- Email: [email protected]
Spin-flip low-energy electron exchange scattering in NiO(100)
Journal of Magnetism and Magnetic Materials 148 (1995) 181-182
~ Jouma! of magnetism and magnetic materials
ELSEVIER
Spin-flip low-energy electron exchange scattering in NiO(100) B. F r o m m e ~'*, A. Hylla a, C. Koch a, E. Kisker ~, A. Gorschliiter b, H. M e r z b a Institut f~r Angewandte Physik der Universitiit Diisseldorf, 40225 Diisseldo~ Germany b Physikalisches lnstitut der Universitiit Miinster, 48149 2Vtiinster, Germany
Abstract Electron exchange processes in NiO have been examined by complete spin-polarized electron energy loss spectroscopy. The measurements show a complicated dependence of the spin-flip exchange scattering on incident angle and energy, unexpected from existing theories and measurements on atoms and ferromagnets. The amount of exchange processes is dramatically enhanced in specular scattering geometry for incident energies around 30, 38 and 100 eV, which is attributed to resonant excitation.
1. Introduction
Spin-polarized electron energy loss spectroscopy (SPEELS) is a successful surface-sensitive method for studying spin-dependent band structures and coupling mechanisms between layers of different types o f magnetism via electron exchange processes [1-4]. But only the complete scattering experhTtent (C-SPEF,LS), with polarized incident electrons a n d polarization analysis of the scattered electrons, provid~',s direct experimental proof of such processes. The partial spin-flip and nonflip intensities ~ e determined from the intensity and the change in electron polarization during scattering. Here, we applied CSPEELS to the antiferromagnetic transition metal compound NiO. First results have been published elsewhere
[5].
2. Results and discussion
In contrast with the 3d metals, the 3d electrons of the Ni 2÷ ions in NiO do not show band-like but localized, quasi-atomic-like character. The octahedral surrounding of the Ni 2+ ions by 0 2 - ions in NiO provides a partial lifting o f the degeneracy of the 3d-states of the Ni ion. The ground state is of 3A2g symmetry (t2g6eg2), and several excited states with triplet and singlet character have been predicted theoretically [6]. Electronic transitions between d-states are strongly dipole forbidden and become only slightly allowed by a weak p - d hybridization. Whereas triplet-triplet transitions might be excited by electron exchange (with spin-flips and nonflips) as well as by direct
* Corresponding author: Fax: +49-211-311-3117; [email protected].
email:
scattering, triplet-singlet transitions can be excited by electron exchange only; spin-flips (ms = 1 --, m s = 0) and nontlips (m s = 0 - , m s ----0) are possible. Our measurements confirm this assumption: all d - d transition-assigned sharp excitation peaks (A, B, C, D in Fig. 1; peak A might be due to a surface state [7]) appear in both the partial flip and nonflip intensities, and the depolarization of the scattered electrons is high. Elastic scattering and dipole-allowed transitions across the insulating band gap are not accompanied by spin-flip processes. The energy and angle dependences of exchange processes seem to be quite different for different materials. For free atoms, experiments and theories show a high amount of exchange processes for low primary electron energies (in the vicinity of the excitation threshold) but for large scattering angles only [8]. For d-excitations in ferromagnetic transition metals a similar angle dependence has been found. The exchange scattering is imposed by smallangle direct dipole scattering, leading to a reduction o f the relative importance o f exchange processes in specular geometry [1,9]. In contrast to free-atom scattering, exchange was ibund in Co at 250 eV primary energy [3]. 3 d - 3 d transitions in antiferromagnetie Cr203 were excited by electron exchange up to a primary energy o f 50 eV [10]. Unpolarized measurements of 4 f - 4 f excitations in Gd seem to show electron exchange at more than 100 eV primary enersy, even in specular geometry [11]. In NiO, exchange scattering is detectable for all d - d excitations up to more than 100 eV primary energy (Fig. 1). Therefore, the exchange excitation between localized, atomic-like states in NiO differs strongly from that of free atoms but seems to agree with that of ferromagnetic solids and rare earth metals. The primary energy and angle-dependent depolarization measurements on NiO exhibit a complicated behavior of
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved $SDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 1 9 9 . 9
182
B. Frolnnneet aL /Journal of Magnetism and Magnetic Materials 148 (1995) 281-182
~.o.
~.
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o81 II
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,cE)/
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0.8
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3. Summary
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Energy Loss (eg) Fig. 1. (a) Spin-integrated energy loss spectrum /(E), nonflip intensity At(E) and flip intensity F(E), and (b) depolarization .D(E) in the band gap region of NiO. Definitions of N(E), F(E) and D(E) are given in Ref. [5]. The measurements were taken in ~l~ecular scattering geometry; the primary energy was 100 eV, The spin-integrated energy loss spectnurn up to 10 eV is given in the inset.
spin-flip exchange processes. Within large energy ranges, the depolarization is small in specular scattering geometry (Fig. 2), but it increases rapidly in off-specular geometry, in agreement with measurememls on free atoms as well as 1.0
........
, ........
~ ........
,........,
.......
, ........
, .........
, ........
,.........,.,.4.._,._
0.8 tO
:= 0.6 N
"~ 0,4 O
80.2 0.0
on ferromagnets. But around 30, 38 and 100 eV the behavior o f spin-flip exchange processes in NiO is completely different. The depolarization is very high in specular geometry; it is scarcely angle dependent and slightly reduced in off-specular geometry. The intensity o f the d - d excitations increases resonance-like at these energies, a behaviour previously also seen in EELS measurements with unpolarized electrons of primary energies around 101) e V [7]. The energies o f the depolarization maxima in specular geometry coincide with energy loss structures, which are attributed to one-electron excitations [12]. Therefore, resonant d - d excitation processes are likely to occur at these energies. The high depolarization in specular geometry shows that these reso. nant excitations lead to the opening of further exchange channels with completely different angular dependences than have been supposed from existing exper'.tments and theories.
EL = l . 6 e V 20 30 40 50 60 70 80 90100110120 Prh'nory Energy (eV)
Fig. 2. Primary energy dependence of the depolarization of the 1.6 eV energy loss structure (C), measured in specular geometry.
Whereas spin-flip excha~age scattering is of negligible influence for dipole-allowed transitions in NiO, it plays an important role in the excitation o f dipole-forbidden d - d transitions with primary electron energies of more than 100 eV. The amount of exchange processes is dramatically enhanced in specular geometry, ff resonant excitation is possible. Acknowledgement: This work was supported by the Deutsche Forsehungsgemeinschaft.
References [1] D. Venus and J. Kirschner, Phys. Rev. B 37 (1988) 2199. [2] T.G. Walker and H. Hopstcr, Phys. Rev. B 48 (1993) 3563. [3] Y.U. Idzerda, D.M. Lind, D.A. Papaconstantopoulos, G.A. Prinz, B.T. Jonker and J J. Krebs, Phys. Rev. Lett. 61 (1988) 1222. [4] Th. Dodt, Dissertation, Uaiversit~t Diisseldoff (1988). [5] B. Fromme, M. Schmitt, E. Kisker, A. Gorsehli~ter and H. Merz, Phys. Rev. B 50 (i994) 1874. [6] A. Fujimori and F. Minami, Phys. Rev. B 30 (1984) 957. [7] A. Gorschli3ter and H. Mer2, Phys. Rev. B 49 (1994) 17293. [8] G.F. Hanne, Phys. Rep. 95 (1983) 95. [9] D.L. Mills, Phys. Rev. B 34 (1986) 6099. [10] H. Hopster, Phys. Rev, B 42 (1990) 2540. [11] J.A.D. Matthew, W.A. Henle, M.G. Ramsey and F.P. Netzer, Phys. Rev. B 43 (1991) 4897. [12] A. Gorschlflter and H. Merz in: Int. Conf. on the Physics of Transition Metals, eds. P.M. Opperieer and J'. Ki~bler (World Scientific, Singapore, 1993), p. 341.
~ Jouma! of magnetism and magnetic materials
ELSEVIER
Spin-flip low-energy electron exchange scattering in NiO(100) B. F r o m m e ~'*, A. Hylla a, C. Koch a, E. Kisker ~, A. Gorschliiter b, H. M e r z b a Institut f~r Angewandte Physik der Universitiit Diisseldorf, 40225 Diisseldo~ Germany b Physikalisches lnstitut der Universitiit Miinster, 48149 2Vtiinster, Germany
Abstract Electron exchange processes in NiO have been examined by complete spin-polarized electron energy loss spectroscopy. The measurements show a complicated dependence of the spin-flip exchange scattering on incident angle and energy, unexpected from existing theories and measurements on atoms and ferromagnets. The amount of exchange processes is dramatically enhanced in specular scattering geometry for incident energies around 30, 38 and 100 eV, which is attributed to resonant excitation.
1. Introduction
Spin-polarized electron energy loss spectroscopy (SPEELS) is a successful surface-sensitive method for studying spin-dependent band structures and coupling mechanisms between layers of different types o f magnetism via electron exchange processes [1-4]. But only the complete scattering experhTtent (C-SPEF,LS), with polarized incident electrons a n d polarization analysis of the scattered electrons, provid~',s direct experimental proof of such processes. The partial spin-flip and nonflip intensities ~ e determined from the intensity and the change in electron polarization during scattering. Here, we applied CSPEELS to the antiferromagnetic transition metal compound NiO. First results have been published elsewhere
[5].
2. Results and discussion
In contrast with the 3d metals, the 3d electrons of the Ni 2÷ ions in NiO do not show band-like but localized, quasi-atomic-like character. The octahedral surrounding of the Ni 2+ ions by 0 2 - ions in NiO provides a partial lifting o f the degeneracy of the 3d-states of the Ni ion. The ground state is of 3A2g symmetry (t2g6eg2), and several excited states with triplet and singlet character have been predicted theoretically [6]. Electronic transitions between d-states are strongly dipole forbidden and become only slightly allowed by a weak p - d hybridization. Whereas triplet-triplet transitions might be excited by electron exchange (with spin-flips and nonflips) as well as by direct
* Corresponding author: Fax: +49-211-311-3117; [email protected].
email:
scattering, triplet-singlet transitions can be excited by electron exchange only; spin-flips (ms = 1 --, m s = 0) and nontlips (m s = 0 - , m s ----0) are possible. Our measurements confirm this assumption: all d - d transition-assigned sharp excitation peaks (A, B, C, D in Fig. 1; peak A might be due to a surface state [7]) appear in both the partial flip and nonflip intensities, and the depolarization of the scattered electrons is high. Elastic scattering and dipole-allowed transitions across the insulating band gap are not accompanied by spin-flip processes. The energy and angle dependences of exchange processes seem to be quite different for different materials. For free atoms, experiments and theories show a high amount of exchange processes for low primary electron energies (in the vicinity of the excitation threshold) but for large scattering angles only [8]. For d-excitations in ferromagnetic transition metals a similar angle dependence has been found. The exchange scattering is imposed by smallangle direct dipole scattering, leading to a reduction o f the relative importance o f exchange processes in specular geometry [1,9]. In contrast to free-atom scattering, exchange was ibund in Co at 250 eV primary energy [3]. 3 d - 3 d transitions in antiferromagnetie Cr203 were excited by electron exchange up to a primary energy o f 50 eV [10]. Unpolarized measurements of 4 f - 4 f excitations in Gd seem to show electron exchange at more than 100 eV primary enersy, even in specular geometry [11]. In NiO, exchange scattering is detectable for all d - d excitations up to more than 100 eV primary energy (Fig. 1). Therefore, the exchange excitation between localized, atomic-like states in NiO differs strongly from that of free atoms but seems to agree with that of ferromagnetic solids and rare earth metals. The primary energy and angle-dependent depolarization measurements on NiO exhibit a complicated behavior of
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved $SDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 1 9 9 . 9
182
B. Frolnnneet aL /Journal of Magnetism and Magnetic Materials 148 (1995) 281-182
~.o.
~.
~
o81 II
c
,cE)/
.~ " , I I ~ . ~ - ~ 0
0.8
\~oo
1
2
3
4
"~0.,t. 0.2
3. Summary
0.0
Energy Loss (eg) Fig. 1. (a) Spin-integrated energy loss spectrum /(E), nonflip intensity At(E) and flip intensity F(E), and (b) depolarization .D(E) in the band gap region of NiO. Definitions of N(E), F(E) and D(E) are given in Ref. [5]. The measurements were taken in ~l~ecular scattering geometry; the primary energy was 100 eV, The spin-integrated energy loss spectnurn up to 10 eV is given in the inset.
spin-flip exchange processes. Within large energy ranges, the depolarization is small in specular scattering geometry (Fig. 2), but it increases rapidly in off-specular geometry, in agreement with measurememls on free atoms as well as 1.0
........
, ........
~ ........
,........,
.......
, ........
, .........
, ........
,.........,.,.4.._,._
0.8 tO
:= 0.6 N
"~ 0,4 O
80.2 0.0
on ferromagnets. But around 30, 38 and 100 eV the behavior o f spin-flip exchange processes in NiO is completely different. The depolarization is very high in specular geometry; it is scarcely angle dependent and slightly reduced in off-specular geometry. The intensity o f the d - d excitations increases resonance-like at these energies, a behaviour previously also seen in EELS measurements with unpolarized electrons of primary energies around 101) e V [7]. The energies o f the depolarization maxima in specular geometry coincide with energy loss structures, which are attributed to one-electron excitations [12]. Therefore, resonant d - d excitation processes are likely to occur at these energies. The high depolarization in specular geometry shows that these reso. nant excitations lead to the opening of further exchange channels with completely different angular dependences than have been supposed from existing exper'.tments and theories.
EL = l . 6 e V 20 30 40 50 60 70 80 90100110120 Prh'nory Energy (eV)
Fig. 2. Primary energy dependence of the depolarization of the 1.6 eV energy loss structure (C), measured in specular geometry.
Whereas spin-flip excha~age scattering is of negligible influence for dipole-allowed transitions in NiO, it plays an important role in the excitation o f dipole-forbidden d - d transitions with primary electron energies of more than 100 eV. The amount of exchange processes is dramatically enhanced in specular geometry, ff resonant excitation is possible. Acknowledgement: This work was supported by the Deutsche Forsehungsgemeinschaft.
References [1] D. Venus and J. Kirschner, Phys. Rev. B 37 (1988) 2199. [2] T.G. Walker and H. Hopstcr, Phys. Rev. B 48 (1993) 3563. [3] Y.U. Idzerda, D.M. Lind, D.A. Papaconstantopoulos, G.A. Prinz, B.T. Jonker and J J. Krebs, Phys. Rev. Lett. 61 (1988) 1222. [4] Th. Dodt, Dissertation, Uaiversit~t Diisseldoff (1988). [5] B. Fromme, M. Schmitt, E. Kisker, A. Gorsehli~ter and H. Merz, Phys. Rev. B 50 (i994) 1874. [6] A. Fujimori and F. Minami, Phys. Rev. B 30 (1984) 957. [7] A. Gorschli3ter and H. Mer2, Phys. Rev. B 49 (1994) 17293. [8] G.F. Hanne, Phys. Rep. 95 (1983) 95. [9] D.L. Mills, Phys. Rev. B 34 (1986) 6099. [10] H. Hopster, Phys. Rev, B 42 (1990) 2540. [11] J.A.D. Matthew, W.A. Henle, M.G. Ramsey and F.P. Netzer, Phys. Rev. B 43 (1991) 4897. [12] A. Gorschlflter and H. Merz in: Int. Conf. on the Physics of Transition Metals, eds. P.M. Opperieer and J'. Ki~bler (World Scientific, Singapore, 1993), p. 341.