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A Mössbauer study of spin-reorientation in YCo4B
0038-1098/92 $5.00 + .00 Pergamon Press plc
Solid State Communications, Vol. 81, No. 1, pp. 121-123, 1992. Printed in Great Britain.
A MOSSBAUER STUDY OF SPIN-REORIENTATION IN YCo4B J.M. Cadogan and Hong-Shuo Li School of Physics, The University of New South Wales, Kensington, N.S.W. 2033 Australia and S.J. Campbell and J. Jing Department of Physics, University College, The University of New South Wales, Campbell, A.C.T. 2600, Australia (Received 25 September 1991 by P. Bur&t)
The occurrence of a plane-axis spin orientation in YCo4B between 4.2 and 293 K has been confirmed by Mtssbauer spectroscopy on a sample of YCo4 B doped with 0.5 wt % s7Fe. The reorientation is driven by the temperature-dependent competition between the magnetoerystalline anisotropies oi' the 2c and 6i cobalt sites. The Mfssbauer spectra indicate a significant preference of S7Fe for occupying the crystallographic 2c site. 1. INTRODUCTION CHANGES IN easy magnetization direction, more commonly known as spin reorientations, are commonplace in rare-earth-transition metal intermetallics. They result from the competition between the magnetoerystalline anisotropies of the rare-earth and transition metal sublattices, driven by the different temperature dependences of these anisotropies. Such reorientations are well accounted for by a model describing the interplay of molecular field and crystal field interactions [e.g. 1, 2]. Not so common are spin reorientations in compounds having a single magnetic sublattice. Such reorientations are driven by the competing anisotropies of atoms within a single sublattice but occupying different crystallographic sites. Magnetization measurements on the hexagonal compound YCo4 B by Dung et aL [3] provided evidence of such a spin orientation at 145K in which only the cobalt sublattice is involved. The magnetic structure was reported to be axial above 145K and planar below 145 K. YCo4B forms in the hexagonal CeCo4 B structure [4] with the space group P6/mmm. Cobalt atoms occupy the 2c and 6i crystallographic sites. Yttrium atoms occupy the la and lb sites and boron atoms occupy the 2d sites. On the basis of crystal field, point-charge model calculations, Dung et aL found easy c-axis anisotropy (Kin > 0) at the 2c sites and planar anisotropy (K~ < 0) at the 6i sites. (/'he anisotropy energy is K~ sin 2 0, where 0 is the angle between the c-axis and the magnetization). Pulsed 59Co N M R experiments by Kapusta et aL [5] showed large orbital contributions
to the cobalt hyperfine fields, indicative of the local cobalt anisotropy, in agreement with the work of Dung et al. [3]. However, other magnetic studies of YCo4B [6, 7] showed no sign of any spin reorientation in this compound. It was the aim of the present work to use Mfssbauer spectroscopy to verify the occurrence, or absence, of a spin reorientation in YCo4B at low temperatures. 2. EXPERIMENTAL DETAILS The sample of YCo4B doped with 57Fe was prepared by argon-arc melting followed by annealing in vacuo at 900°C for two weeks. 57Fe was substituted for Co to the extent of 0.5wt%. Powder X-ray diffraction using CuK~t radiation showed the sample to be single phase with lattice parameters a = 4.99 ,~ and c = 6.87A. 57Fe Mfssbauer spectroscopy was carded out in conventional transmission mode at 4.2 and 293 K using a 57CoRh source. The spectrometer was calibrated using a standard ~t-Fe foil. 3. RESULTS AND DISCUSSION The 4.2 and 293 K Mrssbauer spectra of YCo4 B: 57Fe are shown in Fig. 1. The spectra were fitted with two Lorentzian sextets corresponding to the 2c and 6i sites. The relative areas of the two subspectra for both the 4.2 and 293 K spectra are 40(5)% and 60(5)%. A completely random substitution of 57Fe for Co would yield two sextets with relative areas 25 and 75%. The subspectrum with the larger area has the smaller
121
SPIN-REORIENTATION IN YCo4B
122
Table 2. Polar angles of the hyperfine field in the principal axis system of the electric field gradient (EFG ) of the 6i sites for the possible orientations of the magnetization
100
99 4.2 K 98 .
~
Vol. 81, No. 1
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I
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I
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.
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I
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Magnetization direction
0(°)
~b(°)
[00 1]
90
[100]
0 60
+ 90
1/3 2/3
[21 o]
90 30
+ 90 ___90
1/3 2/3
180
Fraction of intensity 1
I
100
quadrupole terms yields the following expression for the energy levels (using standard notation)
99
eQV~z [3m2 - I(I + 1)] Em = gl#sBhfm + 8 / ( 2 1 - 1)
293K
× [3cos 2 0 -
98
1 + qsin 2 0 c o s 2 4 ] .
The magnetic hyperfine field (Bh/) direction is defined by the polar angles 0, ~b in the principal axis frame of the electric field gradient (EFG). The crystallographic 2c site has the hexagonal point symmetry ~m2 and so Fig. 1. The 57Fe M6ssbauer spectra ofYCo4B : 57Fe at its asymmetry parameter r/ = 0. The 6i site, however, 4.2 and 293 K. has the orthorhombic point symmetry ram, leading to a non-zero 7. We have carried out point-charge hyperfine field, in accord with the N M R data [5] and calculations of the EFG principal axis system at the is thus associated with the 6i sites. It is clear from our 6i sites. Our results indicate that ~/ = 0.8 for the 6i site spectra that iron shows a significant preference for and the 6 atoms of this site are partitioned into two occupying the 2c sites rather than the 6i sites. This groups of 2 and 4 atoms respectively with regard to 0, observation is in agreement with studies ofisostructural 4, when the magnetization lies in the basal plane. We compounds having a much higher iron content [8]. note here that an 57Fe M6ssbauer study of La(Ni, Fe)5 The 57Fe hyperfine parameters deduced from the by Lamloumi et al. [9] found ~/ = 0.7--0.8 for the 3g spectra of Fig. 1 are listed in Table 1. The ratio of the site in this hexagonal CaCu~-P6/mmm structure. The quadrupole splittings for the majority (6i) subspectra 3g site in the CaCu5 structure corresponds to the 6i site at 235 to 4.2 K is - 2 . 1 . This observation provides in the YCo4 B structure. The 0, ~ values for the various unequivocal support for the existence of an axis-plane 6i atoms for different orientations of the magnetizspin reorientation between 293 and 4.2 K. For the case ation are presented in Table 2. Figure 2 shows the when the quadrupole interaction can be treated as a local symmetry around these 6i atoms, including the perturbation on the Zeeman interaction, the nuclear orthogonal mirror planes at these sites. The principal Hamiltonian comprising the Zeeman and electric axes of the EFG tensor are coincident with these mirrors. When the contributions of the various atoms Table 1. Hyperfine parameters deduced from 57Fe in the 6i site are averaged, we find that the quadrupole M6ssbauer spectroscopy in YCo4 B: s7Fe ( Bhl = hyper- splitting at 293 K is a factor of - 2 times that at 4.2 K, fine magnetic field; QS = quadrupolar splitting) in excellent agreement with our results. Further support for the above conclusion can be Site T (K) B,/(T) QS (rams -I) found in the temperature dependence of the 5~Fe hyperfine fields in YCo4B:57Fe. Upon heating from 6i 4.2 15.9 + 0.54 4.2 to 293 K, the hyperfine field at the 2c site is reduced 293 13.5 - 1.15 by 5.1% whereas the hypefline field at the 6i site 2c 4.2 26.9 - 0.17 is reduced by 16.4%. To first approximation, the 293 25.7 - 0.34 anisotropy constant KI varies with temperature as the .
-10
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I
-5
~
.
•
•
I
.
•
i
"
I
0 5 V (mm/s)
.
t
•
i
1
10
Vol. 81, No. 1
SPIN-REORIENTATION IN YCo4 B
123
lographic 6i sites between 4.2 and 293 K is observed which is in full agreement with the behaviour expected for a spin reorientation, based on the point symmetry of that site. In addition, 57Fe shows preferential occupation of the 2c site.
Q
Y
(la,lb)
Q
Co (6i)
•
B (2d)
Acknowledgements - The authors are grateful to the Australian Research Council for its financial support and also the award of an ARC Research Associateship to JJ. HSL acknowledges the award of a PostDoctoral Research Fellowship by The University of New South Wales. We wish to thank Dr A.V.J. Edge for his sample preparation. REFERENCES
YCo4B Fig. 2. The atomic arrangement of YCo4 B projected onto the c-plane (the 2c cobalt sites are omitted for clarity). The orientations of the EFG principal axes at the 6i cobalt sites are marked.
1.
2. cube of the magnetization. Thus, the more rapid decrease in the 6i hyperfine field implies a more rapid fall-off in K~ (6i) than in K~(2c) with increasing temperature. The data presented here suggest a reduction in Kt(6i) of 41%, much greater than the 15% reduction in K~(2c). Hence, the dominant planar anisotropy of the 6i site gives way to the axial anisotropy of the 2c site with increasing temperature and a plane-axis spin reorientation occurs. 4. CONCLUSION The existence of a plane-axis spin reorientation in YCo4B between 4.2 and 293 K has been confirmed by s7Fe M6ssbauer spectroscopy on a sample of Y C o 4 B doped with s7Fe. A change in both sign and magnitude of the quadrupole splitting of 57Fe at the crystal-
3. 4. 5. 6. 7. 8. 9.
J.M.D. Coey, H.S. Li, J.P. Gavigan, J.M. Cadogan & B.P. Hu, in Concerted European Action on Magnets (Edited by I.V. Mitchell, J.M.D. Coey, D. Givord, I.R. Harris and R. Hanitsch), pp. 76-97, Elsevier, London (1989). J.M. Cadogan, J.P. Gavigan, D. Givord & H.S. Li, J. Phys. FIB, 779 (1988). T.T. Dung, N.P. Thuy, N.M. Hong & T.D. Hien, Phys. Status Solidi (a) 106, 201 (1988). Yu.B. Kuz'ma & N.S. Bilonizhko, Kristallografiya 18, 710 (1973); (Sov. Phys. Cryst. 18, 447 (1974)). Cz. Kapusta, N. Spiridis & H. Figiel, J. Magn. Magn. Mater. 83, 153 (1990). A.T. Pedziwiatr, S.Y. Jiang, W.E. Wallace, E. Burzo & V. Pop, J. Magn. Magn. Mater. 66, 69 (1987). F. Spada, C. Abache & H. Oesterreicher, J. Less-Common Metals 99, L21 (1984). Y. Gros, F. Hartmann-Boutron, C. Meyer, M.A. Fr6my & P. Tenaud, J. Magn. Magn. Mater. 74, 319 (1988). J. Lamloumi, A. Percheron-Gcygan, J.C. Achard, G. Jehanno & D. Givord, J. Physique 45, 1643 (1984).
Solid State Communications, Vol. 81, No. 1, pp. 121-123, 1992. Printed in Great Britain.
A MOSSBAUER STUDY OF SPIN-REORIENTATION IN YCo4B J.M. Cadogan and Hong-Shuo Li School of Physics, The University of New South Wales, Kensington, N.S.W. 2033 Australia and S.J. Campbell and J. Jing Department of Physics, University College, The University of New South Wales, Campbell, A.C.T. 2600, Australia (Received 25 September 1991 by P. Bur&t)
The occurrence of a plane-axis spin orientation in YCo4B between 4.2 and 293 K has been confirmed by Mtssbauer spectroscopy on a sample of YCo4 B doped with 0.5 wt % s7Fe. The reorientation is driven by the temperature-dependent competition between the magnetoerystalline anisotropies oi' the 2c and 6i cobalt sites. The Mfssbauer spectra indicate a significant preference of S7Fe for occupying the crystallographic 2c site. 1. INTRODUCTION CHANGES IN easy magnetization direction, more commonly known as spin reorientations, are commonplace in rare-earth-transition metal intermetallics. They result from the competition between the magnetoerystalline anisotropies of the rare-earth and transition metal sublattices, driven by the different temperature dependences of these anisotropies. Such reorientations are well accounted for by a model describing the interplay of molecular field and crystal field interactions [e.g. 1, 2]. Not so common are spin reorientations in compounds having a single magnetic sublattice. Such reorientations are driven by the competing anisotropies of atoms within a single sublattice but occupying different crystallographic sites. Magnetization measurements on the hexagonal compound YCo4 B by Dung et aL [3] provided evidence of such a spin orientation at 145K in which only the cobalt sublattice is involved. The magnetic structure was reported to be axial above 145K and planar below 145 K. YCo4B forms in the hexagonal CeCo4 B structure [4] with the space group P6/mmm. Cobalt atoms occupy the 2c and 6i crystallographic sites. Yttrium atoms occupy the la and lb sites and boron atoms occupy the 2d sites. On the basis of crystal field, point-charge model calculations, Dung et aL found easy c-axis anisotropy (Kin > 0) at the 2c sites and planar anisotropy (K~ < 0) at the 6i sites. (/'he anisotropy energy is K~ sin 2 0, where 0 is the angle between the c-axis and the magnetization). Pulsed 59Co N M R experiments by Kapusta et aL [5] showed large orbital contributions
to the cobalt hyperfine fields, indicative of the local cobalt anisotropy, in agreement with the work of Dung et al. [3]. However, other magnetic studies of YCo4B [6, 7] showed no sign of any spin reorientation in this compound. It was the aim of the present work to use Mfssbauer spectroscopy to verify the occurrence, or absence, of a spin reorientation in YCo4B at low temperatures. 2. EXPERIMENTAL DETAILS The sample of YCo4B doped with 57Fe was prepared by argon-arc melting followed by annealing in vacuo at 900°C for two weeks. 57Fe was substituted for Co to the extent of 0.5wt%. Powder X-ray diffraction using CuK~t radiation showed the sample to be single phase with lattice parameters a = 4.99 ,~ and c = 6.87A. 57Fe Mfssbauer spectroscopy was carded out in conventional transmission mode at 4.2 and 293 K using a 57CoRh source. The spectrometer was calibrated using a standard ~t-Fe foil. 3. RESULTS AND DISCUSSION The 4.2 and 293 K Mrssbauer spectra of YCo4 B: 57Fe are shown in Fig. 1. The spectra were fitted with two Lorentzian sextets corresponding to the 2c and 6i sites. The relative areas of the two subspectra for both the 4.2 and 293 K spectra are 40(5)% and 60(5)%. A completely random substitution of 57Fe for Co would yield two sextets with relative areas 25 and 75%. The subspectrum with the larger area has the smaller
121
SPIN-REORIENTATION IN YCo4B
122
Table 2. Polar angles of the hyperfine field in the principal axis system of the electric field gradient (EFG ) of the 6i sites for the possible orientations of the magnetization
100
99 4.2 K 98 .
~
Vol. 81, No. 1
.
.
.
I
.
.
.
.
I
.
.
.
.
I
.
.
.
.
Magnetization direction
0(°)
~b(°)
[00 1]
90
[100]
0 60
+ 90
1/3 2/3
[21 o]
90 30
+ 90 ___90
1/3 2/3
180
Fraction of intensity 1
I
100
quadrupole terms yields the following expression for the energy levels (using standard notation)
99
eQV~z [3m2 - I(I + 1)] Em = gl#sBhfm + 8 / ( 2 1 - 1)
293K
× [3cos 2 0 -
98
1 + qsin 2 0 c o s 2 4 ] .
The magnetic hyperfine field (Bh/) direction is defined by the polar angles 0, ~b in the principal axis frame of the electric field gradient (EFG). The crystallographic 2c site has the hexagonal point symmetry ~m2 and so Fig. 1. The 57Fe M6ssbauer spectra ofYCo4B : 57Fe at its asymmetry parameter r/ = 0. The 6i site, however, 4.2 and 293 K. has the orthorhombic point symmetry ram, leading to a non-zero 7. We have carried out point-charge hyperfine field, in accord with the N M R data [5] and calculations of the EFG principal axis system at the is thus associated with the 6i sites. It is clear from our 6i sites. Our results indicate that ~/ = 0.8 for the 6i site spectra that iron shows a significant preference for and the 6 atoms of this site are partitioned into two occupying the 2c sites rather than the 6i sites. This groups of 2 and 4 atoms respectively with regard to 0, observation is in agreement with studies ofisostructural 4, when the magnetization lies in the basal plane. We compounds having a much higher iron content [8]. note here that an 57Fe M6ssbauer study of La(Ni, Fe)5 The 57Fe hyperfine parameters deduced from the by Lamloumi et al. [9] found ~/ = 0.7--0.8 for the 3g spectra of Fig. 1 are listed in Table 1. The ratio of the site in this hexagonal CaCu~-P6/mmm structure. The quadrupole splittings for the majority (6i) subspectra 3g site in the CaCu5 structure corresponds to the 6i site at 235 to 4.2 K is - 2 . 1 . This observation provides in the YCo4 B structure. The 0, ~ values for the various unequivocal support for the existence of an axis-plane 6i atoms for different orientations of the magnetizspin reorientation between 293 and 4.2 K. For the case ation are presented in Table 2. Figure 2 shows the when the quadrupole interaction can be treated as a local symmetry around these 6i atoms, including the perturbation on the Zeeman interaction, the nuclear orthogonal mirror planes at these sites. The principal Hamiltonian comprising the Zeeman and electric axes of the EFG tensor are coincident with these mirrors. When the contributions of the various atoms Table 1. Hyperfine parameters deduced from 57Fe in the 6i site are averaged, we find that the quadrupole M6ssbauer spectroscopy in YCo4 B: s7Fe ( Bhl = hyper- splitting at 293 K is a factor of - 2 times that at 4.2 K, fine magnetic field; QS = quadrupolar splitting) in excellent agreement with our results. Further support for the above conclusion can be Site T (K) B,/(T) QS (rams -I) found in the temperature dependence of the 5~Fe hyperfine fields in YCo4B:57Fe. Upon heating from 6i 4.2 15.9 + 0.54 4.2 to 293 K, the hyperfine field at the 2c site is reduced 293 13.5 - 1.15 by 5.1% whereas the hypefline field at the 6i site 2c 4.2 26.9 - 0.17 is reduced by 16.4%. To first approximation, the 293 25.7 - 0.34 anisotropy constant KI varies with temperature as the .
-10
.
.
.
I
-5
~
.
•
•
I
.
•
i
"
I
0 5 V (mm/s)
.
t
•
i
1
10
Vol. 81, No. 1
SPIN-REORIENTATION IN YCo4 B
123
lographic 6i sites between 4.2 and 293 K is observed which is in full agreement with the behaviour expected for a spin reorientation, based on the point symmetry of that site. In addition, 57Fe shows preferential occupation of the 2c site.
Q
Y
(la,lb)
Q
Co (6i)
•
B (2d)
Acknowledgements - The authors are grateful to the Australian Research Council for its financial support and also the award of an ARC Research Associateship to JJ. HSL acknowledges the award of a PostDoctoral Research Fellowship by The University of New South Wales. We wish to thank Dr A.V.J. Edge for his sample preparation. REFERENCES
YCo4B Fig. 2. The atomic arrangement of YCo4 B projected onto the c-plane (the 2c cobalt sites are omitted for clarity). The orientations of the EFG principal axes at the 6i cobalt sites are marked.
1.
2. cube of the magnetization. Thus, the more rapid decrease in the 6i hyperfine field implies a more rapid fall-off in K~ (6i) than in K~(2c) with increasing temperature. The data presented here suggest a reduction in Kt(6i) of 41%, much greater than the 15% reduction in K~(2c). Hence, the dominant planar anisotropy of the 6i site gives way to the axial anisotropy of the 2c site with increasing temperature and a plane-axis spin reorientation occurs. 4. CONCLUSION The existence of a plane-axis spin reorientation in YCo4B between 4.2 and 293 K has been confirmed by s7Fe M6ssbauer spectroscopy on a sample of Y C o 4 B doped with s7Fe. A change in both sign and magnitude of the quadrupole splitting of 57Fe at the crystal-
3. 4. 5. 6. 7. 8. 9.
J.M.D. Coey, H.S. Li, J.P. Gavigan, J.M. Cadogan & B.P. Hu, in Concerted European Action on Magnets (Edited by I.V. Mitchell, J.M.D. Coey, D. Givord, I.R. Harris and R. Hanitsch), pp. 76-97, Elsevier, London (1989). J.M. Cadogan, J.P. Gavigan, D. Givord & H.S. Li, J. Phys. FIB, 779 (1988). T.T. Dung, N.P. Thuy, N.M. Hong & T.D. Hien, Phys. Status Solidi (a) 106, 201 (1988). Yu.B. Kuz'ma & N.S. Bilonizhko, Kristallografiya 18, 710 (1973); (Sov. Phys. Cryst. 18, 447 (1974)). Cz. Kapusta, N. Spiridis & H. Figiel, J. Magn. Magn. Mater. 83, 153 (1990). A.T. Pedziwiatr, S.Y. Jiang, W.E. Wallace, E. Burzo & V. Pop, J. Magn. Magn. Mater. 66, 69 (1987). F. Spada, C. Abache & H. Oesterreicher, J. Less-Common Metals 99, L21 (1984). Y. Gros, F. Hartmann-Boutron, C. Meyer, M.A. Fr6my & P. Tenaud, J. Magn. Magn. Mater. 74, 319 (1988). J. Lamloumi, A. Percheron-Gcygan, J.C. Achard, G. Jehanno & D. Givord, J. Physique 45, 1643 (1984).