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Crystal electric field in RENi10Si2 intermetallics
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1391-1393 North-Holland
iit"
Crystal electric field in RENil0Si 2 intermetallics O. M o z e
a
R. C a c i u f f o
a Istituto I S M del t, Dipartimento di c Philips Research a Dipartimento di
b
K . H . J . B u s c h o w c a n d G. A m o r e t t i d
CNR, Frascati 00044, Italy Scienze dei Materiali e della Terra, Universitd di Ancona, Italy Laboratories, Eindhoven, Netherlands Fisica, Unit,ersitd di Parma, Italy
An insight into the nature of the crystal field interaction of trivalent rare-earth ions in a tetragonal electrostatic environment for non-magnetic compounds which are isomorphous with anisotropic REFea0Si 2 compounds has been undertaken. Crystal field excitations in RENimSi 2 (I4/mmm space group, ThMni2 structure type) compounds, with RE = Er, Tb, Ho and Y, have been investigated by inelastic magnetic neutron scattering. At low temperatures well-defined magnetic transitions are observed for ErNil0Si 2 whilst in compounds with Tb and Ho the observed transitions are not as well resolved and indicate a complex level scheme. Inelastic neutron scattering is a useful technique for studying low lying energy levels of trivalent rare-earth ions in crystals and for studying the magnetic dipole transitions which take place between such levels. Rare-earth intermetallics are by far the most extensively studied systems by this unique technique. R e c e n t work has centred on compounds which are appropriate model systems for investigating the details of the crystal field (CF) interaction in novel ferromagnets with a substantial uniaxial and hence technologically important magnetic anisotropy [1-3]. These are based on the tetragonal ThMn~2 structure where the rare-earth site has 4 / m m m point symmetry [4]. Systems of the type R E M n 4 A I 8 have already been investigated and in E r M n 4 A I 8 [5] the CF parameters obtained by neutron spectroscopy were of approximately the correct sign and magnitude to justify the basal plane orientation of Er magnetic moments in E r F e a A I 8 [6]. These parameters, if extrapolated to apply for Fe-rich compounds, can even also account for the observed non-axial anisotropy observed in ErF%0V 2 [7]. In similar circumstances for TbMn4A1 s a set of C E F parameters which led to a level scheme in agreement with the observed neutron data was obtained whilst in HoMn4A1 s only a qualitative agreement was obtained [8]. These compounds are of central importance in permanent magnet research and therefore deserve detailed investigations by many different macroscopic and microscopic methods. A further relevant model system suitable for investigation is the RENil0Si 2 series. These materials order at very low temperatures due to the absence of a significant magnetic m o m e n t at Ni sites and hence the magnetic behaviour at low temperatures is dominated by the CF interaction. This is a fortunate circumstance as it allows for detailed investigation of this interaction in ThMnl2 compounds. A similar situation arises in both hexagonal and rhombohedral forms
of the RE2Zn17 series. As far as can be determined there are no equivalent non-magnetic compounds which are isomorphous with the Nd2Fe14B structure making the CF interaction in this system difficult to investigate with neutron spectroscopy. The detailed crystal structure of RENimSi2 compounds has recently been investigated by neutron powder diffraction with the Si atoms preferentially occupying the 8f site (fig. 1) [9]. H e n c e an extension of investigations into the nature and type of the crystal field interaction in such materials by neutron spectroscopy would appear to be warranted. This is particularly so in light of the desired further clarification of the anomalous magnetic anisotropy of Er 3+ and Tb 3+ ions in the ThMn12 structure. Samples of ErNil0Si2, TbNil0Si2, HoNil0Si 2 and YNimSi 2, each of approximately 20 grams in weight, were prepared by arc-melting followed by annealing in an argon atmosphere at 1050 ° C for 2 weeks and then at 8 0 0 ° C for a further 4 weeks. X-ray diffraction measurements verified that all samples were essentially
( )
( O
RE 2a
O 8i Ni
~) 8f Ni/Si
• 8j Ni
Fig. 1. Crystal structure of tetragonal RENil0Si2 intermetallic compounds.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
O. Moze et al. / CEF in RENimSi2 interrnetallics
1392
single phase and crystallized in the ThMn 12 structure. Some minority impurity phase was also detected and neutron diffraction measurements previously performed on the YNil0Si 2 sample [9] attributed the impurity as being the cubic YNiliSi 2 phase (NaZn13 structure type, Fm3c). The neutron inelastic scattering measurements were performed on the time focussed crystal analyser spectrometer TFXA at the UK spallation neutron source ISIS, Rutherford Appleton Laboratory, UK. The sample of YN~0Si 2 was also measured in order to provide data for non-magnetic background corrections which arise principally from scattering of the neutrons by lattice vibrations. Measurements for all samples were performed at 20 and 50 K in a closed-cycle displex refrigerator. In most general terms, the thermal neutron magnetic scattering cross-section for a system of N-interacting ions in the dipole approximation for small values of the momentum transfer Q is given by [10]: d ad2~dw
N([ ~1"91e2 )~g2f2(Q)
xl(ilJ±
~ik¢~ ei '_j t 3 e
Ij)12~(E~-E~-hw),
(1)
where hw is the neutron energy transfer, l i) are the 4f-electron eigenstates with energies Es, J ± is the total angular momentum component perpendicular to Q, f(Q) is the single-ion magnetic form factor and gj is the Landd g factor. The crystal electric field Hamiltonian for tetragonal 4 / m m m point symmetry at the 2a rare-earth site, with the z-axis as the quantization axis, contains terms like n0t40 n0r30 nor30 /~.4t~4 and B6406, ^4 where the ~2 ~2, ~4 ~4, ~6 ~6, ~4 ~4 B;,~ are the CF parameters and the 0,~" are the Stevens operator equivalents built up of the total angular momentum operators. Such a Hamiltonian will split the ground state multiplet 7F,5 o f the Tb 3+ ion into 3 doublets and 7 singlets, whilst the 5IsHo3+ multiplet ground state is split into 4 doublets and 9 singlets whilst the ground state of the Er ion is split into 8 doublets. Equation (1) shows implicitly that CEF transitions may be identified by the manner in which their intensities vary with both temperature T and scattering vector Q. The 4f electron eigenstates of the CF Hamiltonian are in principle obtained from the intensities of transitions. In order to determine possible levels and intensities of transitions between CF levels, the observed spectra at 20 K were fitted by a least-squares proccdure to a set of Gaussians. The results for ErNil0Si z at 20 K are displayed in fig. 2. The phonon background as determined by measurements of YNimSi e displayed a more enhanced signal at small energy transfers than corresponding YMn4AI s compounds but in spite of this,
!!
121
;tl
I 3
I I I 4
5
1)'>--- ........ 6
7
Energy transfer (meV) Fig. 2. Inelastic neutron spectra observed at 20 K for ErNiwSi 2.
transitions in all cases could clearly be attributed to magnetic CF transitions. Measurements at the higher temperature were performed on all compounds in order to observe CEF transition from thermally populated excited states. The results of the fitting procedure yielded for the Er compound levels at 2.5, 3.2, 4.0, 5.0 and 5.7 meV with the level at 4.0 meV having the strongest intensity. These transitions can be expected to be transitions from the doublet ground state to further excited doublets. In the Tb and Ho compounds transitions up to about 6 meV were observed but were not resolved with sufficient accuracy for a definitive level scheme to be obtained. Due to inherent instrumental limitations, both low-lying transitions and ones at higher energies could not be observed. This is a limiting factor and does not presently allow a detailed confrontation with the proposed model Hamiltonian. However extension of the measurements to a higher energy resolution for identification of low-lying energy levels appears necessary. Within the dipole approximation, the neutron spectroscopy investigations allow for a microscopic observation of transitions within multiplets of the ground state manifold. Unlike for cubic systems there is no simple quantitative method for unambiguous determination of up to five or more crystal field parameters without some other independent check on microscopic properties which are significantly influenced by the crystal electric field. Measurements of the magnetic susceptibility at low temperatures which are to be undertaken in the near future will allow for a good test of crystal field parameters obtained from the present neutron study. This will ultimately provide a realistic description of the CEF interaction in these and similar compounds. The authors thank J. Tomkinson and M. Adams for assistance during the measurements and the UK Science and Engineering Research Council for provision
O. Moze et al. / CEF in RENi loSi e intermetallics
of n e u t r o n scattering facilities. This work was partly s u p p o r t e d by the E u r o p e a n C o m m i s s i o n within its Research a n d D e v e l o p m e n t p r o g r a m m e B I R E M B R E U 0068. References [1] D.B. de Mooij and K.H.J. Buschow, Philips J. Research 42 (1987) 246. [2] K.H.J. Buschow, D.B. de Mooij, M. Brouha, H.H.A. Smit and R.C. Thiel, IEEE Trans. Magn. MAG-64 (1988) 1611. [3] K.H.J. Buschow, J. Appl. Phys. 63 (1988) 3130. [4] J.V. Florio, R.E. Rundle and A.I. Snow, Acta Crystallogr. 5 (1952) 449.
1393
[5] O. Moze, K.H.J. Buschow, R. Osborn, Z. Bowden and A.D. Taylor, Solid State Commun. 72 (1989) 249. [6] M.O. Bargouth, G. Will and K.H.J. Buschow, J. Magn. Magn. Mater. 6 (1977) 129. [7] O. Moze, L. Pareti, M. Solzi and W.I.F. David, Solid State Commun. 66 (1988) 465. [8] O. Moze, R. Caciuffo and K.H.J. Buschow, Solid State Commun. 76 (1990) 331. [9] O. Moze, R.M. Ibberson and K.H.J. Buschow, Solid State Commun. 78 (1991) 473. [10] P.G. de Gennes, in: Magnetism III, eds. T. Rado and Suhl (Academic Press, New York, 1963) p. 115.
iit"
Crystal electric field in RENil0Si 2 intermetallics O. M o z e
a
R. C a c i u f f o
a Istituto I S M del t, Dipartimento di c Philips Research a Dipartimento di
b
K . H . J . B u s c h o w c a n d G. A m o r e t t i d
CNR, Frascati 00044, Italy Scienze dei Materiali e della Terra, Universitd di Ancona, Italy Laboratories, Eindhoven, Netherlands Fisica, Unit,ersitd di Parma, Italy
An insight into the nature of the crystal field interaction of trivalent rare-earth ions in a tetragonal electrostatic environment for non-magnetic compounds which are isomorphous with anisotropic REFea0Si 2 compounds has been undertaken. Crystal field excitations in RENimSi 2 (I4/mmm space group, ThMni2 structure type) compounds, with RE = Er, Tb, Ho and Y, have been investigated by inelastic magnetic neutron scattering. At low temperatures well-defined magnetic transitions are observed for ErNil0Si 2 whilst in compounds with Tb and Ho the observed transitions are not as well resolved and indicate a complex level scheme. Inelastic neutron scattering is a useful technique for studying low lying energy levels of trivalent rare-earth ions in crystals and for studying the magnetic dipole transitions which take place between such levels. Rare-earth intermetallics are by far the most extensively studied systems by this unique technique. R e c e n t work has centred on compounds which are appropriate model systems for investigating the details of the crystal field (CF) interaction in novel ferromagnets with a substantial uniaxial and hence technologically important magnetic anisotropy [1-3]. These are based on the tetragonal ThMn~2 structure where the rare-earth site has 4 / m m m point symmetry [4]. Systems of the type R E M n 4 A I 8 have already been investigated and in E r M n 4 A I 8 [5] the CF parameters obtained by neutron spectroscopy were of approximately the correct sign and magnitude to justify the basal plane orientation of Er magnetic moments in E r F e a A I 8 [6]. These parameters, if extrapolated to apply for Fe-rich compounds, can even also account for the observed non-axial anisotropy observed in ErF%0V 2 [7]. In similar circumstances for TbMn4A1 s a set of C E F parameters which led to a level scheme in agreement with the observed neutron data was obtained whilst in HoMn4A1 s only a qualitative agreement was obtained [8]. These compounds are of central importance in permanent magnet research and therefore deserve detailed investigations by many different macroscopic and microscopic methods. A further relevant model system suitable for investigation is the RENil0Si 2 series. These materials order at very low temperatures due to the absence of a significant magnetic m o m e n t at Ni sites and hence the magnetic behaviour at low temperatures is dominated by the CF interaction. This is a fortunate circumstance as it allows for detailed investigation of this interaction in ThMnl2 compounds. A similar situation arises in both hexagonal and rhombohedral forms
of the RE2Zn17 series. As far as can be determined there are no equivalent non-magnetic compounds which are isomorphous with the Nd2Fe14B structure making the CF interaction in this system difficult to investigate with neutron spectroscopy. The detailed crystal structure of RENimSi2 compounds has recently been investigated by neutron powder diffraction with the Si atoms preferentially occupying the 8f site (fig. 1) [9]. H e n c e an extension of investigations into the nature and type of the crystal field interaction in such materials by neutron spectroscopy would appear to be warranted. This is particularly so in light of the desired further clarification of the anomalous magnetic anisotropy of Er 3+ and Tb 3+ ions in the ThMn12 structure. Samples of ErNil0Si2, TbNil0Si2, HoNil0Si 2 and YNimSi 2, each of approximately 20 grams in weight, were prepared by arc-melting followed by annealing in an argon atmosphere at 1050 ° C for 2 weeks and then at 8 0 0 ° C for a further 4 weeks. X-ray diffraction measurements verified that all samples were essentially
( )
( O
RE 2a
O 8i Ni
~) 8f Ni/Si
• 8j Ni
Fig. 1. Crystal structure of tetragonal RENil0Si2 intermetallic compounds.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
O. Moze et al. / CEF in RENimSi2 interrnetallics
1392
single phase and crystallized in the ThMn 12 structure. Some minority impurity phase was also detected and neutron diffraction measurements previously performed on the YNil0Si 2 sample [9] attributed the impurity as being the cubic YNiliSi 2 phase (NaZn13 structure type, Fm3c). The neutron inelastic scattering measurements were performed on the time focussed crystal analyser spectrometer TFXA at the UK spallation neutron source ISIS, Rutherford Appleton Laboratory, UK. The sample of YN~0Si 2 was also measured in order to provide data for non-magnetic background corrections which arise principally from scattering of the neutrons by lattice vibrations. Measurements for all samples were performed at 20 and 50 K in a closed-cycle displex refrigerator. In most general terms, the thermal neutron magnetic scattering cross-section for a system of N-interacting ions in the dipole approximation for small values of the momentum transfer Q is given by [10]: d ad2~dw
N([ ~1"91e2 )~g2f2(Q)
xl(ilJ±
~ik¢~ ei '_j t 3 e
Ij)12~(E~-E~-hw),
(1)
where hw is the neutron energy transfer, l i) are the 4f-electron eigenstates with energies Es, J ± is the total angular momentum component perpendicular to Q, f(Q) is the single-ion magnetic form factor and gj is the Landd g factor. The crystal electric field Hamiltonian for tetragonal 4 / m m m point symmetry at the 2a rare-earth site, with the z-axis as the quantization axis, contains terms like n0t40 n0r30 nor30 /~.4t~4 and B6406, ^4 where the ~2 ~2, ~4 ~4, ~6 ~6, ~4 ~4 B;,~ are the CF parameters and the 0,~" are the Stevens operator equivalents built up of the total angular momentum operators. Such a Hamiltonian will split the ground state multiplet 7F,5 o f the Tb 3+ ion into 3 doublets and 7 singlets, whilst the 5IsHo3+ multiplet ground state is split into 4 doublets and 9 singlets whilst the ground state of the Er ion is split into 8 doublets. Equation (1) shows implicitly that CEF transitions may be identified by the manner in which their intensities vary with both temperature T and scattering vector Q. The 4f electron eigenstates of the CF Hamiltonian are in principle obtained from the intensities of transitions. In order to determine possible levels and intensities of transitions between CF levels, the observed spectra at 20 K were fitted by a least-squares proccdure to a set of Gaussians. The results for ErNil0Si z at 20 K are displayed in fig. 2. The phonon background as determined by measurements of YNimSi e displayed a more enhanced signal at small energy transfers than corresponding YMn4AI s compounds but in spite of this,
!!
121
;tl
I 3
I I I 4
5
1)'>--- ........ 6
7
Energy transfer (meV) Fig. 2. Inelastic neutron spectra observed at 20 K for ErNiwSi 2.
transitions in all cases could clearly be attributed to magnetic CF transitions. Measurements at the higher temperature were performed on all compounds in order to observe CEF transition from thermally populated excited states. The results of the fitting procedure yielded for the Er compound levels at 2.5, 3.2, 4.0, 5.0 and 5.7 meV with the level at 4.0 meV having the strongest intensity. These transitions can be expected to be transitions from the doublet ground state to further excited doublets. In the Tb and Ho compounds transitions up to about 6 meV were observed but were not resolved with sufficient accuracy for a definitive level scheme to be obtained. Due to inherent instrumental limitations, both low-lying transitions and ones at higher energies could not be observed. This is a limiting factor and does not presently allow a detailed confrontation with the proposed model Hamiltonian. However extension of the measurements to a higher energy resolution for identification of low-lying energy levels appears necessary. Within the dipole approximation, the neutron spectroscopy investigations allow for a microscopic observation of transitions within multiplets of the ground state manifold. Unlike for cubic systems there is no simple quantitative method for unambiguous determination of up to five or more crystal field parameters without some other independent check on microscopic properties which are significantly influenced by the crystal electric field. Measurements of the magnetic susceptibility at low temperatures which are to be undertaken in the near future will allow for a good test of crystal field parameters obtained from the present neutron study. This will ultimately provide a realistic description of the CEF interaction in these and similar compounds. The authors thank J. Tomkinson and M. Adams for assistance during the measurements and the UK Science and Engineering Research Council for provision
O. Moze et al. / CEF in RENi loSi e intermetallics
of n e u t r o n scattering facilities. This work was partly s u p p o r t e d by the E u r o p e a n C o m m i s s i o n within its Research a n d D e v e l o p m e n t p r o g r a m m e B I R E M B R E U 0068. References [1] D.B. de Mooij and K.H.J. Buschow, Philips J. Research 42 (1987) 246. [2] K.H.J. Buschow, D.B. de Mooij, M. Brouha, H.H.A. Smit and R.C. Thiel, IEEE Trans. Magn. MAG-64 (1988) 1611. [3] K.H.J. Buschow, J. Appl. Phys. 63 (1988) 3130. [4] J.V. Florio, R.E. Rundle and A.I. Snow, Acta Crystallogr. 5 (1952) 449.
1393
[5] O. Moze, K.H.J. Buschow, R. Osborn, Z. Bowden and A.D. Taylor, Solid State Commun. 72 (1989) 249. [6] M.O. Bargouth, G. Will and K.H.J. Buschow, J. Magn. Magn. Mater. 6 (1977) 129. [7] O. Moze, L. Pareti, M. Solzi and W.I.F. David, Solid State Commun. 66 (1988) 465. [8] O. Moze, R. Caciuffo and K.H.J. Buschow, Solid State Commun. 76 (1990) 331. [9] O. Moze, R.M. Ibberson and K.H.J. Buschow, Solid State Commun. 78 (1991) 473. [10] P.G. de Gennes, in: Magnetism III, eds. T. Rado and Suhl (Academic Press, New York, 1963) p. 115.