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Spin-polarized photoemission at interfaces of noble metals with Co and Fe
Journal of Magnetism and Magnetic Materials 121 (1993) 160-162 North-Holland
Spin-polarized photoemission at interfaces of noble metals with Co and Fe D. H a r t m a n n , W. W e b e r , D.A. W e s n e r
1, S.
P o p o v i c a n d G. G i i n t h e r o d t
2. Physikalisches Institut, RWTH Aachen, Templergraben 55, D-5100 Aachen, Germany
The interface systems Cu/Co(0001), Au/Fe(110) and Au/Co(0001) have been studied by angle- and spin-resolved photoemission spectroscopy.All systems show interface states located at binding energies E b = -2.5, -3.3 and -3.2 eV, respectively, none of which exhibit magnetic polarization or spin splitting. Presently the discovery of oscillatory interlayer exchange coupling in multilayer structures fascinates many research groups. F e / C r / F e ( 1 0 0 ) was the first transition metal based sandwich structure showing this oscillatory interlayer exchange coupling between the Fe layers across Cr [1,2]. Also multilayer structures containing noble metals such as F e / C u , C o / C u and F e / A u exhibit oscillatory behavior. Accompanied with oscillatory interlayer exchange the giant magnetoresistance ( G M R ) appears, which reaches up to 50% in C o / C u at 300 K [3], recommending C o / C u multilayers as candidates for magnetic field sensors. The oscillatory interlayer exchange and the G M R hint at magnetic polarization effects and at the possible existence of interface states at the C o / C u interface which might be examined by spin-resolved photoemission spectroscopy. Stimulated by these coupling effects we started to investigate the interfaces of noble metals with Fe and Co by means of angle- and spin-resolved photoemission spectroscopy. We used photon energies of 21.2, 16.85 and 11.83 eV (unpolarized light), an energy resolution of 200 meV and a 100-kV Mott detector for spin analysis. The base pressure of the U H V chamber (1 x 10-1° mbar) rises to 3 x 10 -1° mbar during electron-beam evaporation onto the W ( l l 0 ) substrate. The film quality and growth mode was checked in situ by Correspondence to: Dr. D. Hartmann, 2. Physikalisches Insti-
tut RWTH Aachen, Templergraben 55, D-5100 Aachen, Germany. Tel.: + 49-241/807086; telefax: +49-241/803599. 1 Present address: Lehrstuhi fiir Lasertechnik, RWTH Aachen, Steinbachstrasse 15, DW-5100 Aachen, Germany.
low energy electron diffraction ( L E E D ) and Auger electron spectroscopy (AES). According to recipes published previously we prepared 20 atomic layers (AL) of Fe or Co on the W ( l l 0 ) single crystal substrate held at elevated temperatures (T = 450 K for Fe and T = 400 K for Co) with deposition rates of 2 .&/min [4,5]. The Fe and Co films grow epitaxially with bcc(ll0)- and hcp(0001)-orientation, respectively. Cu or Au was evaporated at 0.5 A L / m i n at 300 K. The L E E D pattern is consistent with a (111) orientation of Cu and Au on Co(0001). One monolayer f c c - A u ( l l l ) on bcc-Fe(ll0) shows the Nishiyama-Wassermann orientational relationship, which means that the [011] fcc direction is parallel to the bcc [001] direction. The C o / C u system, which was the most extensively investigated system, has been checked, by AES. Unfortunately the low-energy Auger lines of Co (53 eV) and Cu (60 eV) interfere. Thus we measured the 716-eV Co Auger line. It shows an exponential decrease (corresponding to e x p ( - d / A ) ) with a decay length A = 12.4 A, as Co is covered by Cu of thickness d. The decay length has the same value as the inelastic mean free path of electrons of 700 eV kinetic energy [6]. This is at least consistent with layer-by-layer growth. The most promising interface system for our investigations proved to be C u ( l l l ) / C o ( 0 0 0 1 ) as judged by the strong oscillatory interlayer exchange and GMR. Fig. 1 shows the evolution of the spin-integrated energy distribution curves (EDCs) of Co(0001) successively covered by up to 5.0 A L of C u ( l l l ) . All spectra were taken at normal emission and under identical conditions
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
D. Hartmann et al. / Spin-polarized photoemission at interfaces of noble metals with Co and Fe
t~
/
Co(0001) + X AL Cu hv = 21.2eV
0o(0001) + i AL Cu hv = 21,2eV
NormalEmission
NormalEmrsslon ~*
%"
• • •
~
,0
~
~,o
~vW• v
Eb = -2,5eV
v v
~v C-
J.L
',7
v•
C2~
iO
5
4
i
i
i
3
2
1
161
7
0
-3
Binding Energy [eV] Fig. l. Spin-integrated EDCs for various Cu overlayer coverages (in atomic layers = AL) on Co(0001), plotted on the same absolute intensity scale.
to get comparable absolute intensities between EDCs. Clearly a C o / C u interface state can be identified as a prominent peak at E b = - 2 . 5 eV for 1 A L Cu on Co. This is emphasized by the dashed line. Addition of another 0.5 A L Cu (total coverage = 1.5 A L Cu) leads to a reduction of the intensity of the interface state and to the development of the 3d bulk-line state at - 2.8 eV. At a coverage of 5 A L Cu only the spectrum of bulk C u ( l l l ) is seen [7]. The two-dimensional character of the interface state has been confirmed by recording EDCs at the different available photon energies (see above). The interface state does not disperse with hu or k ± . Recently, interface states in the systems Pd(lll)/Fe(ll0), Pd(lll)/Co(0001) and P t ( l l l ) / C o ( 0 0 0 1 ) attracted much interest [8]. They are magnetically polarized with an inverted spin splitting of 150-200 meV compared to the exchange splitting of Fe or Co. The spin-resolved photoemission spectrum of the C u ( l l l ) / C o ( 0 0 0 1 ) interface state measured for hu = 21.2 eV is illustrated in fig. 2. Apparently the spin-up-intensity and spin-down-intensity spectra show no spin splitting and their shapes and peak widths are nearly identical. The difference in intensity between the two spin directions is attributed to the positively polarized Co-background. The spin-resolved spectra of 1 A L Cu on Co for binding
2
1
-:qa:n¢{:-_~erav [eV] Fig. 2. Spin-resolved EDCs of the interface state for 1 AL Cu on Co(0001). Solid symbols correspond to the majority-spin direction and open symbols to the minority-spin direction.
energies between E F and - 2 . 3 eV are shown in the upper part of fig. 3. These spectra do not show any significant change compared to the spin-resolved spectra of pure Co shown in the lower part of fig. 3. We identified analogous interface states at A u / F e ( l l 0 ) and Au/Co(0001) interfaces at binding energies of - 3 . 3 and - 3 . 2 eV, respectively. All these interface states including the one in fig. 1 (Cu/Co(0001)) appear in the vicinity of the noble metal d-electron states, suggesting that they
Co(0001) + 1 AL Cu
~v
/
s
V
V
T m co c-
hv = 21,2eV Normal Emission
~ ~-' % ,,,
v Co(0001) 0 k
-2
~~ v 1
0
~indlnc Energy {eV] Fig. 3. Spin-resolved EDCs near the Fermi edge for 1 AL Cu on Co(0001). Solid symbols correspond to the majority-spin direction and open symbols to the minority-spin direction.
162
D. Hartmann et al. / Spin-polarized photoemission at interfaces of noble metals with Co and Fe
have mainly noble metal d-like character. They are not magnetically polarized as confirmed by our spin-resolved photoemission spectra. It should be noted that 1 A L A u ( l l l ) on F e ( l l 0 ) turns the easy axis of F e ( l l 0 ) on a W ( l l 0 ) substrate from the [110] to the [001] direction due to the changed interface anisotropy [9]. This necessitated the rotation of the substrate crystal by 90 ° for our measurement geometry. In conclusion we would like to point out that we discovered interface states at the interfaces of Cu or Au with Fe or Co which at the particular k-point (normal emission) are unpolarized and probably possess noble metal d-like character. Also no significant magnetic change in the spinresolved photoemission spectra (see fig. 3) between E F and - 2.3 eV occurs if Co is covered by 1 AL Cu. This means that no significant magnetic interaction exists between Co and Cu. Probably this is due to the limited overlap of the Co-3d states near E F and the lower lying Cu-3d electronic states at about - 3 eV binding energy which inhibits hybridization and magnetic polarization.
We gratefully acknowledge the support by the Deutsche Forschungsgemeinschaft D F G through Gu 193/2-1 and SFB 341.
References [1] P. Griinberg, R. Schreiber, Y. Pang, M.B. Brodsky and H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [2] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] A. Fert, A. Barth~16my, P. Etienne, S. Lequien, R. Loloee, D.K. Lottis, D.H. Mosca, F. Petroff, W.P. Pratt and P.A. Schroeder, J. Magn. Magn. Mater. 104-107 (1992) 1712 and references therein. [4] U. Gradmann and G. Waller, Surf. Sci. 116 (1982) 539. [5] B.G. Johnson, P.J. Berlowitz, D.W. Goodman and C.H. Bartholomew, Surf. Sci. 217 (1989) 13. [6] M.P. Seah and W.A. Deneh, Surf. Interface Anal. 1 (1979) 2. [7] J.A. Knapp, F.J. Himpsel and D.E. Eastman, Phys. Rev. B 19 (1979) 4952. [8] W. Weber, D.A. Wesner, G. Giintherodt and U. Linke, Phys. Rev. Lett. 66 (1991) 942 and to be published. [9] H.J. Elmers and U. Gradmann, Appl. Phys. A 51 (1990) 255.
Spin-polarized photoemission at interfaces of noble metals with Co and Fe D. H a r t m a n n , W. W e b e r , D.A. W e s n e r
1, S.
P o p o v i c a n d G. G i i n t h e r o d t
2. Physikalisches Institut, RWTH Aachen, Templergraben 55, D-5100 Aachen, Germany
The interface systems Cu/Co(0001), Au/Fe(110) and Au/Co(0001) have been studied by angle- and spin-resolved photoemission spectroscopy.All systems show interface states located at binding energies E b = -2.5, -3.3 and -3.2 eV, respectively, none of which exhibit magnetic polarization or spin splitting. Presently the discovery of oscillatory interlayer exchange coupling in multilayer structures fascinates many research groups. F e / C r / F e ( 1 0 0 ) was the first transition metal based sandwich structure showing this oscillatory interlayer exchange coupling between the Fe layers across Cr [1,2]. Also multilayer structures containing noble metals such as F e / C u , C o / C u and F e / A u exhibit oscillatory behavior. Accompanied with oscillatory interlayer exchange the giant magnetoresistance ( G M R ) appears, which reaches up to 50% in C o / C u at 300 K [3], recommending C o / C u multilayers as candidates for magnetic field sensors. The oscillatory interlayer exchange and the G M R hint at magnetic polarization effects and at the possible existence of interface states at the C o / C u interface which might be examined by spin-resolved photoemission spectroscopy. Stimulated by these coupling effects we started to investigate the interfaces of noble metals with Fe and Co by means of angle- and spin-resolved photoemission spectroscopy. We used photon energies of 21.2, 16.85 and 11.83 eV (unpolarized light), an energy resolution of 200 meV and a 100-kV Mott detector for spin analysis. The base pressure of the U H V chamber (1 x 10-1° mbar) rises to 3 x 10 -1° mbar during electron-beam evaporation onto the W ( l l 0 ) substrate. The film quality and growth mode was checked in situ by Correspondence to: Dr. D. Hartmann, 2. Physikalisches Insti-
tut RWTH Aachen, Templergraben 55, D-5100 Aachen, Germany. Tel.: + 49-241/807086; telefax: +49-241/803599. 1 Present address: Lehrstuhi fiir Lasertechnik, RWTH Aachen, Steinbachstrasse 15, DW-5100 Aachen, Germany.
low energy electron diffraction ( L E E D ) and Auger electron spectroscopy (AES). According to recipes published previously we prepared 20 atomic layers (AL) of Fe or Co on the W ( l l 0 ) single crystal substrate held at elevated temperatures (T = 450 K for Fe and T = 400 K for Co) with deposition rates of 2 .&/min [4,5]. The Fe and Co films grow epitaxially with bcc(ll0)- and hcp(0001)-orientation, respectively. Cu or Au was evaporated at 0.5 A L / m i n at 300 K. The L E E D pattern is consistent with a (111) orientation of Cu and Au on Co(0001). One monolayer f c c - A u ( l l l ) on bcc-Fe(ll0) shows the Nishiyama-Wassermann orientational relationship, which means that the [011] fcc direction is parallel to the bcc [001] direction. The C o / C u system, which was the most extensively investigated system, has been checked, by AES. Unfortunately the low-energy Auger lines of Co (53 eV) and Cu (60 eV) interfere. Thus we measured the 716-eV Co Auger line. It shows an exponential decrease (corresponding to e x p ( - d / A ) ) with a decay length A = 12.4 A, as Co is covered by Cu of thickness d. The decay length has the same value as the inelastic mean free path of electrons of 700 eV kinetic energy [6]. This is at least consistent with layer-by-layer growth. The most promising interface system for our investigations proved to be C u ( l l l ) / C o ( 0 0 0 1 ) as judged by the strong oscillatory interlayer exchange and GMR. Fig. 1 shows the evolution of the spin-integrated energy distribution curves (EDCs) of Co(0001) successively covered by up to 5.0 A L of C u ( l l l ) . All spectra were taken at normal emission and under identical conditions
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
D. Hartmann et al. / Spin-polarized photoemission at interfaces of noble metals with Co and Fe
t~
/
Co(0001) + X AL Cu hv = 21.2eV
0o(0001) + i AL Cu hv = 21,2eV
NormalEmission
NormalEmrsslon ~*
%"
• • •
~
,0
~
~,o
~vW• v
Eb = -2,5eV
v v
~v C-
J.L
',7
v•
C2~
iO
5
4
i
i
i
3
2
1
161
7
0
-3
Binding Energy [eV] Fig. l. Spin-integrated EDCs for various Cu overlayer coverages (in atomic layers = AL) on Co(0001), plotted on the same absolute intensity scale.
to get comparable absolute intensities between EDCs. Clearly a C o / C u interface state can be identified as a prominent peak at E b = - 2 . 5 eV for 1 A L Cu on Co. This is emphasized by the dashed line. Addition of another 0.5 A L Cu (total coverage = 1.5 A L Cu) leads to a reduction of the intensity of the interface state and to the development of the 3d bulk-line state at - 2.8 eV. At a coverage of 5 A L Cu only the spectrum of bulk C u ( l l l ) is seen [7]. The two-dimensional character of the interface state has been confirmed by recording EDCs at the different available photon energies (see above). The interface state does not disperse with hu or k ± . Recently, interface states in the systems Pd(lll)/Fe(ll0), Pd(lll)/Co(0001) and P t ( l l l ) / C o ( 0 0 0 1 ) attracted much interest [8]. They are magnetically polarized with an inverted spin splitting of 150-200 meV compared to the exchange splitting of Fe or Co. The spin-resolved photoemission spectrum of the C u ( l l l ) / C o ( 0 0 0 1 ) interface state measured for hu = 21.2 eV is illustrated in fig. 2. Apparently the spin-up-intensity and spin-down-intensity spectra show no spin splitting and their shapes and peak widths are nearly identical. The difference in intensity between the two spin directions is attributed to the positively polarized Co-background. The spin-resolved spectra of 1 A L Cu on Co for binding
2
1
-:qa:n¢{:-_~erav [eV] Fig. 2. Spin-resolved EDCs of the interface state for 1 AL Cu on Co(0001). Solid symbols correspond to the majority-spin direction and open symbols to the minority-spin direction.
energies between E F and - 2 . 3 eV are shown in the upper part of fig. 3. These spectra do not show any significant change compared to the spin-resolved spectra of pure Co shown in the lower part of fig. 3. We identified analogous interface states at A u / F e ( l l 0 ) and Au/Co(0001) interfaces at binding energies of - 3 . 3 and - 3 . 2 eV, respectively. All these interface states including the one in fig. 1 (Cu/Co(0001)) appear in the vicinity of the noble metal d-electron states, suggesting that they
Co(0001) + 1 AL Cu
~v
/
s
V
V
T m co c-
hv = 21,2eV Normal Emission
~ ~-' % ,,,
v Co(0001) 0 k
-2
~~ v 1
0
~indlnc Energy {eV] Fig. 3. Spin-resolved EDCs near the Fermi edge for 1 AL Cu on Co(0001). Solid symbols correspond to the majority-spin direction and open symbols to the minority-spin direction.
162
D. Hartmann et al. / Spin-polarized photoemission at interfaces of noble metals with Co and Fe
have mainly noble metal d-like character. They are not magnetically polarized as confirmed by our spin-resolved photoemission spectra. It should be noted that 1 A L A u ( l l l ) on F e ( l l 0 ) turns the easy axis of F e ( l l 0 ) on a W ( l l 0 ) substrate from the [110] to the [001] direction due to the changed interface anisotropy [9]. This necessitated the rotation of the substrate crystal by 90 ° for our measurement geometry. In conclusion we would like to point out that we discovered interface states at the interfaces of Cu or Au with Fe or Co which at the particular k-point (normal emission) are unpolarized and probably possess noble metal d-like character. Also no significant magnetic change in the spinresolved photoemission spectra (see fig. 3) between E F and - 2.3 eV occurs if Co is covered by 1 AL Cu. This means that no significant magnetic interaction exists between Co and Cu. Probably this is due to the limited overlap of the Co-3d states near E F and the lower lying Cu-3d electronic states at about - 3 eV binding energy which inhibits hybridization and magnetic polarization.
We gratefully acknowledge the support by the Deutsche Forschungsgemeinschaft D F G through Gu 193/2-1 and SFB 341.
References [1] P. Griinberg, R. Schreiber, Y. Pang, M.B. Brodsky and H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [2] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] A. Fert, A. Barth~16my, P. Etienne, S. Lequien, R. Loloee, D.K. Lottis, D.H. Mosca, F. Petroff, W.P. Pratt and P.A. Schroeder, J. Magn. Magn. Mater. 104-107 (1992) 1712 and references therein. [4] U. Gradmann and G. Waller, Surf. Sci. 116 (1982) 539. [5] B.G. Johnson, P.J. Berlowitz, D.W. Goodman and C.H. Bartholomew, Surf. Sci. 217 (1989) 13. [6] M.P. Seah and W.A. Deneh, Surf. Interface Anal. 1 (1979) 2. [7] J.A. Knapp, F.J. Himpsel and D.E. Eastman, Phys. Rev. B 19 (1979) 4952. [8] W. Weber, D.A. Wesner, G. Giintherodt and U. Linke, Phys. Rev. Lett. 66 (1991) 942 and to be published. [9] H.J. Elmers and U. Gradmann, Appl. Phys. A 51 (1990) 255.