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Coercive field of Sm2Fe17Nx epoxy resin bonded magnets
Journal of Magnetism and Magnetic Materials 135 (1994) 221-225
ELSEVIER
Coercive field of Sm,Fe,,N,
A
journal al magnetism
IH A
Fifgnglic materials
epoxy resin bonded magnets
Jifan Hu a~*,Fuming Yang a, Ruwen Zhao a, Zhenxi Wang a, Shenjun Yu b, Shouzeng Zhou b, Boping Hu ‘, Yizhong Wang ’ aInstituteof Physics, Chinese Academy of Sciences, Beijing 100080, China b Department of Materials Science and Engineering, University of Science and Technology, Beijing 100083, China ’ San Huan Research Laboratory, Chinese Academy of Sciences, Beijing 100080, China
(Received 15 July 1993; in revised form 10 November
1993)
Abstract The field dependence of the coercive field for isotropic and aligned Sm,Fe17N, epoxy resin bonded magnets are investigated. Results indicate that the coercive field of the isotropic magnet is larger than that of the aligned magnet. For small magnetizing fields, the coercive field of the sample magnetized antiparallel to the alignment direction is larger than that of the sample magnetized parallel to the alignment direction of the magnets. When the magnetizing field is large enough, the coercivities obtained after magnetizing the aligned sample in parallel and antiparallel directions are the same. The microstructural parameter derived from the temperature dependence of the coercive field implies that both nucleation and pinning mechanism could be responsible for the magnetic hardening of the Sm,Fe,,N, epoxy resin bonded magnets.
isotropic and aligned Sm,Fe,,N, bonded magnets are investigated.
1. Introduction Recently it has been found that the Sm,Fe,,N, phase has a large anisotropy field, a reasonable saturation magnetization and a relative high Curie temperature [ll. Different methods have been used to developed a practical nitride magnet, such as the mechanical alloying process [2-31, the hydrogen decrepitation process [4], the epoxy resin [51 or metal bonded methods [6-111 and the explosion sintering technique [12]. In the present work, the field dependence of the coercivity for
* Corresponding
author.
0304-8853/94/$07.00 0 1994 Elsevier SSDI 0304-8853(94)00058-Y
Science
epoxy resin
2. Experiments Primary Sm,Fe,, alloys were prepared by arcmelting under argon atmosphere and subsequent vacuum annealing at 1050°C for 24 h. X-ray analysis indicates that the alloy is a single phase. The sample then was crushed and nitrogenated at 500°C for 4 h. The nitride powders were then mixed with epoxy resin and bonded. The aligned magnet was obtained by aligning the SmFeN powder, while bonding in a magnetic field up to 2 T for several hours. The field dependence and
B.V. All rights reserved
J. Hu et al. /Journal
222
the temperature were measured
3. Results
of Magnetism and Magnetic Materials 135 (1994) 221-225 1.4
dependence of the coercive field with a pulsed high magnetic field.
I
I
I
,
(a)
and discussion
3.1. Field dependence
of the coerciuity
It has been found that the dependence of the coercivity on the maximum applied field is mainly dominated by the distribution of the critical fields originating from the grain misalignment. The magnetizing field is required to overcome the critical field of the individual grains [13]. Such critical fields may reflect the effects of the anisotropy barrier and the stray fields due to the sharp corners of the grains and the interactions at the boundaries. Fig. 1 shows the field dependence of the coercive field H, (see Fig. l(a)> and the remanence ratio B,/B$, where B, is the remanence and B,!! is the saturated remanence (see Fig. l(b)). The coercivity increases sharply while increasing the magnetic field up to 2 T and then increases slowly with increasing magnetic field. It is evident from Fig. l(b) that the remanence increases with increasing magnetizing field. Above a magnetizing field of about 3 T, the remanence increases slowly and approaches its saturation value. The fact that the coercivity increases slowly with the magnetizing field above 2 T may be due to overcoming the barrier of the critical field originating from the existence of a small amount of the grains with large angles between the direction of the external field and the c-axis of the misaligned grain, whose saturation magnetization is difficult to reach at a small magnetizing field. Fig. 2 shows the field dependence of the coercive field H, (see Fig. 2(a)) and the remanence ratio B,/B$ (see Fig. 2(b)) for the aligned magnet, respectively. The external demagnetizing field is applied to the sample in the directions antiparallel and parallel to the alignment direction alternatively after magnetizing the sample parallel and antiparallel to the alignment direction, which are labeled in Fig. 2. It can be seen that the coercivity after magnetizing the sample antiparallel to the alignment direction is larger than that after mag-
0
2
4
6
8
10
1.2
1.0
0.8
Eh :
0.6 co’ 0.4
0.2
0.0
Y 0
I
I
I
I
I
2
4
6
8
10
j-paw(T) Fig. 1. Magnetizing field dependence (b) remanence ratio for isotropic bonded magnet.
of (a) the coercivity and Sm,Fe,,N, epoxy resin
netizing the sample parallel to the alignment direction below the magnetizing field of 3 T, even though the magnetizing field antiparallel to the alignment direction is smaller than that parallel to the alignment direction (comparing the coercivity at a magnetizing field of 1.5 T antiparallel to the alignment direction with the coercivity at a magnetizing field of 2 T parallel to the alignment
J. Hu et al. /Journal
1.0
223
of Magnetism and Magnetic Materials 135 (I 994) 221-225
coercivity than the well aligned grain. Such behavior can also be seen by comparing the saturated coercivity of the aligned and isotropic SmFeN samples from Figs. l(a) and 2(a). For the aligned sample, the coercivity obtained after magnetizing the sample in parallel and antiparallel directions is the same when the magnetizing field is large enough, where the coercivity reaches its saturation value.
I
3.2. Temperature dependence of the coercivity
0.0’ 0
2
6
4
According to the well established micromagnetic theory [14-161, the coercivity controlled by both nucleation and pinning mechanism depends sensitively on the microstructure. The microstructural parameters CYand N,, can be deduced from the relation
6
)+Happ(T)
1.0
H, = a2K,/M,
!
or K/M,
0.6 -
‘rn& \ L 0.6 m
0.0
- N,,M,,
= a2K,/Ms’
- Nerr,
with the temperature dependence of K,, MS and K, is the anisotropy constant, MS is the saturation magnetization. The parameter (Y describes the effect of the reduced anisotropy at the grains and the strength of the exchange and magnetostatic coupling between neighboring grains, the parameter N,, represents the effects of the demagnetization fields of nonmagnetic
H,, where -
I 0
1
I
I
1
2
3
I
4 )+,Happ
I
5 CT)
I
I
6
7
I 6
Fig. 2. Magnetizing field dependence of (a) the coercivity and (b) remanence ratio for aligned Sm,Fe,,N, epoxy resin bonded magnet. The arrow indicates the case where the magnetizing field is applied upon the sample antiparallel to the alignment direction.
1.2
1.0 E xv
0.6
9
direction). It is obvious that the remanence ratio is smaller at a larger magnetizing field antiparallel to the alignment direction than that at a relatively small magnetizing field parallel to the alignment direction. The relative large coercivity after magnetizing the sample antiparallel to the alignment direction is mainly due to the reason that the misaligned grain maybe has a larger
0.6
0.4 250
300
350
400
450
500
T 6) Fig. 3. The temperature dependence of the coercivity Sm,Fe,,N, epoxy resin bonded magnet.
of the
224
J. Hu et al. /Journal
of Magnetism and Magnetic Materials 135 (1994) 221-225
SmFeN magnet by Wendhausen et al. [lo] may be due to the reaction between Zn powder and the Sm,Fe,,N, grain surface, which improves the surface behavior of the Sm,Fe,,N, grain and therefore decreases the demagnetization factor.
4. Conclusions
0.3 ’ 8.0
I
1
I
I
I
I
8.5
9.0
9.5
10.0
10.5
11.0
2K,/ MS2 Fig. 4. Data
H, /M,
vs 2K, /M:
fitted with one straight
line.
phases and local demagnetization fields at sharp edges and corners of single domain particle. The temperature dependence of K, and MS for Sm,Fe,,N, have been derived by fitting the magnetization curve of isotropic magnet based on the theory of Stoner-Wohlfarth [17]. The temperature dependence of the coercivity of Sm,Fe,,N, epoxy resin bonded magnet is shown in Fig. 3. Data HJM, vs 2K,/M,’ were fitted by one straight line with CY= 0.31 and N,, = 2.37 (Fig. 4). Since the maximum of the microstructural parameter cx of the pinning field can reach the value of 0.3 [15,16], both the pinning mechanism and the nucleation mechanism could be responsible to the magnetic hardening of SmFeN epoxy resin bonded magnet. Further investigations to clarify the hardening mechanism of resin bonded SmFeN magnet are needed. The large effective demagnetization factor N = 2.37 mainly comes from the strong stray field of very sharp corners of Sm,Fe,,N, grain considering the different shape of each Sm,Fe,,N, grain in our resin bonded SmFeN magnet. Another inhomogeneity in grain center due to the unsaturated nitrogen absorption and the nonmagnetic pole also give contributions to the effective demagnetization factor. It is obvious that the large stray field plays an important role in the reduction of an ideal nucleation field of a Sm,Fe,,N, magnet [18]. The relative small effective demagnetization factor N,,, = 0.43-1.1 obtained in a Zn-bonded
Based on our above experimental results, it can be concluded that the coercive field of the isotropic SmFeN magnet is larger than that of the aligned SmFeN magnet. For the small magnetizing field the coercive field after magnetizing the sample antiparallel to the alignment direction is larger than the coercive field after magnetizing the sample parallel to alignment direction for an aligned magnet. When the magnetizing field is large enough, the coercivities obtained after magnetizing the aligned sample in parallel and antiparallel directions are the same. The microstructural parameter (Y derived from the temperature dependence of the coercive field is about 0.31, which implies that both nucleation and pinning mechanism could be responsible to the magnetic hardening of the Sm,Fe,,N, epoxy resin bonded magnets.
References 111J.M.D.
Coey and H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. 121K. Schnitzke, L. Schultz, J. Wecker and M. Katter, Appl. Phys. Lett. 57 (1990) 2853. [31 J. Ding, P.G. McCormick and R. Street, Appl. Phys. Lett. 61 (1992) 2721. [41 Jun Yang, Shouzeng Zhou, Maocai Zhang, Fobiao Li, Junhua Zhao and Run Wang, Mater. Lett., 12 (1991) 242. 151 Y.L. Liu, D.W. Wang, B.P. Hu, J.L. Gao, Q. Song, L. Liu, X.L. Rao, J.M. Xu, G.C. Liu, F. Cao, H. Li, L. Yin, N. Tang, M.J. Yu, T. Gus, Z.X. Wang, Kexue TongBao 36 (1991) 1850. 161 Y. Otani, A. Moukarika, H. Sun, J.M.D. Coey, E. Devlin and I.R. Harris, J. Appl. Phys. 69 (1991) 6735. [71 M.Q. Huang, L.Y. Zhang, B.M. Ma, Y. Zheng, J.M. Elbicki, W.E. wallace and S.G. Sankar, J. Appl. Phys. 70 (1991) 6027. LB1M. Endoh, M. Iwata and M. Tokunaga, J. Appl. Phys. 70 (1991) 6030. [91 K.-H. Miiller, G. Leitner, W. Pitschke, P.A.P. Wend-
J. Hu et al. /Journal
of Magnetism and Magnetic Materials 135 (1994) 221-225
hausen, A. Handstein and D. Eckert, Phys. Stat. Sol. (al 133 (1992) K37. [lo] P.A.P. Wendhausen, K.-H. Miiller, A. Handstein, D. Eckert, W. Pitschke and Bo-Ping Hu, FC-05, INTERMAG’93, Stockholm (1993). [ll] Jifan Hu, I. KIeinschroth, R. Reisser, H. Kronmiiller and Shouzeng Zhou, Phys. Stat. Sol. (a) 138 (1993) 257. [12] B.P. Hu, X.L. Rao, J.M. Xu, G.C. Liu, Y.Z. Wang, X.L. Dong, D.X. Zhang and M. Cai, J. Appl. Phys. 74 (1993) 489. [13] Jifan Hu, H. Gerth, A. Forkl, G. Martinek, X.C. Kou and H. Kronmiiller, Phys. Stat. Sol. (a) 137 (1993) 227.
225
[14] H. Kronmiiller, Proc. 7th Inter. Workshop on Re-Co-Permanent Magnets, Eds. X. Pan, W. Ho and C. Yu (China Acad. Publ. Beijing, China, 1983) p. 339. [15] H. Kronmiiller, J. Magnetic Sot. Japan 15 (1991) 6. [16] H. Kronmiiller, in: Supermagnets, Hardmagnetic materials, Eds. J. Long and F. Granjean (IUuwer Academic Publishers, Dordrecht, 1990) p. 461. [17] Jifan Hu, X.C. Kou, H. Kronmiiller and Shouzeng Zhou, Phys. Stat. Sol. 134 (19921 499. [18] Jifan Hu, X.C. Kou, T. Dragon, H. Kronmiiller and Bo-ping Hu, Phys. Stat. Sol. (a) 139 (1993) 199.
ELSEVIER
Coercive field of Sm,Fe,,N,
A
journal al magnetism
IH A
Fifgnglic materials
epoxy resin bonded magnets
Jifan Hu a~*,Fuming Yang a, Ruwen Zhao a, Zhenxi Wang a, Shenjun Yu b, Shouzeng Zhou b, Boping Hu ‘, Yizhong Wang ’ aInstituteof Physics, Chinese Academy of Sciences, Beijing 100080, China b Department of Materials Science and Engineering, University of Science and Technology, Beijing 100083, China ’ San Huan Research Laboratory, Chinese Academy of Sciences, Beijing 100080, China
(Received 15 July 1993; in revised form 10 November
1993)
Abstract The field dependence of the coercive field for isotropic and aligned Sm,Fe17N, epoxy resin bonded magnets are investigated. Results indicate that the coercive field of the isotropic magnet is larger than that of the aligned magnet. For small magnetizing fields, the coercive field of the sample magnetized antiparallel to the alignment direction is larger than that of the sample magnetized parallel to the alignment direction of the magnets. When the magnetizing field is large enough, the coercivities obtained after magnetizing the aligned sample in parallel and antiparallel directions are the same. The microstructural parameter derived from the temperature dependence of the coercive field implies that both nucleation and pinning mechanism could be responsible for the magnetic hardening of the Sm,Fe,,N, epoxy resin bonded magnets.
isotropic and aligned Sm,Fe,,N, bonded magnets are investigated.
1. Introduction Recently it has been found that the Sm,Fe,,N, phase has a large anisotropy field, a reasonable saturation magnetization and a relative high Curie temperature [ll. Different methods have been used to developed a practical nitride magnet, such as the mechanical alloying process [2-31, the hydrogen decrepitation process [4], the epoxy resin [51 or metal bonded methods [6-111 and the explosion sintering technique [12]. In the present work, the field dependence of the coercivity for
* Corresponding
author.
0304-8853/94/$07.00 0 1994 Elsevier SSDI 0304-8853(94)00058-Y
Science
epoxy resin
2. Experiments Primary Sm,Fe,, alloys were prepared by arcmelting under argon atmosphere and subsequent vacuum annealing at 1050°C for 24 h. X-ray analysis indicates that the alloy is a single phase. The sample then was crushed and nitrogenated at 500°C for 4 h. The nitride powders were then mixed with epoxy resin and bonded. The aligned magnet was obtained by aligning the SmFeN powder, while bonding in a magnetic field up to 2 T for several hours. The field dependence and
B.V. All rights reserved
J. Hu et al. /Journal
222
the temperature were measured
3. Results
of Magnetism and Magnetic Materials 135 (1994) 221-225 1.4
dependence of the coercive field with a pulsed high magnetic field.
I
I
I
,
(a)
and discussion
3.1. Field dependence
of the coerciuity
It has been found that the dependence of the coercivity on the maximum applied field is mainly dominated by the distribution of the critical fields originating from the grain misalignment. The magnetizing field is required to overcome the critical field of the individual grains [13]. Such critical fields may reflect the effects of the anisotropy barrier and the stray fields due to the sharp corners of the grains and the interactions at the boundaries. Fig. 1 shows the field dependence of the coercive field H, (see Fig. l(a)> and the remanence ratio B,/B$, where B, is the remanence and B,!! is the saturated remanence (see Fig. l(b)). The coercivity increases sharply while increasing the magnetic field up to 2 T and then increases slowly with increasing magnetic field. It is evident from Fig. l(b) that the remanence increases with increasing magnetizing field. Above a magnetizing field of about 3 T, the remanence increases slowly and approaches its saturation value. The fact that the coercivity increases slowly with the magnetizing field above 2 T may be due to overcoming the barrier of the critical field originating from the existence of a small amount of the grains with large angles between the direction of the external field and the c-axis of the misaligned grain, whose saturation magnetization is difficult to reach at a small magnetizing field. Fig. 2 shows the field dependence of the coercive field H, (see Fig. 2(a)) and the remanence ratio B,/B$ (see Fig. 2(b)) for the aligned magnet, respectively. The external demagnetizing field is applied to the sample in the directions antiparallel and parallel to the alignment direction alternatively after magnetizing the sample parallel and antiparallel to the alignment direction, which are labeled in Fig. 2. It can be seen that the coercivity after magnetizing the sample antiparallel to the alignment direction is larger than that after mag-
0
2
4
6
8
10
1.2
1.0
0.8
Eh :
0.6 co’ 0.4
0.2
0.0
Y 0
I
I
I
I
I
2
4
6
8
10
j-paw(T) Fig. 1. Magnetizing field dependence (b) remanence ratio for isotropic bonded magnet.
of (a) the coercivity and Sm,Fe,,N, epoxy resin
netizing the sample parallel to the alignment direction below the magnetizing field of 3 T, even though the magnetizing field antiparallel to the alignment direction is smaller than that parallel to the alignment direction (comparing the coercivity at a magnetizing field of 1.5 T antiparallel to the alignment direction with the coercivity at a magnetizing field of 2 T parallel to the alignment
J. Hu et al. /Journal
1.0
223
of Magnetism and Magnetic Materials 135 (I 994) 221-225
coercivity than the well aligned grain. Such behavior can also be seen by comparing the saturated coercivity of the aligned and isotropic SmFeN samples from Figs. l(a) and 2(a). For the aligned sample, the coercivity obtained after magnetizing the sample in parallel and antiparallel directions is the same when the magnetizing field is large enough, where the coercivity reaches its saturation value.
I
3.2. Temperature dependence of the coercivity
0.0’ 0
2
6
4
According to the well established micromagnetic theory [14-161, the coercivity controlled by both nucleation and pinning mechanism depends sensitively on the microstructure. The microstructural parameters CYand N,, can be deduced from the relation
6
)+Happ(T)
1.0
H, = a2K,/M,
!
or K/M,
0.6 -
‘rn& \ L 0.6 m
0.0
- N,,M,,
= a2K,/Ms’
- Nerr,
with the temperature dependence of K,, MS and K, is the anisotropy constant, MS is the saturation magnetization. The parameter (Y describes the effect of the reduced anisotropy at the grains and the strength of the exchange and magnetostatic coupling between neighboring grains, the parameter N,, represents the effects of the demagnetization fields of nonmagnetic
H,, where -
I 0
1
I
I
1
2
3
I
4 )+,Happ
I
5 CT)
I
I
6
7
I 6
Fig. 2. Magnetizing field dependence of (a) the coercivity and (b) remanence ratio for aligned Sm,Fe,,N, epoxy resin bonded magnet. The arrow indicates the case where the magnetizing field is applied upon the sample antiparallel to the alignment direction.
1.2
1.0 E xv
0.6
9
direction). It is obvious that the remanence ratio is smaller at a larger magnetizing field antiparallel to the alignment direction than that at a relatively small magnetizing field parallel to the alignment direction. The relative large coercivity after magnetizing the sample antiparallel to the alignment direction is mainly due to the reason that the misaligned grain maybe has a larger
0.6
0.4 250
300
350
400
450
500
T 6) Fig. 3. The temperature dependence of the coercivity Sm,Fe,,N, epoxy resin bonded magnet.
of the
224
J. Hu et al. /Journal
of Magnetism and Magnetic Materials 135 (1994) 221-225
SmFeN magnet by Wendhausen et al. [lo] may be due to the reaction between Zn powder and the Sm,Fe,,N, grain surface, which improves the surface behavior of the Sm,Fe,,N, grain and therefore decreases the demagnetization factor.
4. Conclusions
0.3 ’ 8.0
I
1
I
I
I
I
8.5
9.0
9.5
10.0
10.5
11.0
2K,/ MS2 Fig. 4. Data
H, /M,
vs 2K, /M:
fitted with one straight
line.
phases and local demagnetization fields at sharp edges and corners of single domain particle. The temperature dependence of K, and MS for Sm,Fe,,N, have been derived by fitting the magnetization curve of isotropic magnet based on the theory of Stoner-Wohlfarth [17]. The temperature dependence of the coercivity of Sm,Fe,,N, epoxy resin bonded magnet is shown in Fig. 3. Data HJM, vs 2K,/M,’ were fitted by one straight line with CY= 0.31 and N,, = 2.37 (Fig. 4). Since the maximum of the microstructural parameter cx of the pinning field can reach the value of 0.3 [15,16], both the pinning mechanism and the nucleation mechanism could be responsible to the magnetic hardening of SmFeN epoxy resin bonded magnet. Further investigations to clarify the hardening mechanism of resin bonded SmFeN magnet are needed. The large effective demagnetization factor N = 2.37 mainly comes from the strong stray field of very sharp corners of Sm,Fe,,N, grain considering the different shape of each Sm,Fe,,N, grain in our resin bonded SmFeN magnet. Another inhomogeneity in grain center due to the unsaturated nitrogen absorption and the nonmagnetic pole also give contributions to the effective demagnetization factor. It is obvious that the large stray field plays an important role in the reduction of an ideal nucleation field of a Sm,Fe,,N, magnet [18]. The relative small effective demagnetization factor N,,, = 0.43-1.1 obtained in a Zn-bonded
Based on our above experimental results, it can be concluded that the coercive field of the isotropic SmFeN magnet is larger than that of the aligned SmFeN magnet. For the small magnetizing field the coercive field after magnetizing the sample antiparallel to the alignment direction is larger than the coercive field after magnetizing the sample parallel to alignment direction for an aligned magnet. When the magnetizing field is large enough, the coercivities obtained after magnetizing the aligned sample in parallel and antiparallel directions are the same. The microstructural parameter (Y derived from the temperature dependence of the coercive field is about 0.31, which implies that both nucleation and pinning mechanism could be responsible to the magnetic hardening of the Sm,Fe,,N, epoxy resin bonded magnets.
References 111J.M.D.
Coey and H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. 121K. Schnitzke, L. Schultz, J. Wecker and M. Katter, Appl. Phys. Lett. 57 (1990) 2853. [31 J. Ding, P.G. McCormick and R. Street, Appl. Phys. Lett. 61 (1992) 2721. [41 Jun Yang, Shouzeng Zhou, Maocai Zhang, Fobiao Li, Junhua Zhao and Run Wang, Mater. Lett., 12 (1991) 242. 151 Y.L. Liu, D.W. Wang, B.P. Hu, J.L. Gao, Q. Song, L. Liu, X.L. Rao, J.M. Xu, G.C. Liu, F. Cao, H. Li, L. Yin, N. Tang, M.J. Yu, T. Gus, Z.X. Wang, Kexue TongBao 36 (1991) 1850. 161 Y. Otani, A. Moukarika, H. Sun, J.M.D. Coey, E. Devlin and I.R. Harris, J. Appl. Phys. 69 (1991) 6735. [71 M.Q. Huang, L.Y. Zhang, B.M. Ma, Y. Zheng, J.M. Elbicki, W.E. wallace and S.G. Sankar, J. Appl. Phys. 70 (1991) 6027. LB1M. Endoh, M. Iwata and M. Tokunaga, J. Appl. Phys. 70 (1991) 6030. [91 K.-H. Miiller, G. Leitner, W. Pitschke, P.A.P. Wend-
J. Hu et al. /Journal
of Magnetism and Magnetic Materials 135 (1994) 221-225
hausen, A. Handstein and D. Eckert, Phys. Stat. Sol. (al 133 (1992) K37. [lo] P.A.P. Wendhausen, K.-H. Miiller, A. Handstein, D. Eckert, W. Pitschke and Bo-Ping Hu, FC-05, INTERMAG’93, Stockholm (1993). [ll] Jifan Hu, I. KIeinschroth, R. Reisser, H. Kronmiiller and Shouzeng Zhou, Phys. Stat. Sol. (a) 138 (1993) 257. [12] B.P. Hu, X.L. Rao, J.M. Xu, G.C. Liu, Y.Z. Wang, X.L. Dong, D.X. Zhang and M. Cai, J. Appl. Phys. 74 (1993) 489. [13] Jifan Hu, H. Gerth, A. Forkl, G. Martinek, X.C. Kou and H. Kronmiiller, Phys. Stat. Sol. (a) 137 (1993) 227.
225
[14] H. Kronmiiller, Proc. 7th Inter. Workshop on Re-Co-Permanent Magnets, Eds. X. Pan, W. Ho and C. Yu (China Acad. Publ. Beijing, China, 1983) p. 339. [15] H. Kronmiiller, J. Magnetic Sot. Japan 15 (1991) 6. [16] H. Kronmiiller, in: Supermagnets, Hardmagnetic materials, Eds. J. Long and F. Granjean (IUuwer Academic Publishers, Dordrecht, 1990) p. 461. [17] Jifan Hu, X.C. Kou, H. Kronmiiller and Shouzeng Zhou, Phys. Stat. Sol. 134 (19921 499. [18] Jifan Hu, X.C. Kou, T. Dragon, H. Kronmiiller and Bo-ping Hu, Phys. Stat. Sol. (a) 139 (1993) 199.