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Magnetic anisotropy in the system La2Fe14 − xCoxB and its relation to the system Nd2Fe14 − xCoxB
Journal of the Less-Common
Metals, 136 (1988) 361 - 373
367
MAGNETIC ANISOTROPY IN THE SYSTEM LazFe14_,Co,B ITS RELATION TO THE SYSTEM Nd2Fei4 _,Co,B R. GR&XNGER
AND
and H. KIRCHMAYR
Institute for Experimental
Physics, University of Technology,
Vienna (Austria)
K. H. J. BUSCHOW Philips Research Laboratories,
5600 JA Eindhoven
(The Netherlands)
(Received June 22,1987)
Summary We have studied the system La,Fe,,_,Co,B by means of X-ray diffraction, magnetic measurements and singular point detection (SPD) measurements. We present experimental data on the concentration dependence of the lattice constants, Curie temperatures, saturation magnetization and anisotropy fields. Attention is mainly given to the determination of anisotropy fields, which were also studied as a function of temperature in the range from 4.2 K to the corresponding Curie temperatures. The experimental information obtained for the system La,Fe14_,Co,B was used to distinguish the relative anisotropy contributions of the neodymium sublattice and 3d sublattice in NdZFei4_,Co,B. 1. Introduction In a previous investigation [l] we studied the magnetic properties of the system NdzFe14_,Co,B and showed that compounds with low cobalt concentration give rise to only one spin reorientation below T,, whereas compounds with high cobalt concentration exhibit two spin reorientations below T,. The additional spin reorientation in the latter compounds occurs at fairly high temperatures and originates from the difference in sign and the difference in temperature dependence between the contributions of the neodymium sublattice anisotropy and the 3d sublattice anisotropy. In this investigation we also addressed the question of how far substitution of cobalt for iron in NdzFei4B influences the anisotropy field and its temperature dependence in concentration ranges and temperature ranges of interest for permanent magnet applications of these materials. We showed that cobalt substitution generally lowers the room temperature anisotropy field, without offering a satisfactory explanation for this effect. In the present investigation we have focused our attention on the magnetic properties of the system LazFe14_,Co,B. Since lanthanum is non@ Elsevier Sequoia/Printed in The Netherlands
368
magnetic the study of this system enables us to obtain conclusive experimental information on the 3d sublattice anisotropy as a function of temperature and concentration. The results obtained from this investigation will be used to distinguish the relative contributions of the neodymium and 3d sublattice anisotropies in Nd,Fe,,_,Co,B. It will be shown that the reduction of the room temperature anisotropy with increasing x in NdzFeis_,Co,B is not caused by a reduction in crystal field interaction, but is mainly due to the strong decrease of the exchange field experienced by the neodymium moments when cobalt substitutes for iron in Nd,Fe&
2. Experimental
details
Samples of LazFe,, _,Co,B compounds were prepared by arc melting under purified argon gas from starting materials of at least 99.9% purity. They were vacuum annealed for 3 weeks at about 850 “C. After this treatment the samples were examined by X-ray diffraction and found to be approximately single phase, the content of impurity phases being of the order of a few per cent. The Curie temperatures of the various compounds were derived from measurements of the temperature dependence of the magnetization u by means of the Faraday method. The values of T, were obtained by plotting u2 us. T and extrapolating to a2 + 0. The anisotropy field HA was measured in a pulsed field system from 4.2 K up to the Curie temperature using the singular point detection (SPD) technique. This method leads to a singularity at H = HA in experimental curves of d2M/dt2 us. H (M is the magnetization) if the c axis is the easy magnetization direction.
3. Results and discussion X-ray diffraction showed that all La2Fei4_,Co,B compounds investigated have the tetragonal Nd2Fe,,B-type structure [ 21. The lattice constants are plotted as a function of concentration in Fig. 1. Both the a and the c axis decrease with increasing cobalt concentration in such a way that c/a varies only slightly for small x and decreases somewhat more strongly for large x. The concentration dependence of the Curie temperature is shown in Fig. 2. At low cobalt concentrations, in particular, the values of T, vary strongly with 3t, which may be associated with a preferred site occupation, as discussed in ref. 3. Included in this figure is the concentration dependence of the room temperature saturation magnetization, measured on magnetically aligned powder samples. Results of singular point detection measurements are shown in Fig. 3. In an earlier investigation we had already found that HA in LazFeieB behaves anomalously, i.e. it increases with temperature in the range 4 - 350 K and
369 900 :\
‘1
“. -1220
890-
0
4
2
6
10
8
12
14
Y
Fig. 1. Concentration dependence of the lattice constants in LazFelJ _,Co,B.
1000
200
160
4c
600 La,Fe,,_,Co,B
0
IL 0
1
2
1
4
1
6
I
I
8
10
500
I
12
14
Y
Fig. 2. Concentration
dependence of the Curie temperature ature saturation magnetization U, in La~Fe~,+,Co,B.
T,
and room
temper-
370 I
I
Fig.
I
I
3, Temperature dependence series LazFe IJ_~CO~B.
I
I
I
of the anisotropy
1
field
I
in various
compounds of the
decreases at higher temperatures [4]. This tendency of HA to increase with temperature is also shown by LazFe,&ozB, albeit the corresponding temperature range is far more limited here than in LazFei4B. The temperatures at which HA goes to zero for the different compounds in Fig. 3 correspond to the Curie temperatures of these compounds (see also Fig. 2). The concentration dependences of the anisotropy fields measured at 4.2 K and at 300 K can be compared in Fig. 4. It may be seen that at both temperatures HA(x) gives rise to a maximum. In general, it may be said that the development of a maximum becomes more pronounced when the temperature corresponding to these curves decreases. The occurrence of maxima in the HA(x) curves is intimately connected with the fact the HA in the ironrich compounds shows an anomalous temperature dependence in the low temperature regime. It is possible that this anomalous temperature depenwith the development of a considerable dence of HA is closely connected spontaneous volume magnetostriction [ 51. This may lead to a decrease in HA with temperature and may counteract the normal tendency of HA to increase with the magnetization, i.e. with decreasing temperature. The anisotropy field HA is related to the anisotropy constants K1, Kz .... K, by means of the expression LJSHA = K1 + 2Kz 2 where J, is the magnetic polarization. This latter quantity can be obtained from u, and the density. Using the X-ray density derived from the data given in Fig. 1 together with the u, data of Fig, 2 we have calculated room temperature values of i J,H, and plotted this quantity as a function of concentration in the bottom part of Fig. 4 (full triangles). In the likely event that Kz can be neglected with respect to K1, this curve represents the concentra-
Fig. 4. Concentration dependence of the anisotropy fields HA at 4.2 K (full circles) and 300 K (full squares) in LazFe14_,Co,B. The curve shown in the bottom part of the figure represents the concentration dependence of K1 = LHAJs at 300 K in La~Fer+,Co, (full triangles). The curve in the top part of th e figure (open circles) represents the concentration dependence of the neodymium sublattice contribution to the anisotropy energy iH*J, (for more details see text).
tion dependence of K1 for LaaFelq_,Co,B. From published data of the concentration dependence of HA [l] and CT,[2] in NdzFe14-,Co,B we have calculated the concentration dependence of iJ,H, for this series of compounds. In this case, too, +H,J, represents mainly K1 since Kz can practically be neglected at room temperature (K, = is K,) [3,6]. The neodymium sublattice contribution may now be obtained after subtraction of the iHAJs values found for La2Fe14_xC~uB, the latter being regarded as representative of the 3d sublattice contribution. The curve of iH,J, obtained in this way is shown in the top part of Fig. 4 (open circles). It may be seen that the anisotropy energy associated with the single-ion contribution of neodymium in NdZFe14_xC~XB remains almost constant at the lowest cobalt concentrations but decreases strongly at higher cobalt concentrations. Extrapolation to Nd,Co,& would lead to a value more than a factor of 2 lower than in NdzFe14B. It is well known that the single-ion contribution to the anisotropy in Nd,Fe,,B is crystal field induced and originates from the electrostatic interaction of the asymmetric 4f electron charge cloud with effective charges surrounding it [ 3,61. The leading term in materials of uniaxial symmetry is of second order. This holds a fortiori when room temperature values of the anisotropy energy are considered. To a good approximation one may therefore take the anisotropy energy as proportional to the second-order crystal field parameter A,“. It has been shown on several occasions that the sign and magnitude of A?” on a given rare earth site in a given crystal structure can be probed by means of “‘Gd Mijssbauer spectroscopy. Results of such measurements are now available for Gd,Fe,,B as well as for Gd,C!o,& [7,8]. It
372
follows from these measurements that the AZ0 values in RZCoi4B are the same (within 20%) as the values of the corresponding compounds R*Fe,,B. This means that on the basis of the effect of AZ0alone one would not expect such a strong concentration dependence of the rare earth anisotropy as follows from the results shown in the top part of Fig. 4. Honma et al. [9] have analysed the magnetocrystalline anisotropy in NdzFei4B in terms of crystal field interaction and exchange interactions and showed that the anisotropy arises through a combined action of both types of interaction. Experimentally it is found that the 3d-4f coupling constant JR -3d in R,Co,,B is roughly twice that in R2Fe,&, but owing to a smaller 3d moment in the former compound the exchange field H,, a JR -3&M acting on the 4f moment remains approximately the same in both series of compounds [lo]. For completeness we note that the concentration dependence of T, is not a measure of the variation in the 3d-4f exchange energy in NdzFei4_xCo,B, since T, reflects mainly the change in the 3d-3d exchange energy in these materials. One may state therefore that the concentration dependence of the rare earth anisotropy in NdZFe14_XCoXB cannot be explained by corresponding changes in ASoand He,. 4. Concluding remarks We have studied the magnetocrystalline anisotropy in the pseudoternary series LazFei4_,Co,B and found that the room temperature anisotropy constant K, changes its sign at approximately x = 11.5. This sign change occurs at a slightly higher concentration than found in the series Y,Fei4_,Co,B (x = 9.5) [ll]. By combining the results obtained in the course of the present investigation with those obtained previously for NdzFeiQ_,Co,B we were able to separate the contributions to the room temperature anisotropy in Nd,Fer4_,Co,B. In the concentration range of technological interest (i.e. for relatively small cobalt concentrations) both contributions vary only little with concentration. In this concentration range and at room temperature the relative 3d sublattice contribution to the anisotropy is still a substantial proportion of the total anisotropy. It is roughly three times smaller than the neodymium sublattice contribution. The strong decrease in the latter anisotropy at higher cobalt concentrations cannot satisfactorily be explained in terms of the exchange field and the second-order crystal field parameter AZ0 alone. This means that it is necessary to include higher order crystal field terms for describing the rare earth anisotropy in Nd2Fei4_xCoxB, even at room temperature. Our results suggest that it is mainly the concentration dependence of these higher order terms which is responsible for the change in the rare earth anisotropy in this series. References 1 R. GrSssinger, R. Krewenka, X. K. Sun, R. Eibler, H. R. Kirchmayr and K. H. J. Buschow, J. Less-Common Met., 124 (1986) 165.
373 2 J. F. Herb&, J. J. Croat, F. E. Pinkerton and W. B. Yelon, Phys. Rev. B, 29 (1984) 4176. 3 K. H. J. Buschow, Materials Science Reps., 1 (1986) 3. 4 R. Griissinger, X. K. Sun, R. Eibler, K. H. J. Buschow and H. R. Kirchmayr, J. Magn. Magn. Mater., 58 (1986) 55. 5 R. Grijssinger and K. H. J. Buschow, J. Less-Common Met., 135 (1987) 39. 6 J. M. D. Coey, J. Less-Common Met., 126 (1986) 21. 7 M. Boge, G. Czjzek, D. Givord, C. Jeandey, H. S. Li and J. L. Oddon, J. Phys. F, 16 (1986) L67. 8 H. H. A. Smit, R. C. Thiel and K. H. J. Buschow, Physica, to be published. 9 H. Honma, M. Shimotomai and M. Doyama, Proc. 5th Znt. Conf. on Crystalline Field and Anomalous Mixing Effects in f-Electron Systems, Sendai, April 1985. 10 K. H. J. Buschow, D. B. de Mooij, S. Sinnema, R. J. Radwanski and J. J. H. Franse, J. Magn. Magn. Mater., 51 (1985) 221. 11 F. Bolzoni, J. M. D. Coey, J. Gavigan, D. Givord, 0. Moze, L. Pareti and T. Viadieu, J. Magn. Magn. Mater., 65 (1987) 123.
Metals, 136 (1988) 361 - 373
367
MAGNETIC ANISOTROPY IN THE SYSTEM LazFe14_,Co,B ITS RELATION TO THE SYSTEM Nd2Fei4 _,Co,B R. GR&XNGER
AND
and H. KIRCHMAYR
Institute for Experimental
Physics, University of Technology,
Vienna (Austria)
K. H. J. BUSCHOW Philips Research Laboratories,
5600 JA Eindhoven
(The Netherlands)
(Received June 22,1987)
Summary We have studied the system La,Fe,,_,Co,B by means of X-ray diffraction, magnetic measurements and singular point detection (SPD) measurements. We present experimental data on the concentration dependence of the lattice constants, Curie temperatures, saturation magnetization and anisotropy fields. Attention is mainly given to the determination of anisotropy fields, which were also studied as a function of temperature in the range from 4.2 K to the corresponding Curie temperatures. The experimental information obtained for the system La,Fe14_,Co,B was used to distinguish the relative anisotropy contributions of the neodymium sublattice and 3d sublattice in NdZFei4_,Co,B. 1. Introduction In a previous investigation [l] we studied the magnetic properties of the system NdzFe14_,Co,B and showed that compounds with low cobalt concentration give rise to only one spin reorientation below T,, whereas compounds with high cobalt concentration exhibit two spin reorientations below T,. The additional spin reorientation in the latter compounds occurs at fairly high temperatures and originates from the difference in sign and the difference in temperature dependence between the contributions of the neodymium sublattice anisotropy and the 3d sublattice anisotropy. In this investigation we also addressed the question of how far substitution of cobalt for iron in NdzFei4B influences the anisotropy field and its temperature dependence in concentration ranges and temperature ranges of interest for permanent magnet applications of these materials. We showed that cobalt substitution generally lowers the room temperature anisotropy field, without offering a satisfactory explanation for this effect. In the present investigation we have focused our attention on the magnetic properties of the system LazFe14_,Co,B. Since lanthanum is non@ Elsevier Sequoia/Printed in The Netherlands
368
magnetic the study of this system enables us to obtain conclusive experimental information on the 3d sublattice anisotropy as a function of temperature and concentration. The results obtained from this investigation will be used to distinguish the relative contributions of the neodymium and 3d sublattice anisotropies in Nd,Fe,,_,Co,B. It will be shown that the reduction of the room temperature anisotropy with increasing x in NdzFeis_,Co,B is not caused by a reduction in crystal field interaction, but is mainly due to the strong decrease of the exchange field experienced by the neodymium moments when cobalt substitutes for iron in Nd,Fe&
2. Experimental
details
Samples of LazFe,, _,Co,B compounds were prepared by arc melting under purified argon gas from starting materials of at least 99.9% purity. They were vacuum annealed for 3 weeks at about 850 “C. After this treatment the samples were examined by X-ray diffraction and found to be approximately single phase, the content of impurity phases being of the order of a few per cent. The Curie temperatures of the various compounds were derived from measurements of the temperature dependence of the magnetization u by means of the Faraday method. The values of T, were obtained by plotting u2 us. T and extrapolating to a2 + 0. The anisotropy field HA was measured in a pulsed field system from 4.2 K up to the Curie temperature using the singular point detection (SPD) technique. This method leads to a singularity at H = HA in experimental curves of d2M/dt2 us. H (M is the magnetization) if the c axis is the easy magnetization direction.
3. Results and discussion X-ray diffraction showed that all La2Fei4_,Co,B compounds investigated have the tetragonal Nd2Fe,,B-type structure [ 21. The lattice constants are plotted as a function of concentration in Fig. 1. Both the a and the c axis decrease with increasing cobalt concentration in such a way that c/a varies only slightly for small x and decreases somewhat more strongly for large x. The concentration dependence of the Curie temperature is shown in Fig. 2. At low cobalt concentrations, in particular, the values of T, vary strongly with 3t, which may be associated with a preferred site occupation, as discussed in ref. 3. Included in this figure is the concentration dependence of the room temperature saturation magnetization, measured on magnetically aligned powder samples. Results of singular point detection measurements are shown in Fig. 3. In an earlier investigation we had already found that HA in LazFeieB behaves anomalously, i.e. it increases with temperature in the range 4 - 350 K and
369 900 :\
‘1
“. -1220
890-
0
4
2
6
10
8
12
14
Y
Fig. 1. Concentration dependence of the lattice constants in LazFelJ _,Co,B.
1000
200
160
4c
600 La,Fe,,_,Co,B
0
IL 0
1
2
1
4
1
6
I
I
8
10
500
I
12
14
Y
Fig. 2. Concentration
dependence of the Curie temperature ature saturation magnetization U, in La~Fe~,+,Co,B.
T,
and room
temper-
370 I
I
Fig.
I
I
3, Temperature dependence series LazFe IJ_~CO~B.
I
I
I
of the anisotropy
1
field
I
in various
compounds of the
decreases at higher temperatures [4]. This tendency of HA to increase with temperature is also shown by LazFe,&ozB, albeit the corresponding temperature range is far more limited here than in LazFei4B. The temperatures at which HA goes to zero for the different compounds in Fig. 3 correspond to the Curie temperatures of these compounds (see also Fig. 2). The concentration dependences of the anisotropy fields measured at 4.2 K and at 300 K can be compared in Fig. 4. It may be seen that at both temperatures HA(x) gives rise to a maximum. In general, it may be said that the development of a maximum becomes more pronounced when the temperature corresponding to these curves decreases. The occurrence of maxima in the HA(x) curves is intimately connected with the fact the HA in the ironrich compounds shows an anomalous temperature dependence in the low temperature regime. It is possible that this anomalous temperature depenwith the development of a considerable dence of HA is closely connected spontaneous volume magnetostriction [ 51. This may lead to a decrease in HA with temperature and may counteract the normal tendency of HA to increase with the magnetization, i.e. with decreasing temperature. The anisotropy field HA is related to the anisotropy constants K1, Kz .... K, by means of the expression LJSHA = K1 + 2Kz 2 where J, is the magnetic polarization. This latter quantity can be obtained from u, and the density. Using the X-ray density derived from the data given in Fig. 1 together with the u, data of Fig, 2 we have calculated room temperature values of i J,H, and plotted this quantity as a function of concentration in the bottom part of Fig. 4 (full triangles). In the likely event that Kz can be neglected with respect to K1, this curve represents the concentra-
Fig. 4. Concentration dependence of the anisotropy fields HA at 4.2 K (full circles) and 300 K (full squares) in LazFe14_,Co,B. The curve shown in the bottom part of the figure represents the concentration dependence of K1 = LHAJs at 300 K in La~Fer+,Co, (full triangles). The curve in the top part of th e figure (open circles) represents the concentration dependence of the neodymium sublattice contribution to the anisotropy energy iH*J, (for more details see text).
tion dependence of K1 for LaaFelq_,Co,B. From published data of the concentration dependence of HA [l] and CT,[2] in NdzFe14-,Co,B we have calculated the concentration dependence of iJ,H, for this series of compounds. In this case, too, +H,J, represents mainly K1 since Kz can practically be neglected at room temperature (K, = is K,) [3,6]. The neodymium sublattice contribution may now be obtained after subtraction of the iHAJs values found for La2Fe14_xC~uB, the latter being regarded as representative of the 3d sublattice contribution. The curve of iH,J, obtained in this way is shown in the top part of Fig. 4 (open circles). It may be seen that the anisotropy energy associated with the single-ion contribution of neodymium in NdZFe14_xC~XB remains almost constant at the lowest cobalt concentrations but decreases strongly at higher cobalt concentrations. Extrapolation to Nd,Co,& would lead to a value more than a factor of 2 lower than in NdzFe14B. It is well known that the single-ion contribution to the anisotropy in Nd,Fe,,B is crystal field induced and originates from the electrostatic interaction of the asymmetric 4f electron charge cloud with effective charges surrounding it [ 3,61. The leading term in materials of uniaxial symmetry is of second order. This holds a fortiori when room temperature values of the anisotropy energy are considered. To a good approximation one may therefore take the anisotropy energy as proportional to the second-order crystal field parameter A,“. It has been shown on several occasions that the sign and magnitude of A?” on a given rare earth site in a given crystal structure can be probed by means of “‘Gd Mijssbauer spectroscopy. Results of such measurements are now available for Gd,Fe,,B as well as for Gd,C!o,& [7,8]. It
372
follows from these measurements that the AZ0 values in RZCoi4B are the same (within 20%) as the values of the corresponding compounds R*Fe,,B. This means that on the basis of the effect of AZ0alone one would not expect such a strong concentration dependence of the rare earth anisotropy as follows from the results shown in the top part of Fig. 4. Honma et al. [9] have analysed the magnetocrystalline anisotropy in NdzFei4B in terms of crystal field interaction and exchange interactions and showed that the anisotropy arises through a combined action of both types of interaction. Experimentally it is found that the 3d-4f coupling constant JR -3d in R,Co,,B is roughly twice that in R2Fe,&, but owing to a smaller 3d moment in the former compound the exchange field H,, a JR -3&M acting on the 4f moment remains approximately the same in both series of compounds [lo]. For completeness we note that the concentration dependence of T, is not a measure of the variation in the 3d-4f exchange energy in NdzFei4_xCo,B, since T, reflects mainly the change in the 3d-3d exchange energy in these materials. One may state therefore that the concentration dependence of the rare earth anisotropy in NdZFe14_XCoXB cannot be explained by corresponding changes in ASoand He,. 4. Concluding remarks We have studied the magnetocrystalline anisotropy in the pseudoternary series LazFei4_,Co,B and found that the room temperature anisotropy constant K, changes its sign at approximately x = 11.5. This sign change occurs at a slightly higher concentration than found in the series Y,Fei4_,Co,B (x = 9.5) [ll]. By combining the results obtained in the course of the present investigation with those obtained previously for NdzFeiQ_,Co,B we were able to separate the contributions to the room temperature anisotropy in Nd,Fer4_,Co,B. In the concentration range of technological interest (i.e. for relatively small cobalt concentrations) both contributions vary only little with concentration. In this concentration range and at room temperature the relative 3d sublattice contribution to the anisotropy is still a substantial proportion of the total anisotropy. It is roughly three times smaller than the neodymium sublattice contribution. The strong decrease in the latter anisotropy at higher cobalt concentrations cannot satisfactorily be explained in terms of the exchange field and the second-order crystal field parameter AZ0 alone. This means that it is necessary to include higher order crystal field terms for describing the rare earth anisotropy in Nd2Fei4_xCoxB, even at room temperature. Our results suggest that it is mainly the concentration dependence of these higher order terms which is responsible for the change in the rare earth anisotropy in this series. References 1 R. GrSssinger, R. Krewenka, X. K. Sun, R. Eibler, H. R. Kirchmayr and K. H. J. Buschow, J. Less-Common Met., 124 (1986) 165.
373 2 J. F. Herb&, J. J. Croat, F. E. Pinkerton and W. B. Yelon, Phys. Rev. B, 29 (1984) 4176. 3 K. H. J. Buschow, Materials Science Reps., 1 (1986) 3. 4 R. Griissinger, X. K. Sun, R. Eibler, K. H. J. Buschow and H. R. Kirchmayr, J. Magn. Magn. Mater., 58 (1986) 55. 5 R. Grijssinger and K. H. J. Buschow, J. Less-Common Met., 135 (1987) 39. 6 J. M. D. Coey, J. Less-Common Met., 126 (1986) 21. 7 M. Boge, G. Czjzek, D. Givord, C. Jeandey, H. S. Li and J. L. Oddon, J. Phys. F, 16 (1986) L67. 8 H. H. A. Smit, R. C. Thiel and K. H. J. Buschow, Physica, to be published. 9 H. Honma, M. Shimotomai and M. Doyama, Proc. 5th Znt. Conf. on Crystalline Field and Anomalous Mixing Effects in f-Electron Systems, Sendai, April 1985. 10 K. H. J. Buschow, D. B. de Mooij, S. Sinnema, R. J. Radwanski and J. J. H. Franse, J. Magn. Magn. Mater., 51 (1985) 221. 11 F. Bolzoni, J. M. D. Coey, J. Gavigan, D. Givord, 0. Moze, L. Pareti and T. Viadieu, J. Magn. Magn. Mater., 65 (1987) 123.