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Competing interactions in hexagonal PrxNd1−xCo5 pseudobinary intermetallic compounds
136
Journal
of Magnetism
and Magnetic
Materials
83 (1990) 136- 138 North-Holland
COMPETING INTERACTIONS IN HEXAGONAL INTERMETALLIC COMPOUNDS 0. MOZE.
G. MARUSI,
M.R. IBARRA
“, M. SOLZI
fi,Nd,
and
_ $20~ PSEUDOBINARY
L. PARETI
IsrrrutoMASPEC de1 CNR, 43100 Parma, Itub ‘I ICMA-CSIC, Unioersidad de Zaragoza, 50009 Zurcrgora, Spurn Measurements of the cone magnetic structure in Pr,Nd,_,Co, compounds show that the spin reorientation transition temperature from easy plane to easy cone and the low temperature value of the angle between the easy magnetization directlon and the c-axis both decrease with decreasing Nd concentration. The observed behaviour of the magnetic anisotropy implies a particularly complex competition between a non-axial Pr and planar Nd anisotropy as well as the large Co axial anisotropy.
1. Introduction
2. Experimental
Rare earth transition metal compounds of the structure type RECo, (RE = Rare Earth) have been intensively investigated principally because of their importance as permanent magnet materials. They also display a wealth of exotic magnetic transitions which are primarily due to the different temperature dependence of the RE and Co sublattice anisotropies. It is of particular interest for the magnetic anisotropy of the individual sublattices to be well understood as it is this which determines whether or not the RE magnetization is collinear with that of the Co sublattice. In PrCo, the Pr moment commences to tilt away from the c-axis at 107 K and drags the Co moment with it giving rise to a situation where the total magnetization has a cone of easy axes persisting down to low temperatures. with the cone angle increasing to a final value of about 20” [l]. In the case of NdCo, the competition between the planar anisotropy of the Nd ion and the axial Co anisotropy leads to a transition from a collinear easy axis above 300 K to an easy plane ferromagnet below 200 K [2]. In the easy cone region the RE and Co moments are presumably no longer collinear as the RE-Co exchange coupling is not large enough to overcome the difference in their respective anisotropy energies. Classical calculations [3,4] suggest that such a non-collinearity can lead to an anomalous field dependence of the magnetization (first order magnetization processes, FOMP) and large high order anisotropy constants [5]. Such a non-collinear structure has been observed in HoCo, (61. A preliminary investigation of the composition and temperature dependence of the cone magnetic structure in hexagonal Pr, Nd, _xCo, compounds is reported here. The observed spin reorientation transitions indicate a complex competition between Pr, Nd and Co anisotropies. 0304-8853/90/$03.50 (North-Holland)
C Elsevier Science Publishers B.V.
details
Compounds of Pr,Nd, _.,Co, with x = 0.8, 0.6, 0.5, 0.4 and 0.2 were melted in an arc furnace under an argon atmosphere. The resulting buttons were annealed in argon at 1000° C for 6 days. X-ray diffraction was used for identification of the CaCu, structure. Polar magnetization plots between 293 and 77 K on powders aligned in a magnetic field of I.9 T enabled the composition and temperature dependence of the cone angle to be obtained. The respective spin reorientation transition temperatures were verified by carrying out measurements of the temperature dependence of the initial susceptibility in an applied magnetic field of 5 mT. The anisotropy field was measured by use of the singular point detection technique [7]. 3. Results
and discussion
All the observed positions and intensities of the X-ray diffraction lines were consistent with the presence of the CaCu, structure (space group P6/mmm) (81. The refined lattice parameters were in excellent agreement with previous structural investigations of PrCo, and NdCo, compounds [9]. The measured composition and temperature dependence of the value of the angle 0 between the easy magnetization direction and the c-axis is displayed in fig. 1 together with the results of previous investigations for PrCo, and NdCo, [2]. The spin reorientation transition temperature from easy cone to easy axis is found to increase with increasing Nd composition. The extrapolated low temperature value of 0 increases with increasing Nd composition resulting in an easy plane configuration at 77 K for x = 0.2 and 0.4. The anisotropy field H, (measured with the applied magnetic field perpendicular to the c-axis to that H, = [2K, + 4K, + 6K,]/M,) decreases with increasing Nd composition
0. Maze et al. / Competing interactions in Pr, Nd,
$0,
137
Fig. 1. Temperature and composition dependence of the angle 0 between the easy magnetization direction and the c-axis for Pr,Nd, _,Co, compounds. Lines are guides to the eye. The full circles are for the present data whilst the open circles are data for single crystal PrCo, and NdCo, compounds taken from
ref. [2].
2. A preliminary magnetic phase diagram, obtained on the basis of the polar magnetization plots and the initial susceptibility measurements, is displayed in fig. 3. The temperature interval over which the transition from easy-axis to easy-plane takes place for Nd rich compounds increases with decreasing Nd composition. The easy-axis to easy-plane reorientation also takes place at higher temperatures for the Nd rich compounds due to the dominance of the 2nd order crystal field term 8:. These two facts taken together with the observed
and
decreasing
temperature
as displayed
in fig.
‘55
Fig. 3. Magnetic phase diagram for Pr,Nd, _,Cos compounds. Full circles are points obtained from the polar magnetization measurements whilst the open circles are points obtained from the initial susceptibility measurements.
of the anisotropy field with increasing Pr composition are strong indications as to the dominance of crystal field terms of higher orders with increasing Pr composition since these terms have a strong temperature dependence. Further measurements are in progress in order to determine the value of the cone angle at low temperatures for Pr rich compounds. Applications of a model incorporating crystal field and exchange terms to the temperature and composition dependence of the spin reorientation transition will ultimately provide a sensitive test for the nature of the competing interactions between Pr and Nd anisotropies in this structure. In addition a definite comparison can be made between crystal field parameters as determined by other techniques such as, for example, inelastic neutron scattering. This should in principle give a more precise viewpoint as the origin of the mechanism which gives rise to the anomalous nature of the Pr anisotropy in the RECo, structure [lo] and the non-collinearity between RE and Co sublattices. increase
References
T(K) Fig. 2. Composition and temperature dependence sotropy field H, for Pr,Nd,_,Co, compounds. guides to the eye.
of the aniLines are
VI G. Hoffer and K. Strnat, IEEE Trans. Magn. MAG-2 (1966) 487. PI E. Tatsumoto. T. Okamoto, H. Fujii and C. Inoue, J. de Phys. 32 (1971) Cl-550. [31 Yu.P. Irkhin and E.V. Rozenfeld. Sov. Phys. Solid State 16 (1974) 310. [41 S. Rinaldi and L. Pareti, J. Appl. Phys. 50 (1979) 7719. [51 G. Asti, F. Bolzoni, F. Leccabue, R. Pannizieri. L. Pareti and S. Rinaldi, J. Magn. Magn. Mat. 15-18 (1980) 561.
138
0. More et al. / Competing wtteractions WI PC, Nd,
[6] B. Decrop. J. Deportes and R. Lemaire. J. Less-Common Metals 94 (1983) 199. [7] G. Asti and S. Rinaldi. J. Appl. Phys. 45 (1974) 3600. [8] J.H. Wernick and S. Geller, Acta. Cryst. 12 (1959) 662.
Jo,
[9] W.A.J. Velge and K.H.J. Buschow. J. Appl. Phys. 39 (1969) 1717. [lo] L. Par&, 0. Maze, M. Solzi and F. Bolzoni, J. Appl. Phya. 63 (1988) 172.
Journal
of Magnetism
and Magnetic
Materials
83 (1990) 136- 138 North-Holland
COMPETING INTERACTIONS IN HEXAGONAL INTERMETALLIC COMPOUNDS 0. MOZE.
G. MARUSI,
M.R. IBARRA
“, M. SOLZI
fi,Nd,
and
_ $20~ PSEUDOBINARY
L. PARETI
IsrrrutoMASPEC de1 CNR, 43100 Parma, Itub ‘I ICMA-CSIC, Unioersidad de Zaragoza, 50009 Zurcrgora, Spurn Measurements of the cone magnetic structure in Pr,Nd,_,Co, compounds show that the spin reorientation transition temperature from easy plane to easy cone and the low temperature value of the angle between the easy magnetization directlon and the c-axis both decrease with decreasing Nd concentration. The observed behaviour of the magnetic anisotropy implies a particularly complex competition between a non-axial Pr and planar Nd anisotropy as well as the large Co axial anisotropy.
1. Introduction
2. Experimental
Rare earth transition metal compounds of the structure type RECo, (RE = Rare Earth) have been intensively investigated principally because of their importance as permanent magnet materials. They also display a wealth of exotic magnetic transitions which are primarily due to the different temperature dependence of the RE and Co sublattice anisotropies. It is of particular interest for the magnetic anisotropy of the individual sublattices to be well understood as it is this which determines whether or not the RE magnetization is collinear with that of the Co sublattice. In PrCo, the Pr moment commences to tilt away from the c-axis at 107 K and drags the Co moment with it giving rise to a situation where the total magnetization has a cone of easy axes persisting down to low temperatures. with the cone angle increasing to a final value of about 20” [l]. In the case of NdCo, the competition between the planar anisotropy of the Nd ion and the axial Co anisotropy leads to a transition from a collinear easy axis above 300 K to an easy plane ferromagnet below 200 K [2]. In the easy cone region the RE and Co moments are presumably no longer collinear as the RE-Co exchange coupling is not large enough to overcome the difference in their respective anisotropy energies. Classical calculations [3,4] suggest that such a non-collinearity can lead to an anomalous field dependence of the magnetization (first order magnetization processes, FOMP) and large high order anisotropy constants [5]. Such a non-collinear structure has been observed in HoCo, (61. A preliminary investigation of the composition and temperature dependence of the cone magnetic structure in hexagonal Pr, Nd, _xCo, compounds is reported here. The observed spin reorientation transitions indicate a complex competition between Pr, Nd and Co anisotropies. 0304-8853/90/$03.50 (North-Holland)
C Elsevier Science Publishers B.V.
details
Compounds of Pr,Nd, _.,Co, with x = 0.8, 0.6, 0.5, 0.4 and 0.2 were melted in an arc furnace under an argon atmosphere. The resulting buttons were annealed in argon at 1000° C for 6 days. X-ray diffraction was used for identification of the CaCu, structure. Polar magnetization plots between 293 and 77 K on powders aligned in a magnetic field of I.9 T enabled the composition and temperature dependence of the cone angle to be obtained. The respective spin reorientation transition temperatures were verified by carrying out measurements of the temperature dependence of the initial susceptibility in an applied magnetic field of 5 mT. The anisotropy field was measured by use of the singular point detection technique [7]. 3. Results
and discussion
All the observed positions and intensities of the X-ray diffraction lines were consistent with the presence of the CaCu, structure (space group P6/mmm) (81. The refined lattice parameters were in excellent agreement with previous structural investigations of PrCo, and NdCo, compounds [9]. The measured composition and temperature dependence of the value of the angle 0 between the easy magnetization direction and the c-axis is displayed in fig. 1 together with the results of previous investigations for PrCo, and NdCo, [2]. The spin reorientation transition temperature from easy cone to easy axis is found to increase with increasing Nd composition. The extrapolated low temperature value of 0 increases with increasing Nd composition resulting in an easy plane configuration at 77 K for x = 0.2 and 0.4. The anisotropy field H, (measured with the applied magnetic field perpendicular to the c-axis to that H, = [2K, + 4K, + 6K,]/M,) decreases with increasing Nd composition
0. Maze et al. / Competing interactions in Pr, Nd,
$0,
137
Fig. 1. Temperature and composition dependence of the angle 0 between the easy magnetization direction and the c-axis for Pr,Nd, _,Co, compounds. Lines are guides to the eye. The full circles are for the present data whilst the open circles are data for single crystal PrCo, and NdCo, compounds taken from
ref. [2].
2. A preliminary magnetic phase diagram, obtained on the basis of the polar magnetization plots and the initial susceptibility measurements, is displayed in fig. 3. The temperature interval over which the transition from easy-axis to easy-plane takes place for Nd rich compounds increases with decreasing Nd composition. The easy-axis to easy-plane reorientation also takes place at higher temperatures for the Nd rich compounds due to the dominance of the 2nd order crystal field term 8:. These two facts taken together with the observed
and
decreasing
temperature
as displayed
in fig.
‘55
Fig. 3. Magnetic phase diagram for Pr,Nd, _,Cos compounds. Full circles are points obtained from the polar magnetization measurements whilst the open circles are points obtained from the initial susceptibility measurements.
of the anisotropy field with increasing Pr composition are strong indications as to the dominance of crystal field terms of higher orders with increasing Pr composition since these terms have a strong temperature dependence. Further measurements are in progress in order to determine the value of the cone angle at low temperatures for Pr rich compounds. Applications of a model incorporating crystal field and exchange terms to the temperature and composition dependence of the spin reorientation transition will ultimately provide a sensitive test for the nature of the competing interactions between Pr and Nd anisotropies in this structure. In addition a definite comparison can be made between crystal field parameters as determined by other techniques such as, for example, inelastic neutron scattering. This should in principle give a more precise viewpoint as the origin of the mechanism which gives rise to the anomalous nature of the Pr anisotropy in the RECo, structure [lo] and the non-collinearity between RE and Co sublattices. increase
References
T(K) Fig. 2. Composition and temperature dependence sotropy field H, for Pr,Nd,_,Co, compounds. guides to the eye.
of the aniLines are
VI G. Hoffer and K. Strnat, IEEE Trans. Magn. MAG-2 (1966) 487. PI E. Tatsumoto. T. Okamoto, H. Fujii and C. Inoue, J. de Phys. 32 (1971) Cl-550. [31 Yu.P. Irkhin and E.V. Rozenfeld. Sov. Phys. Solid State 16 (1974) 310. [41 S. Rinaldi and L. Pareti, J. Appl. Phys. 50 (1979) 7719. [51 G. Asti, F. Bolzoni, F. Leccabue, R. Pannizieri. L. Pareti and S. Rinaldi, J. Magn. Magn. Mat. 15-18 (1980) 561.
138
0. More et al. / Competing wtteractions WI PC, Nd,
[6] B. Decrop. J. Deportes and R. Lemaire. J. Less-Common Metals 94 (1983) 199. [7] G. Asti and S. Rinaldi. J. Appl. Phys. 45 (1974) 3600. [8] J.H. Wernick and S. Geller, Acta. Cryst. 12 (1959) 662.
Jo,
[9] W.A.J. Velge and K.H.J. Buschow. J. Appl. Phys. 39 (1969) 1717. [lo] L. Par&, 0. Maze, M. Solzi and F. Bolzoni, J. Appl. Phya. 63 (1988) 172.