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Structure and magnetism of the YFe10−xCoxV2 system
Journal of Magnetism and Magnetic North-Holland, Amsterdam
STRUCTURE
Materials
82 (1989) 239-242
AND MAGNETISM
239
OF THE YFe 10_ ,Co,V, SYSTEM
M. JURCZYK Institute
of Molecular Physics, Polish Academy
of Sciences, Smoluchowskiego
17/19,
60-l 79 Poznari, Poland
and O.D. CHISTJAKOV A.A. Baikov Institute
Received
of Metallurgy,
USSR Academy
of Sciences, Lenin Prospect 49, II 7911 Moscow,
USSR
19 July 1989
YFe,,_,Co,V2 compounds (0 d x d 10) have been investigated by X-ray and magnetometry techniques. They crystallize in tetragonal structure of the ThMn,,-type. The lattice constants decrease monotonically with the increasing cobalt concentration. Saturation magnetization at 77 and 297 K reaches a maximum around x = 2 and gradually decrease for higher cobalt content. YFe,,_,Co,V, alloys have a Curie temperature of around 547 K for x = 0, reaching the value of 671 K for x = 5 and then decreasing again to 611 K for x = 10. These compounds exhibit uniaxial anisotropy for x Q 2, at 297 K. For higher cobalt content anisotropy becomes planar. The anisotropy field for the YFe,,,V, alloy is 32 and 79 kOe et 297 and 77 K, respectively, which is decreased in the presence of cobalt.
1. Introduction It has recently been reported in the literature that ternary compounds based on the tetragonal ThMn,,-type structure are formed when yttrium or rare earth (R) and iron are combined with a small amount of M = Si, Ti, V, Cr, MO or W [l-3]. All these alloys can be represented as RFe,,_,M,. Detailed investigations of the systems YFe,,_,V, showed that there is a substantial range of solid solubility extending approximately from x = 1.5 to x = 3 [4]. A strong decrease of the Curie temperature with decreasing Fe concentration was found in these compounds. Magnetic measurements made on those RFe,,M, alloys in which R is nonmagnetic have shown that the Fe sublattice anisotropy favours an easy magnetization direction parallel to the c-axis. Some of these materials may be considered as starting materials for permanent magnet applications [3]. For example, at 4.2 K, the YFe,,V, alloy has a Curie 0304-8853/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
temperature of 532 K and the magnetization per formula unit is 16.15~~ [l]. The anisotropy field found in this compound equals about 40 kOe [l] at 4.2 K. As in R,Fe,,B, it has been found that the addition of Co to the RFe,,M, system increases the value of the Curie temperature [3]. So far, the effect of Co substitution was studied in the SmTiFe,,_,Co, system [5,6]. NdCosSi, [7] and RCo,,V, [8] compounds were also investigated. Ohashi et al. [5] reported that the SmTiFe,,_,Co, alloys (ThMn,,-type structure) to be stable up to x = 11. The last results are in contrast with those of Cheng et al. [6]. These investigations show that single-phase materials exhibiting a ThMn,,-type structure can be formed only for x < 2. Substitution of Co for some of the Fe in SmTiFe,, _ $0, beyond x = 2 leads to the appearance of a considerable amount of extraneous phases (primarily of 2 : 17 stoichiometry) [6]. Our investigation indicates that YFe,,_,Co,V, B.V.
240
M. Jurczyk, O.D. Chistjakov / Structure of YFe,,
compounds of ThMn,,-type structure are formed in the concentration region from x = 0 to 10. The structural and magnetic properties of these alloys have not been reported. In the course of our search for new magnetic materials, it was of interest to investigate magnetic properties of these new systems. In this paper we present detailed information on crystal structure and on the magnetic properties of the YFe,, _ ,Co,V, system.
2. Experimental All the compounds were prepared by induction melting of stoichiometric amounts of the constituent element in a water-cooled copper boat, under an atmosphere of argon. The purity of the materials was (in mass’%) Y (99.9) Fe, Co (99.99) and V (99.7). The ingots were inverted and melted several times to insure homogeneity. The appropriate stoichiometry was determined by monitoring the weight of ingots after every melt. The determination of composition by weight is accurate to 0.01 at%. After melting, the samples were heat-treated at around 850 o C for two weeks and quenched in water. Intermetallic compounds of YFe,,_,Co,V, with x = 0, 1, 2, 3, 5, 7.5 and 10 were prepared. X-ray diffraction experiments with FeK, radiation were made to determine the crystallographic structure. Optical microscopy and scanning electron microscopy equipped with an X-ray microanalyser have been used to identify the single phase. The electron probe analyses were performed by a Camebax-Microanalyzer. A vibrating sample magnetometer with an external field up to 30 kOe was used for determination of the saturation magnetization (MS) and anisotropy field (HA). Measurements were made at 77 and 297 K. M, values were obtained from measurements on loose powders, sieved to 40 l~,rn, the actual values being obtained from Honda (M vs. l/H) plots. The anisotropy measurements were performed on the same powders, aligned in wax. The Curie temperatures were determined by conventional method by plotting M* vs. T and extrapolating the steep part of the curve to M2 = 0.
,CoxV2 system
3. Results and discussion
3.1. Structural
properties
The X-ray diffraction patterns of the annealed materials showed that for the entire composition range (x = O-10) the materials were isostructural. They crystallize in the body-centered tetragonal ThMn,,-type crystal structure. The measured lattice parameters a and c were found to decrease monotonically with increasing cobalt concentration in YFe,,_,Co,V, (fig. 1). Structural data for YFe,,,V, alloy are in good agreement with the results obtained by De Boer et al. [l]. The decrease of lattice parameters most probably occurs because cobalt atoms are smaller in size than iron atoms. All the YFe r0 ~ ,Co,V, compounds are examined by optical microscopy, also. Single-phase microstructure was observed for Co-concentrations of x > 3. In the composition range 0 G x G 2 except the ThMn,, structure a few percent of bee Fe-V phase has been observed. The presence of bee Fe-V phase in YFe,,_,V,, samples with relatively low vanadium concentration has also been reported by De Mooij and Buschow [4].
4.7&l,_
.- ‘-0,
l\.
4.74
,
4.70 ;, 0
2 x
4
In
(cl
,
‘.
6
,
,
6
10
YFe,0_xCoxV2
Fig. 1. The composition dependence of the lattice parameters and c, determined at room temperature.
a
M. Jurczyk, O.D. Chistjakov / Structure of YFe,, _ ,Co,V,
241
system
1 I
0
2
x in x in
YFe
lo-xcoxv2
Fig. 3. The composition
Fig. 2. The composition dependence of the saturation moments determined at room temperature and at liquid nitrogen temperature, respectively.
3.2. Magnetic properties
The composition dependence of the saturation moment of the YFe,,_,CoJz system, at 77 and 297 K is plotted in fig. 2. Increasing the cobalt content leads initially to an increase in moment, M,, giving a maximum at x = 2. At higher Co contents, M, decrease. The occurrence of a maximum in M, is not only characteristic of Fe-Co binary alloys [9], but also in the R(Fe, Co),, R,(Fe, Co),, and R,(Fe, Co),,B systems [lo-121. This can, as for the Slater-Pauling curve, be explained in terms of the rigid band model in which holes are present in both 3d sub-bands in the Fe-rich samples. The values of the saturation magnetization (a,) at 77 K, are listed in table 1. Included in table 1 are the values of the corresponding saturation Table 1 Magnetic
characteristics
Composition
of YFe,,_,Co,V,
alloys at 77 K
MS
p3d
x
;L2,W
hJf.4
h/34
0 1 2 3 5 7.5 10
114.2 118.1 122.0 114.8 98.3 75.2 51.5
15.32 15.91 16.50 15.59 13.46 10.40 7.19
1.53 1.59 1.65 1.56 1.35 1.04 0.72
I
I
4
6 YFe
dependence
I
6
I
10
J
lo-xcoxv2 of the Curie temperature.
moments (M,) and the values of the corresponding moment per 3d atom (~1~~).As shown in table 1, the mean 3d moment decreases monotonically (for x > 2) as more cobalt is introduced to the YFe,,_,Co,V, system. Yttrium has no magnetic moment. The calculations yield the 3d moment to be 1.53~~ for x = 0 (iron moment) and 0.72~~ for x = 10 (cobalt moment) at 77 K. Verhoef et al. [13] observed similar mean iron moments in the RFe,,_,M, systems. On the other hand, the mean cobalt moment calculated for YCo,,V, (0.72~~) is close to the value obtained in YCo,,$ [14], YCo, or YCo, [15] compounds (= 0.5~~). But, this is lower than the corresponding values in Co metal, RCo,, R&o,, [15] and R,Co,,B [16]. The composition dependence of the Curie temperatures, T,, are plotted in fig. 3. The YFelO_,Co,V, alloys have a Curie temperature around 547 K for x = 0, reaching the value of 671 K for x = 5 and then decreasing again to 611 K for x = 10. This behavior is almost the same as observed in the case of the Y(Fe, Co), system and can be explained in the same way [lo], in terms of a rigid band model. The T, data for YFe,,V, are in reasonable agreement with those of De Boer et al. [l]. It is interesting to note that a very similar maximum in T, was also reported for crystallized Nd,(Fe,_,Co,),,Ti, ribbons with the ThMn,,type structure [17]. Separate X-ray measurements were made on the field oriented (H,,, = 10 kOe) powdered samples of YFe,,_,Co,V, system, in order to determine the direction of easy magnetization. The
242
M. Jurczyk,
l
0. D. Chistjakov
R.T.
o LN
2
/ Structure of YFe,,) _ .Co,V,
system
Magnetic measurements on other rare earth containing RFe,,-,Co,V, alloys are in progress. Some of these appear to be attractive candidates for permanent magnet fabrication. They have large saturation magnetization and strong magnetocrystalline anisotropy field.
Acknowledgement x in
YFe
lo-xcoxv2
Fig. 4. The composition dependence of the anisotropy fields, determined at room temperature and at liquid nitrogen temperature, respectively.
crystallographic axes direction of easy magnetization were determined by comparing the intensity ratios of the (002) (400) (321) and (330) planes in the random and aligned powders. From the results of these measurements it was concluded that at room temperature the YFe,,_,Co,V, compounds exhibit uniaxial anisotropy for x G 2. For a higher cobalt content (3 G x G 10) the anisotropy becomes planar. Anisotropy fields HA at 77 and 297 K are plotted as a function of x in fig. 4. HA decrease linearly in the composition range of 0 < x G 2 for all the alloys. The anisotropy field for the YFe,,V, compound is 32 and 79 kOe at 297 and 77 K, respectively, which were found to be higher than those reported earlier [l].
4. Conclusions The YFe,,~$o,V, compounds crystallize in the body-centered tetragonal ThMn,,-type crystal structure in the concentration range x = 0 to 10. The lattice constants decrease monotonically with the increasing cobalt concentration. A characteristics maximum in the composition dependence of the saturation magnetization for x = 2 is observed. The determined 3d moment ranges from 1.65 to 0.72pJatom. The Curie temperatures for these materials are comparable to that of Nd,Fe,,B. The anisotropy was found to be strongly dependent on the cobalt content in YFe,,_,Co,V,. These compounds exhibit uniaxial anisotropy for x Q 2.
This research has been partly supported by the Polish Academy of Sciences from the program RPBP 01.8.
References [l] F.R. de Boer, Y.K. Huang, D.B. de Mooij and K.H.J. Buschow, J. Less-Common Metals 135 (1987) 199. [2] K. Ohashi, Y. Tawara, R. Osugi, J. Sakurai and Y. Komura, J. Less-Common Metals 139 (1988) Ll. [3] K.H.J. Buschow, D.B. de Mooij, M. Brouha, H.H.A. Smit and R.C. Thiel, IEEE Trans. Magn. MAG-24 (1988) 1611. [4] D.B. de Mooij and H.K.J. Buschow, J. Less-Common Metals 136 (1988) 207. [5] K. Ohashi, T. Yokoyama, R. Osugi and Y. Tawara, IEEE Trans. Magn. MAG-23 (1987) 3101. [6] S.F. Cheng, V.K. Sinha, Y. Xu, J.M. Elbicki, E.B. Boltich, W.E. Wallace, S.G. Sankar and D.E. Laughlin, J. Magn. Magn. Mat. 75 (1988) 330. [7] S.K. Malik, L.Y. Zhang, W.E. Wallace and S.G. Sankar, J. Magn. Magn. Mat. 78 (1989) L6. [8] M. Jurczyk, to be published. [9] A.R. Victoria and L.M. Falicov, Phys. Rev. B 30 (1984) 259. [lo] E. Burzo, Solid State Commun. 25 (1978) 525. [ll] K.N.R. Taylor and C.A. Poldy. J. Phys. F 5 (1975) 1593. [12] Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett. 46 (1985) 308. [13] R. Verhoef, F.R. de Boer, Zhang Zhi-dong and K.H.J. Buschow, J. Magn. Magn. Mat. 75 (1988) 319. [14] M. Jurczyk, A.T. Pedziwiatr and W.E. Wallace, J. Magn. Magn. Mat. 67 (1987) Ll. [15] W.E. Wallace, Rare Earth Intermetallics (Academic Press, New York, 1973). [16] K.H.J. Buschow, D.B. de Mooij, S. Sinnema, R.J. Radwanski and J.J.M. Franse, J. Magn. Magn. Mat. 51 (1985) 211. [17] E.W. Singleton, J. Strzeszewski, G.C. Hadjipanayis and D.J. Sellmyer, J. Appl. Phys. 64 (1988) 5717.
STRUCTURE
Materials
82 (1989) 239-242
AND MAGNETISM
239
OF THE YFe 10_ ,Co,V, SYSTEM
M. JURCZYK Institute
of Molecular Physics, Polish Academy
of Sciences, Smoluchowskiego
17/19,
60-l 79 Poznari, Poland
and O.D. CHISTJAKOV A.A. Baikov Institute
Received
of Metallurgy,
USSR Academy
of Sciences, Lenin Prospect 49, II 7911 Moscow,
USSR
19 July 1989
YFe,,_,Co,V2 compounds (0 d x d 10) have been investigated by X-ray and magnetometry techniques. They crystallize in tetragonal structure of the ThMn,,-type. The lattice constants decrease monotonically with the increasing cobalt concentration. Saturation magnetization at 77 and 297 K reaches a maximum around x = 2 and gradually decrease for higher cobalt content. YFe,,_,Co,V, alloys have a Curie temperature of around 547 K for x = 0, reaching the value of 671 K for x = 5 and then decreasing again to 611 K for x = 10. These compounds exhibit uniaxial anisotropy for x Q 2, at 297 K. For higher cobalt content anisotropy becomes planar. The anisotropy field for the YFe,,,V, alloy is 32 and 79 kOe et 297 and 77 K, respectively, which is decreased in the presence of cobalt.
1. Introduction It has recently been reported in the literature that ternary compounds based on the tetragonal ThMn,,-type structure are formed when yttrium or rare earth (R) and iron are combined with a small amount of M = Si, Ti, V, Cr, MO or W [l-3]. All these alloys can be represented as RFe,,_,M,. Detailed investigations of the systems YFe,,_,V, showed that there is a substantial range of solid solubility extending approximately from x = 1.5 to x = 3 [4]. A strong decrease of the Curie temperature with decreasing Fe concentration was found in these compounds. Magnetic measurements made on those RFe,,M, alloys in which R is nonmagnetic have shown that the Fe sublattice anisotropy favours an easy magnetization direction parallel to the c-axis. Some of these materials may be considered as starting materials for permanent magnet applications [3]. For example, at 4.2 K, the YFe,,V, alloy has a Curie 0304-8853/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
temperature of 532 K and the magnetization per formula unit is 16.15~~ [l]. The anisotropy field found in this compound equals about 40 kOe [l] at 4.2 K. As in R,Fe,,B, it has been found that the addition of Co to the RFe,,M, system increases the value of the Curie temperature [3]. So far, the effect of Co substitution was studied in the SmTiFe,,_,Co, system [5,6]. NdCosSi, [7] and RCo,,V, [8] compounds were also investigated. Ohashi et al. [5] reported that the SmTiFe,,_,Co, alloys (ThMn,,-type structure) to be stable up to x = 11. The last results are in contrast with those of Cheng et al. [6]. These investigations show that single-phase materials exhibiting a ThMn,,-type structure can be formed only for x < 2. Substitution of Co for some of the Fe in SmTiFe,, _ $0, beyond x = 2 leads to the appearance of a considerable amount of extraneous phases (primarily of 2 : 17 stoichiometry) [6]. Our investigation indicates that YFe,,_,Co,V, B.V.
240
M. Jurczyk, O.D. Chistjakov / Structure of YFe,,
compounds of ThMn,,-type structure are formed in the concentration region from x = 0 to 10. The structural and magnetic properties of these alloys have not been reported. In the course of our search for new magnetic materials, it was of interest to investigate magnetic properties of these new systems. In this paper we present detailed information on crystal structure and on the magnetic properties of the YFe,, _ ,Co,V, system.
2. Experimental All the compounds were prepared by induction melting of stoichiometric amounts of the constituent element in a water-cooled copper boat, under an atmosphere of argon. The purity of the materials was (in mass’%) Y (99.9) Fe, Co (99.99) and V (99.7). The ingots were inverted and melted several times to insure homogeneity. The appropriate stoichiometry was determined by monitoring the weight of ingots after every melt. The determination of composition by weight is accurate to 0.01 at%. After melting, the samples were heat-treated at around 850 o C for two weeks and quenched in water. Intermetallic compounds of YFe,,_,Co,V, with x = 0, 1, 2, 3, 5, 7.5 and 10 were prepared. X-ray diffraction experiments with FeK, radiation were made to determine the crystallographic structure. Optical microscopy and scanning electron microscopy equipped with an X-ray microanalyser have been used to identify the single phase. The electron probe analyses were performed by a Camebax-Microanalyzer. A vibrating sample magnetometer with an external field up to 30 kOe was used for determination of the saturation magnetization (MS) and anisotropy field (HA). Measurements were made at 77 and 297 K. M, values were obtained from measurements on loose powders, sieved to 40 l~,rn, the actual values being obtained from Honda (M vs. l/H) plots. The anisotropy measurements were performed on the same powders, aligned in wax. The Curie temperatures were determined by conventional method by plotting M* vs. T and extrapolating the steep part of the curve to M2 = 0.
,CoxV2 system
3. Results and discussion
3.1. Structural
properties
The X-ray diffraction patterns of the annealed materials showed that for the entire composition range (x = O-10) the materials were isostructural. They crystallize in the body-centered tetragonal ThMn,,-type crystal structure. The measured lattice parameters a and c were found to decrease monotonically with increasing cobalt concentration in YFe,,_,Co,V, (fig. 1). Structural data for YFe,,,V, alloy are in good agreement with the results obtained by De Boer et al. [l]. The decrease of lattice parameters most probably occurs because cobalt atoms are smaller in size than iron atoms. All the YFe r0 ~ ,Co,V, compounds are examined by optical microscopy, also. Single-phase microstructure was observed for Co-concentrations of x > 3. In the composition range 0 G x G 2 except the ThMn,, structure a few percent of bee Fe-V phase has been observed. The presence of bee Fe-V phase in YFe,,_,V,, samples with relatively low vanadium concentration has also been reported by De Mooij and Buschow [4].
4.7&l,_
.- ‘-0,
l\.
4.74
,
4.70 ;, 0
2 x
4
In
(cl
,
‘.
6
,
,
6
10
YFe,0_xCoxV2
Fig. 1. The composition dependence of the lattice parameters and c, determined at room temperature.
a
M. Jurczyk, O.D. Chistjakov / Structure of YFe,, _ ,Co,V,
241
system
1 I
0
2
x in x in
YFe
lo-xcoxv2
Fig. 3. The composition
Fig. 2. The composition dependence of the saturation moments determined at room temperature and at liquid nitrogen temperature, respectively.
3.2. Magnetic properties
The composition dependence of the saturation moment of the YFe,,_,CoJz system, at 77 and 297 K is plotted in fig. 2. Increasing the cobalt content leads initially to an increase in moment, M,, giving a maximum at x = 2. At higher Co contents, M, decrease. The occurrence of a maximum in M, is not only characteristic of Fe-Co binary alloys [9], but also in the R(Fe, Co),, R,(Fe, Co),, and R,(Fe, Co),,B systems [lo-121. This can, as for the Slater-Pauling curve, be explained in terms of the rigid band model in which holes are present in both 3d sub-bands in the Fe-rich samples. The values of the saturation magnetization (a,) at 77 K, are listed in table 1. Included in table 1 are the values of the corresponding saturation Table 1 Magnetic
characteristics
Composition
of YFe,,_,Co,V,
alloys at 77 K
MS
p3d
x
;L2,W
hJf.4
h/34
0 1 2 3 5 7.5 10
114.2 118.1 122.0 114.8 98.3 75.2 51.5
15.32 15.91 16.50 15.59 13.46 10.40 7.19
1.53 1.59 1.65 1.56 1.35 1.04 0.72
I
I
4
6 YFe
dependence
I
6
I
10
J
lo-xcoxv2 of the Curie temperature.
moments (M,) and the values of the corresponding moment per 3d atom (~1~~).As shown in table 1, the mean 3d moment decreases monotonically (for x > 2) as more cobalt is introduced to the YFe,,_,Co,V, system. Yttrium has no magnetic moment. The calculations yield the 3d moment to be 1.53~~ for x = 0 (iron moment) and 0.72~~ for x = 10 (cobalt moment) at 77 K. Verhoef et al. [13] observed similar mean iron moments in the RFe,,_,M, systems. On the other hand, the mean cobalt moment calculated for YCo,,V, (0.72~~) is close to the value obtained in YCo,,$ [14], YCo, or YCo, [15] compounds (= 0.5~~). But, this is lower than the corresponding values in Co metal, RCo,, R&o,, [15] and R,Co,,B [16]. The composition dependence of the Curie temperatures, T,, are plotted in fig. 3. The YFelO_,Co,V, alloys have a Curie temperature around 547 K for x = 0, reaching the value of 671 K for x = 5 and then decreasing again to 611 K for x = 10. This behavior is almost the same as observed in the case of the Y(Fe, Co), system and can be explained in the same way [lo], in terms of a rigid band model. The T, data for YFe,,V, are in reasonable agreement with those of De Boer et al. [l]. It is interesting to note that a very similar maximum in T, was also reported for crystallized Nd,(Fe,_,Co,),,Ti, ribbons with the ThMn,,type structure [17]. Separate X-ray measurements were made on the field oriented (H,,, = 10 kOe) powdered samples of YFe,,_,Co,V, system, in order to determine the direction of easy magnetization. The
242
M. Jurczyk,
l
0. D. Chistjakov
R.T.
o LN
2
/ Structure of YFe,,) _ .Co,V,
system
Magnetic measurements on other rare earth containing RFe,,-,Co,V, alloys are in progress. Some of these appear to be attractive candidates for permanent magnet fabrication. They have large saturation magnetization and strong magnetocrystalline anisotropy field.
Acknowledgement x in
YFe
lo-xcoxv2
Fig. 4. The composition dependence of the anisotropy fields, determined at room temperature and at liquid nitrogen temperature, respectively.
crystallographic axes direction of easy magnetization were determined by comparing the intensity ratios of the (002) (400) (321) and (330) planes in the random and aligned powders. From the results of these measurements it was concluded that at room temperature the YFe,,_,Co,V, compounds exhibit uniaxial anisotropy for x G 2. For a higher cobalt content (3 G x G 10) the anisotropy becomes planar. Anisotropy fields HA at 77 and 297 K are plotted as a function of x in fig. 4. HA decrease linearly in the composition range of 0 < x G 2 for all the alloys. The anisotropy field for the YFe,,V, compound is 32 and 79 kOe at 297 and 77 K, respectively, which were found to be higher than those reported earlier [l].
4. Conclusions The YFe,,~$o,V, compounds crystallize in the body-centered tetragonal ThMn,,-type crystal structure in the concentration range x = 0 to 10. The lattice constants decrease monotonically with the increasing cobalt concentration. A characteristics maximum in the composition dependence of the saturation magnetization for x = 2 is observed. The determined 3d moment ranges from 1.65 to 0.72pJatom. The Curie temperatures for these materials are comparable to that of Nd,Fe,,B. The anisotropy was found to be strongly dependent on the cobalt content in YFe,,_,Co,V,. These compounds exhibit uniaxial anisotropy for x Q 2.
This research has been partly supported by the Polish Academy of Sciences from the program RPBP 01.8.
References [l] F.R. de Boer, Y.K. Huang, D.B. de Mooij and K.H.J. Buschow, J. Less-Common Metals 135 (1987) 199. [2] K. Ohashi, Y. Tawara, R. Osugi, J. Sakurai and Y. Komura, J. Less-Common Metals 139 (1988) Ll. [3] K.H.J. Buschow, D.B. de Mooij, M. Brouha, H.H.A. Smit and R.C. Thiel, IEEE Trans. Magn. MAG-24 (1988) 1611. [4] D.B. de Mooij and H.K.J. Buschow, J. Less-Common Metals 136 (1988) 207. [5] K. Ohashi, T. Yokoyama, R. Osugi and Y. Tawara, IEEE Trans. Magn. MAG-23 (1987) 3101. [6] S.F. Cheng, V.K. Sinha, Y. Xu, J.M. Elbicki, E.B. Boltich, W.E. Wallace, S.G. Sankar and D.E. Laughlin, J. Magn. Magn. Mat. 75 (1988) 330. [7] S.K. Malik, L.Y. Zhang, W.E. Wallace and S.G. Sankar, J. Magn. Magn. Mat. 78 (1989) L6. [8] M. Jurczyk, to be published. [9] A.R. Victoria and L.M. Falicov, Phys. Rev. B 30 (1984) 259. [lo] E. Burzo, Solid State Commun. 25 (1978) 525. [ll] K.N.R. Taylor and C.A. Poldy. J. Phys. F 5 (1975) 1593. [12] Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett. 46 (1985) 308. [13] R. Verhoef, F.R. de Boer, Zhang Zhi-dong and K.H.J. Buschow, J. Magn. Magn. Mat. 75 (1988) 319. [14] M. Jurczyk, A.T. Pedziwiatr and W.E. Wallace, J. Magn. Magn. Mat. 67 (1987) Ll. [15] W.E. Wallace, Rare Earth Intermetallics (Academic Press, New York, 1973). [16] K.H.J. Buschow, D.B. de Mooij, S. Sinnema, R.J. Radwanski and J.J.M. Franse, J. Magn. Magn. Mat. 51 (1985) 211. [17] E.W. Singleton, J. Strzeszewski, G.C. Hadjipanayis and D.J. Sellmyer, J. Appl. Phys. 64 (1988) 5717.