy in this system. Moreover, investigation of this series may also be helpful for applicational purposes, because in nature Nd is much more abundant, and therefore cheaper, than Sm. In the present investigation, we have focused our attention on the crystal structure and magnetic properties of the (Sm,_,Nd,),Fe,,N, series, es0921-4526/92/$05.00
0
1992 - Elsevier
Science
Publishers
pecially on magnetization and magnetocrystalline anisotropy, in order to investigate the possibility of substitution of some Nd for Sm in Sm,Fe,,N, in the application of this material as a starting material in the permanent-magnet production.
2. Experimental
methods
host compounds were All (Sm,_,Nd,),Fe,, prepared by arc melting starting elements with purity of 99% or higher, followed by annealing in vacuum at 1150°C for 4 h. The ingots were pulverized to an average particle size of 25 Frn and the powder samples obtained were heated in purified N, under a pressure of about 1 atm at 550°C for 4 h to form the nitride compounds (Sm,_.Nd,),Fe,,N,. The value of y was determined to be between 2 and 3 by weighing. X-ray diffraction was employed to determine the structure of the nitrides. X-ray diffraction patterns of magnetically aligned powder samples
B.V. All rights
reserved
M.J.
Yu et al. I High field magnetization process of (Sm, Nd)?Fe,,N,
obtained at room temperature were used to determine the EMD of the nitrides. The Curie temperatures T, were derived from M-T curves measured in a Faraday balance. Magnetization curves were measured with an extracting-sample magnetometer in applied fields up to 7 T at temperatures between 4.2 K and room temperature. The high-field magnetization in applied fields up to 35 T was measured at 4.2 and 77 K in the High Field Installation at the University of Amsterdam [7]. The magnetic isotherms were recorded with the external field applied either parallel or perpendicular to the alignment direction of cylindrical samples which were prepared by aligning powder particles at room temperature in a magnetic field of 1 T applied parallel and perpendicular to the cylinder axis and by fixing their direction with epoxy resin. The anisotropy fields B, were derived from the intersection point of the two curves, measured with the field parallel and perpendicular to the alignment direction. The spin-reorientation temperatures T,, were determined from AC-susceptibility measurements below room temperature and from u-T curves between room temperature and the Curie temperature.
3. Results and discussion X-ray diffraction patterns showed that, like their host compounds, all (Sm,_,Nd,),Fe,,N, compounds crystallize in the rhombohedral Th,Zn,, structure. In all compounds, a small amount of a-Fe was detected. The X-ray diffraction patterns of (Sm,,,Nd,.,),Fe,, and are shown in fig. 1. Com(Sm,.,NdO.,),Fe,,N, pared with the host, the peak positions of the nitride exhibit a clear shift toward lower angles, indicating the lattice expansion upon nitrogenation. As the nitrogen atoms enter the host lattice interstitially [6], the crystal structure is retained and only lattice expansion takes place upon nitrogenation. The lattice parameters a and c of the (Sm,_,Nd,),Fe,,N, compounds change very little with x. The average a and c values for the nitrides are 8.77 and 12.65 A, respectively.
compounds
30"
40’
239
0”
28 Fig.
1. X-ray
diffractions
patterns
of (a) (Sm,,Nd,,6)zFel,
and (b) (Sm, ,Nd,, &Fe,,N,.
After nitrogenation the average cell volume expansion AVW amounts to 6.6%. Figure 2 shows the temperature dependence of the AC-susceptibility x for several compositions between 4.2 K and room temperature. Indications for spin reorientations were found in the form of maxima or jumps in the x(T) curves. For x = 0.4 and 0.6 clear maxima are observed at about 120 and 240 K, respectively. No transition was found for the compound with x = 0.7. For the cases x = 0.8 and 0.9, the x(T) curves exhibit anomalies again, this time in the form of jumps. The estimated T,, values for x = 0.8 and 0.9 are 170 and 210 K, respectively. In the tentative spin phase diagram in fig. 3, the T,, values deduced from the AC-susceptibility data are represented by the open circles. The values for x = 0.4 and 0.8 are in good agreement with the T,, values obtained from u(T) measurements. The values for 0.8 and 0.9 should certainly be associated with another spin reorientation. Therefore, tentatively, we suggest, as indicated in fig. 3, that the transition in the compounds with x = 0.4 and 0.6 corresponds to a reorientation from a uniaxial to a cone spin structure, whereas the transitions in the compounds with x = 0.8 and 0.9 are from a cone to a planar spin structure.
M.J.
240
Yu et al. I High field magnetization process of (Sm. Nd)?Fe,,N,,
ISml-xNd,)
0
2Fe17Ny
L
0
300
200
100 T
[Kl
Fig. 2. Temperature dependence of the AC susceptibility (in arbitrary units) of (Sm,-,Nd,),Fe,,N, compounds with x = 0.4, 0.6, 0.7, 0.8 and 0.9.
which sensitively depends on the distance between Fe-Fe atoms. The strong increase of the Curie temperature may partly be explained in terms of the lattice expansion of the nitrides which leads to an increase in the average nearest neighbor Fe-Fe distance and, therefore, to a decrease of the contribution of the negative FeFe interactions. In fig. 4, the high-field magnetization curves of the (Sm,PXNd,),Fe,,N, compounds are shown, measured at 4.2 K with the external magnetic fields up to 35 T applied either parallel or perpendicular to the alignment direction of the samples. Values for the saturation magnetization a, were derived from the easy direction magnetization curves by means of w versus l/B plots. The us values have been corrected for the contribution of the (Y-Fe impurity phase to the magnetization, which could be deduced from high temperature magnetization measurements. The concentration dependence of the saturation magnetization at 4.2 K is shown in fig. 5. us increases
300
The Curie temperatures of the (Sm,_,Nd,),Fe,,N, compounds, also indicated in fig. 3, are as much as 340 K higher than those of their hosts and change only very little with X. The Curie temperature is dominated by the strength of the exchange coupling between Fe-Fe moments
compounds
250
I
-
,
I
I
(Sml-xNdx)
I
2Fe17Ny
-
800 2 -
600
+ 400 01
0
200 n 0.2
0.4
Fig. 3. Spin phase
diagram
“0.0
0.6
0.8
X
of (Sm,+,Nd,)ZFe,,N,.
1.0
10
20
a [Tl
30
40
Fig. 4. Magnetization curves at 4.2 K for magnetically aligned powder samples of (Sm,+,Nd,),Fe,,N, compounds for two directions of the external field, parallel (0) and perpendicular (LJ) to the alignment direction.
M.J.
Yu et al. I High field magnetization process of (Sm, Nd),Fe,,N,
40
Eml-xNdxl
t
Eiml-xNd,) T
0 0.0
0.2
0.4
241
compounds
2Fe17Ny
2Fe17Ny = 4.2
0.6
K
0.0
1.0
5.0
X
Fig. 5. Concentration dependence of the saturation magnetization of (Sm,_,Nd,)?Fe,,N, compounds at 4.2 K.
rapidly from 162.5 Am*/kg for x = 0 to 193.0 Am*/kg as x increases from 0 to 0.6. For x larger than 0.6, a, maintains constant. The observed concentration dependence of us may be connected with preferential substitution of Nd for Sm. The substituted Nd atoms may preferentially substitute for Sm atoms at sites where the Sm moment is antiferromagnetically coupled with the Fe moments, which obviously leads to an increase in a,. For x larger than 0.6, the Sm atoms with negative exchange coupling with Fe moments have completely been substituted, so that as remains unchanged due to the small difference between Sm and Nd moments. From the magnetization curves in fig. 4 it can be seen that Sm,Fe,,N, has the largest anisotropy field B,, amounting to 32 T. With increasing Nd content B, decreases until x = 0.6. For x = 0.7 the two magnetization curves measured with the field parallel and perpendicular to the alignment direction coincide, showing that the anisotropy has disappeared. For x > 0.7, the planar anisotropy increases. Figure 6 shows the concentration dependence of B,. X-ray diffraction patterns of aligned powder samples (fig. 7) gave good evidence for the change of anisotropy with x at room temperature. It can be seen in fig. 7 that in the (Sm,_,Nd,),Fe,,N, series at room temperature the uniaxial anisotropy retains up to x = 0.6. When x reaches 0.7, the compound be-
0.2
0.4
0.6
0.8
1.0
X
Fig. 6. Anisotropy at 4.2 K.
field of (Sm,_,Nd,),Fe,,N,
compounds
Fig. 7. Room temperature X-ray diffraction patterns of magnetically aligned (Sm,_,Nd,),Fe,,NY powder samples with x = 0.6, 0.7 and 0.8.
comes nearly isotropic. For x = 0.8, cone anisotropy appears. The observation that at room temperature the easy magnetization direction remains along the c axis up to x = 0.7 is consistent with the spin phase diagram in fig. 3.
242
M.J.’ Yu et al. I High field magnetization process of (Sm, Nd)?Fe,,N,.
4. Conclusions
Substitution of Nd for Sm in Sm,Fe,,N, leads to a moderate decrease of T, and to an increase of c>. However, it has a large effect on the magnetocrystalline anisotropy. In the (Sm, _X)2Fe,,Ny compounds, low temperature and large x lead to a phase transition from uniaxial anisotropy to planar anisotropy. At room temperature, the phase-transition boundary is at approximately x = 0.7. These results suggest that it is possible to replace some Sm with Nd in Sm,Fe,,N, for practical application of these compounds in permanent-magnet production.
compounds
been carried out within the scientific exchan: program between China and The Netherlanc
References [II J.M.D. (1990)
PI K.H.J.
[31
[41 [51
Acknowledgements
bl
We are grateful to J.F. Liu, S.Q. Ji, W.G. Lin and J.R. Zhang for their valuable suggestions and kind help. The present investigation has
[71
Coey L251.
and Hong
Sun, J. Magn.
Magn.
Mater.
Buschow, R. Coehoorn, D.B. de Mooij, K. Waard and T.H. Jacobs, J. Magn. Magn. Mater. (1990) L35. J.M.D. Coey, Hong Sun and Yoshichilca Otani, Proc. t Internat. Symp. on Magnetic Anisotropy and Coerciv in Rare-Earth-Transition Metal Alloys, S1.3, Pittsburg USA (1990). Bo-ping Hu, Hong-shuo Li, Hong Sun and J.M.D. Cot J. Phys. Condens. Mat. 3 (1991) 3983. M. Katter, J. Wecker and L. Schultz, J. Magn. Mag Mater. 92 (1990) L14. R.M. Ibberson, 0. Moze, T.H. Jacobs and K.H.J. Bt chow, J. Phys. 3 (1991) 1219. R. Gersdorf, F.R. de Boer, J.C. Wolfrat, F.A. Mull and L.W. Roeland, in: High Field Magnetism, ed. I Date (North-Holland, Amsterdam, 1983) p. 277.