- Email: [email protected]
Magnetic characteristics of R2Fe14 − xMnxB systems (R ≡ Y, Nd, Pr and Gd)
Journal
of the Less-Common
MAGNETIC
Metals,
124
CHARACTERISTICS
(1986)
55
55
- 60
OF RzFe,,
_xMn,B
SYSTEMS
(R = Y, Nd, Pr AND Gd) M. Q. HUANG,
E. B. BOLTICH
Magnetics Technology ment, Carnegie-Mellon
and W. E. WALLACE
Center and Metallurgical University, Pittsburgh,
Engineering and Materials PA 15213 (U.S.A.)
Science
Depart-
E. OSWALD Department (Received
of Chemistry, November
University
26, 1985;
of Pittsburgh,
in revised
Pittsburgh,
form January
PA 15260
(U.S.A.)
20, 1986)
Summary Magnetic properties of R2Fe14_,Mn,B systems (R = Y, Nd, Pr, Gd) were investigated over a large temperature range. All the compounds studied crystallized in the tetragonal RzFe14B structure. The substitution of iron by manganese rapidly decreased the Curie temperatures and saturation moments. The anisotropy field HA at 77 K is found to increase, especially for R = Pr compounds. The spin-reorientation temperature is lowered by manganese substitution for the Ndz(Fe, Mn)14B systems, and the cone angle 8 is also reduced.
1. Introduction The discovery by Sagawa et al. [l, 21 and others [3] that a new permanent magnet produced from an Nd-Fe-B alloy had an energy product of about 40 MG Oe initiated a very exciting field of research and technological application. Several studies have been made recently of the magnetic behavior of the (R, R’)2Fe14B and R2(Fe,T)14B [4 - 71 systems (T = transition metals) in an effort to elucidate the nature of the exchange interaction in these systems and to extend the range of application of the Nd-Fe-B magnets. This paper reports the results of an investigation of magnetism in the R2Fe14_,MnXB systems (R = Y, Nd, Pr, Gd).
2. Experimental details Samples were prepared by induction melting the constituent R (purity, 99.99%), iron (purity, 99.95%), manganese (purity, 99.95%) and boron (purity, 99.98%) in a water-cooled boat under a titanium-gettered flowing 0022-5088/86/$3,50
0 Elsevier
Sequoia/Printed
in The Netherlands
56
argon atmosphere. Excess manganese was added to replace that lost by evaporation. The samples were then heat treated at around 900 “C for one week. This produced a single-phase material (within the detection limits of thermomagnetic analysis (TMA) and X-ray diffraction). Magnetic measurements were made by standard methods in fields to 20 kOe. The measurements at 77 K and room temperature were performed with a PAR vibrating sample magnetometer. All other measurements were made by the Faraday technique. The anisotropy fields were measured on powders aligned in wax. The spin-reorientation temperatures were obtained from the inflection point in the M(T) curves of rough powders which had been aligned in a field of 20 kOe at 4.2 K before the measurement. Measurements were made in a field of 2 kOe.
3. Results and discussion Structural and magnetic properties of selected compounds are presented in Tables 1 and 2 and in Figs. 1 and 2. All of the compounds crystallize in the tetragonal R2Fe14B structure. The lattice parameters decrease slightly when iron is partly replaced by manganese (x = 1 - 2). 3.1. Magnetic moments The composition dependencies of the saturation moment are shown in Table 1. The moments were found to decrease sharply with manganese substitution for all these systems. The rate of decrease was larger than that TABLE Lattice pounds
1 parameters, saturation (R = Y, Nd, Pr, Gd)
Compound
moments
Lattice parameters a @I
and curie temperatures
of R2Fe14_,Mn,B
Curie
P,(C(*(f.U.)-l)
c (8)
About 300 K
77K
com-
4.2 K experimental
4.2 K calculated
ature W
YzFel4B
8.174
12.054
27.5
30.4
31.6
Y2FelsMnB Y2FelzMn2B
8.762 8.735
12.052 12.023
18.4 9.3
23.3 15.6
25.8 18.1
565 479 374
Nd2Fe 14B Nd2Fe13MnB Nd2Fe12Mn2B
8.707 8.797 8.786
12.203 12.196 12.188
32.2 24.0 14.3
36.4 30.8 23.4
38.2 32.5 25.3
32.4 24.7
589 496 324
PrzFel4B PrzFeIsMnB PrzFelzMnzB
8.814 8.800 8.790
12.253 12.237 12.228
30.1 22.3 12.4
34.8 29.4 22.3
36.3 31.6 23.9
30.6 22.9
566 471 366
Gd2Fe14B Gd2Fe13MnB Gd2Fe12Mn2B
8.798 8.790 8.786
12.099 12.105 12.103
17.3 11.8 6.5
17.8 13.0 7.3
18.8 14.2 8.5
13.0 5.3
669 560 443
57 TABLE
2
Anisotropy fields and (R = Y, Nd, Pr, Gd) Compound
spin-reorientation
temperatures
of RzFer4_,Mn,B
DOMa
HA&@)
77 K
About 300 K
T,(K)
About 300 K
77 K
axis axis axis
axis axis axis
YzFersMnB YZFer2Mn2B
27.3 28.1 17.1
Nd2Fer4B Nd*FersMnB Nd*FerzMnB
70.1 68.2 50.6
-
axis axis axis
cone cone cone
Prz Fe 14B PrzFersMnB PrzFer2Mn2B
79.3 10.8 45.6
172 210 245
axis axis axis
axis axis axis
Gd2Fe14B Gd2FeraMnB Gd2FerzMn2B
28.6 41.9 45.1
axis axis axis
axis axis axis
YzFeG
aDOM signifies
2
direction
4
6
21.0 34.8 34.4
23.7 41.2 68.2
compounds
134 95 70
of magnetization.
8
IO
12
14
16
H, (KOe)
Fig. 1. Magnetization us. field strength dicular (90”) to the c axis.
for magnetic
field applied
parallel
(0”) and perpen-
expected by a simple dilution mode. The Mn-Fe distances are in the region less than 2.8 A, where antiferromagnetic coupling is expected [S] and a probable explanation is that the manganese atoms couple antiparallel to iron, resulting in a rapid decrease in the saturation moment. An antiferromagnetic coupling of the manganese and iron sublattice has been observed in the R6Fe2s-R6Mn2s phases [ 91. The manganese moment computed by assuming collinear structures and using the observed moments of Y2Fe14_,Mn,B is 3.5 to 4.5 pg (Mn atom)-‘, approximately equal to the value of Mn’+ moment
58
0 T (“K)
Fig. 2. Magnetization us. temperature for NdzFelzMnzB. The decline in magnetization between 50 and 100 K is ascribed to the spin-reorientation temperature of this alloy.
[lo]. If one assumes that the R-Fe coupling remains the same as in the R2Fei4B compounds, it is evident that neodymium and praseodymium couple antiferromagnetically with manganese, while gadolinium couples ferromagnetically. If we postulate that Y, FelL) _,Mn,B adequately represents the moment of 3d atoms in R2Fei4_,Mn,B systems the moments at 4.2 K for R2Fe1+,Mn,B compounds (R = Nd, Pr and Gd) were computed assuming a collinear structure and using the rare earth moments obtained from the RzFe14B systems [ 111. Results are given in column 7 of Table 1. There is no significant difference between the computed moments and the experimental values for R = Nd, Pr compounds, since the measured magnetization is uncertain by 2 to 3%. In the case of Gd2Fe14_,Mn,B the computed moments are lower than the experimental values. Perhaps in these systems the assumed coupling mode does not obtain. 3.2. Curie temperature The composition dependencies of the Curie temperatures T, of R,Fe14_,Mn,B systems are shown in Table 1. For all these sytems a similar composition dependence is observed. T, decreases rapidly at the rate of about 100 K (Mn atom)-i when iron is replaced by manganese, indicating a weakening of the Fe-Fe magnetic interaction. However, T, increases significantly at a rate of about 80 K (Co atom)-’ with substitution of iron by cobalt [ 6, 71. As mentioned before [ 51, the increase in T, of R2 Fei4 _-xCo,B systems can be correlated with the increase in the positive interaction between 3d metals and decrease of negative ones, cobalt replacing a fraction of Fe(8j,), Fe(8j,) or Fe(lGk,) atoms. In R,Fei4_,Mn,B systems it is postulated that the manganese atoms also replace a portion of the Sj and 16k1 Fe atoms
59
and the manganese couple antiparallel to the iron, resulting in a rapid decrease of the Curie temperature as in the R,(Fe, Mn),, and R,(Mn, Co),? systems. 3.3. Anisotropies and spin reorientation The directions of magnetization and anisotropy fields at 77 K and room temperature are presented in Table 2. The results for the yttrium and gadolinium systems show a s~bst~tial anisotropy field, indicating significant 3d sublattice contributions. On substituting iron by manganese, HA at 77 K increases for both Y and Gd compounds; at room temperature HA also increases for R = Gd, but decreases for R = Y. The incorporation of manganese into the iron sublattice substantially strengthens the uniaxial anisotropy of these systems at lower temperature, as is observed in R2(Co, Fe, Mn)i, systems 1121. The different behavior of HA at room temperature for yttrium and gadolinium compounds may be a consequence of the effect of the magnetic gadolinium ion on the band structure in Gd1Fe,4_,Mn,B compounds. In the R,Fei,_,Mn,B (R = Nd, Pr) systems, HA measured at room temperature decreases with increasing X. In this respect these systems behave similarly to Y,Fe14_,Mn,B. The magnetic behavior of the Ndz Fei4 _,Mn,B systems is generally similar to that of Nd2Fe14B, except that the interaction which is responsible for the tipping of the magnetization vector away from the c axis at low temperatures is weakened when iron is partially replaced by manganese. Magnetization versus applied field for Nd~Fe~~Mn~B in the easy and hard directions is plotted in Fig. 1. These data can be used to estimate the cone angle 8. 8 = tan-‘(M,/M,,), where M, and Mg, are the spontaneous moments parallel and perpendicular to the unique axis, obtained by back-extrapolating the linear portion of the curves to zero applied field. 8 w 10”. This contrasts with 8 = 30” for NdzFe14B. The reduction of B by 20” is one indication of the influence of manganese alloying. The reduction in spin-reorientation temperature T, from about 140 K for Nd,Fei4B to about 65 K for NdpFelZMnzB (see Fig. 2) is another indication that the interaction which causes the magnetization to lie off the c axis is weakened when iron is replaced by manganese. The reductions observed for T, and 8 are consistent. The spin-reorientation phenomenon in NdzFe14B has recently been interpreted by Boltich and Wallace [13] on a crystal field model using the Hamiltonian ;fc = BSO$ + BzOg + 2(g - l)/[email protected]. Careful examination of their formalism shows that exchange has the effect of pulling the magnetization vector away from the c axis. In other words, 0 > 0 because of the effect of exchange. As exchange is weakened, 8 relaxes toward zero. Replacement of iron by manganese weakens exchange, as indicated by the decline in T,. Thus, it is expected that 8 will be reduced as iron is replaced by manganese since under these circumstances exchange competes less well with the crystal field interaction and the 3d sublattice anisotropies which direct the magnetization along the c axis, This is one factor which reduces 8. The other is the
60
enhanced 3d sublattice placed by manganese.
anisotropy
(see Table 2) which results as iron is re-
Acknowledgment This work was supported Office.
by a contract
with the U.S. Army Research
References 1 M. Sagawa, S. Fujimura, N. Tagawa, H. Tamamoto and Y. Matsuura, J. Appl. Phys., 55 (1984) 2083. 2 M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga, IEEE Trans. Mug., 20 (1984) 1584. 3 J. J. Croat, J. F. Herb&, R. W. Lee and F. E. Pinkerton, J. Appl. Phys., 55 (1984) 2078. 4 M. Q. Huang, E. B. Boltich, E. Oswald and W. E. Wallace, in X. Guangxian and X. Jimei (eds.), New Frontiers in Rare Earth Science and Applications, Science Press, Beijing, 1985, p. 974. 5 E. Burzo, E. B. Boltich, M. Q. Huang and W. E. Wallace, Proc. 4th Int. Symp. on Magnetic Anisotropy and Coercivity in Rare Earth-Transition Metal Alloys, Dayton, Ohio, May, 9, 1985, p. 771, University of Dayton, Magnetics, KL-365, Dayton, Ohio 45469, U.S.A. 6 Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett., 46 (3) (1985). 7 M. Q. Huang, E. B. Boltich, W. E. Wallace and E. Oswald, J. Magn. Magn. Mater., in the press. 8 M. Merches, W. E. Wallace and R. S. Craig, J. Magn. Magn. Muter., 24 (1981) 97. 9 H. R. Kirchmayr and W. Steiner, J. Phys., Paris, 32 (1971) Cl-665-7. 10 W. E. Wallace, Rare Earth Zntermetallics, Academic Press, New York, 1973, p. 190. 11 E. B. Boltich, E. Oswald, M. Q. Huang, S. Hirosawa, W. E. Wallace and E. Burzo, J. Appl. Phys., 57 (1985) 4106. 12 W. E. Wallace and K. S. V. L. Narasimhan, in E. C. Subbarao and W. E. Wallace (eds.), The Science and Technology of Rare Earth Materials, Academic Press, New York, 1980, p. 329. 13 E. B. Boltich and W. E. Wallace, Solid State Commun., 55 (1985) 529.
of the Less-Common
MAGNETIC
Metals,
124
CHARACTERISTICS
(1986)
55
55
- 60
OF RzFe,,
_xMn,B
SYSTEMS
(R = Y, Nd, Pr AND Gd) M. Q. HUANG,
E. B. BOLTICH
Magnetics Technology ment, Carnegie-Mellon
and W. E. WALLACE
Center and Metallurgical University, Pittsburgh,
Engineering and Materials PA 15213 (U.S.A.)
Science
Depart-
E. OSWALD Department (Received
of Chemistry, November
University
26, 1985;
of Pittsburgh,
in revised
Pittsburgh,
form January
PA 15260
(U.S.A.)
20, 1986)
Summary Magnetic properties of R2Fe14_,Mn,B systems (R = Y, Nd, Pr, Gd) were investigated over a large temperature range. All the compounds studied crystallized in the tetragonal RzFe14B structure. The substitution of iron by manganese rapidly decreased the Curie temperatures and saturation moments. The anisotropy field HA at 77 K is found to increase, especially for R = Pr compounds. The spin-reorientation temperature is lowered by manganese substitution for the Ndz(Fe, Mn)14B systems, and the cone angle 8 is also reduced.
1. Introduction The discovery by Sagawa et al. [l, 21 and others [3] that a new permanent magnet produced from an Nd-Fe-B alloy had an energy product of about 40 MG Oe initiated a very exciting field of research and technological application. Several studies have been made recently of the magnetic behavior of the (R, R’)2Fe14B and R2(Fe,T)14B [4 - 71 systems (T = transition metals) in an effort to elucidate the nature of the exchange interaction in these systems and to extend the range of application of the Nd-Fe-B magnets. This paper reports the results of an investigation of magnetism in the R2Fe14_,MnXB systems (R = Y, Nd, Pr, Gd).
2. Experimental details Samples were prepared by induction melting the constituent R (purity, 99.99%), iron (purity, 99.95%), manganese (purity, 99.95%) and boron (purity, 99.98%) in a water-cooled boat under a titanium-gettered flowing 0022-5088/86/$3,50
0 Elsevier
Sequoia/Printed
in The Netherlands
56
argon atmosphere. Excess manganese was added to replace that lost by evaporation. The samples were then heat treated at around 900 “C for one week. This produced a single-phase material (within the detection limits of thermomagnetic analysis (TMA) and X-ray diffraction). Magnetic measurements were made by standard methods in fields to 20 kOe. The measurements at 77 K and room temperature were performed with a PAR vibrating sample magnetometer. All other measurements were made by the Faraday technique. The anisotropy fields were measured on powders aligned in wax. The spin-reorientation temperatures were obtained from the inflection point in the M(T) curves of rough powders which had been aligned in a field of 20 kOe at 4.2 K before the measurement. Measurements were made in a field of 2 kOe.
3. Results and discussion Structural and magnetic properties of selected compounds are presented in Tables 1 and 2 and in Figs. 1 and 2. All of the compounds crystallize in the tetragonal R2Fe14B structure. The lattice parameters decrease slightly when iron is partly replaced by manganese (x = 1 - 2). 3.1. Magnetic moments The composition dependencies of the saturation moment are shown in Table 1. The moments were found to decrease sharply with manganese substitution for all these systems. The rate of decrease was larger than that TABLE Lattice pounds
1 parameters, saturation (R = Y, Nd, Pr, Gd)
Compound
moments
Lattice parameters a @I
and curie temperatures
of R2Fe14_,Mn,B
Curie
P,(C(*(f.U.)-l)
c (8)
About 300 K
77K
com-
4.2 K experimental
4.2 K calculated
ature W
YzFel4B
8.174
12.054
27.5
30.4
31.6
Y2FelsMnB Y2FelzMn2B
8.762 8.735
12.052 12.023
18.4 9.3
23.3 15.6
25.8 18.1
565 479 374
Nd2Fe 14B Nd2Fe13MnB Nd2Fe12Mn2B
8.707 8.797 8.786
12.203 12.196 12.188
32.2 24.0 14.3
36.4 30.8 23.4
38.2 32.5 25.3
32.4 24.7
589 496 324
PrzFel4B PrzFeIsMnB PrzFelzMnzB
8.814 8.800 8.790
12.253 12.237 12.228
30.1 22.3 12.4
34.8 29.4 22.3
36.3 31.6 23.9
30.6 22.9
566 471 366
Gd2Fe14B Gd2Fe13MnB Gd2Fe12Mn2B
8.798 8.790 8.786
12.099 12.105 12.103
17.3 11.8 6.5
17.8 13.0 7.3
18.8 14.2 8.5
13.0 5.3
669 560 443
57 TABLE
2
Anisotropy fields and (R = Y, Nd, Pr, Gd) Compound
spin-reorientation
temperatures
of RzFer4_,Mn,B
DOMa
HA&@)
77 K
About 300 K
T,(K)
About 300 K
77 K
axis axis axis
axis axis axis
YzFersMnB YZFer2Mn2B
27.3 28.1 17.1
Nd2Fer4B Nd*FersMnB Nd*FerzMnB
70.1 68.2 50.6
-
axis axis axis
cone cone cone
Prz Fe 14B PrzFersMnB PrzFer2Mn2B
79.3 10.8 45.6
172 210 245
axis axis axis
axis axis axis
Gd2Fe14B Gd2FeraMnB Gd2FerzMn2B
28.6 41.9 45.1
axis axis axis
axis axis axis
YzFeG
aDOM signifies
2
direction
4
6
21.0 34.8 34.4
23.7 41.2 68.2
compounds
134 95 70
of magnetization.
8
IO
12
14
16
H, (KOe)
Fig. 1. Magnetization us. field strength dicular (90”) to the c axis.
for magnetic
field applied
parallel
(0”) and perpen-
expected by a simple dilution mode. The Mn-Fe distances are in the region less than 2.8 A, where antiferromagnetic coupling is expected [S] and a probable explanation is that the manganese atoms couple antiparallel to iron, resulting in a rapid decrease in the saturation moment. An antiferromagnetic coupling of the manganese and iron sublattice has been observed in the R6Fe2s-R6Mn2s phases [ 91. The manganese moment computed by assuming collinear structures and using the observed moments of Y2Fe14_,Mn,B is 3.5 to 4.5 pg (Mn atom)-‘, approximately equal to the value of Mn’+ moment
58
0 T (“K)
Fig. 2. Magnetization us. temperature for NdzFelzMnzB. The decline in magnetization between 50 and 100 K is ascribed to the spin-reorientation temperature of this alloy.
[lo]. If one assumes that the R-Fe coupling remains the same as in the R2Fei4B compounds, it is evident that neodymium and praseodymium couple antiferromagnetically with manganese, while gadolinium couples ferromagnetically. If we postulate that Y, FelL) _,Mn,B adequately represents the moment of 3d atoms in R2Fei4_,Mn,B systems the moments at 4.2 K for R2Fe1+,Mn,B compounds (R = Nd, Pr and Gd) were computed assuming a collinear structure and using the rare earth moments obtained from the RzFe14B systems [ 111. Results are given in column 7 of Table 1. There is no significant difference between the computed moments and the experimental values for R = Nd, Pr compounds, since the measured magnetization is uncertain by 2 to 3%. In the case of Gd2Fe14_,Mn,B the computed moments are lower than the experimental values. Perhaps in these systems the assumed coupling mode does not obtain. 3.2. Curie temperature The composition dependencies of the Curie temperatures T, of R,Fe14_,Mn,B systems are shown in Table 1. For all these sytems a similar composition dependence is observed. T, decreases rapidly at the rate of about 100 K (Mn atom)-i when iron is replaced by manganese, indicating a weakening of the Fe-Fe magnetic interaction. However, T, increases significantly at a rate of about 80 K (Co atom)-’ with substitution of iron by cobalt [ 6, 71. As mentioned before [ 51, the increase in T, of R2 Fei4 _-xCo,B systems can be correlated with the increase in the positive interaction between 3d metals and decrease of negative ones, cobalt replacing a fraction of Fe(8j,), Fe(8j,) or Fe(lGk,) atoms. In R,Fei4_,Mn,B systems it is postulated that the manganese atoms also replace a portion of the Sj and 16k1 Fe atoms
59
and the manganese couple antiparallel to the iron, resulting in a rapid decrease of the Curie temperature as in the R,(Fe, Mn),, and R,(Mn, Co),? systems. 3.3. Anisotropies and spin reorientation The directions of magnetization and anisotropy fields at 77 K and room temperature are presented in Table 2. The results for the yttrium and gadolinium systems show a s~bst~tial anisotropy field, indicating significant 3d sublattice contributions. On substituting iron by manganese, HA at 77 K increases for both Y and Gd compounds; at room temperature HA also increases for R = Gd, but decreases for R = Y. The incorporation of manganese into the iron sublattice substantially strengthens the uniaxial anisotropy of these systems at lower temperature, as is observed in R2(Co, Fe, Mn)i, systems 1121. The different behavior of HA at room temperature for yttrium and gadolinium compounds may be a consequence of the effect of the magnetic gadolinium ion on the band structure in Gd1Fe,4_,Mn,B compounds. In the R,Fei,_,Mn,B (R = Nd, Pr) systems, HA measured at room temperature decreases with increasing X. In this respect these systems behave similarly to Y,Fe14_,Mn,B. The magnetic behavior of the Ndz Fei4 _,Mn,B systems is generally similar to that of Nd2Fe14B, except that the interaction which is responsible for the tipping of the magnetization vector away from the c axis at low temperatures is weakened when iron is partially replaced by manganese. Magnetization versus applied field for Nd~Fe~~Mn~B in the easy and hard directions is plotted in Fig. 1. These data can be used to estimate the cone angle 8. 8 = tan-‘(M,/M,,), where M, and Mg, are the spontaneous moments parallel and perpendicular to the unique axis, obtained by back-extrapolating the linear portion of the curves to zero applied field. 8 w 10”. This contrasts with 8 = 30” for NdzFe14B. The reduction of B by 20” is one indication of the influence of manganese alloying. The reduction in spin-reorientation temperature T, from about 140 K for Nd,Fei4B to about 65 K for NdpFelZMnzB (see Fig. 2) is another indication that the interaction which causes the magnetization to lie off the c axis is weakened when iron is replaced by manganese. The reductions observed for T, and 8 are consistent. The spin-reorientation phenomenon in NdzFe14B has recently been interpreted by Boltich and Wallace [13] on a crystal field model using the Hamiltonian ;fc = BSO$ + BzOg + 2(g - l)/[email protected]. Careful examination of their formalism shows that exchange has the effect of pulling the magnetization vector away from the c axis. In other words, 0 > 0 because of the effect of exchange. As exchange is weakened, 8 relaxes toward zero. Replacement of iron by manganese weakens exchange, as indicated by the decline in T,. Thus, it is expected that 8 will be reduced as iron is replaced by manganese since under these circumstances exchange competes less well with the crystal field interaction and the 3d sublattice anisotropies which direct the magnetization along the c axis, This is one factor which reduces 8. The other is the
60
enhanced 3d sublattice placed by manganese.
anisotropy
(see Table 2) which results as iron is re-
Acknowledgment This work was supported Office.
by a contract
with the U.S. Army Research
References 1 M. Sagawa, S. Fujimura, N. Tagawa, H. Tamamoto and Y. Matsuura, J. Appl. Phys., 55 (1984) 2083. 2 M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga, IEEE Trans. Mug., 20 (1984) 1584. 3 J. J. Croat, J. F. Herb&, R. W. Lee and F. E. Pinkerton, J. Appl. Phys., 55 (1984) 2078. 4 M. Q. Huang, E. B. Boltich, E. Oswald and W. E. Wallace, in X. Guangxian and X. Jimei (eds.), New Frontiers in Rare Earth Science and Applications, Science Press, Beijing, 1985, p. 974. 5 E. Burzo, E. B. Boltich, M. Q. Huang and W. E. Wallace, Proc. 4th Int. Symp. on Magnetic Anisotropy and Coercivity in Rare Earth-Transition Metal Alloys, Dayton, Ohio, May, 9, 1985, p. 771, University of Dayton, Magnetics, KL-365, Dayton, Ohio 45469, U.S.A. 6 Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett., 46 (3) (1985). 7 M. Q. Huang, E. B. Boltich, W. E. Wallace and E. Oswald, J. Magn. Magn. Mater., in the press. 8 M. Merches, W. E. Wallace and R. S. Craig, J. Magn. Magn. Muter., 24 (1981) 97. 9 H. R. Kirchmayr and W. Steiner, J. Phys., Paris, 32 (1971) Cl-665-7. 10 W. E. Wallace, Rare Earth Zntermetallics, Academic Press, New York, 1973, p. 190. 11 E. B. Boltich, E. Oswald, M. Q. Huang, S. Hirosawa, W. E. Wallace and E. Burzo, J. Appl. Phys., 57 (1985) 4106. 12 W. E. Wallace and K. S. V. L. Narasimhan, in E. C. Subbarao and W. E. Wallace (eds.), The Science and Technology of Rare Earth Materials, Academic Press, New York, 1980, p. 329. 13 E. B. Boltich and W. E. Wallace, Solid State Commun., 55 (1985) 529.