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Au(1 1 1) studied by spin-resolved photoelectron spectroscopy
Applied Surface Science 169±170 (2001) 176±179
Electronic structures of magnetic ultrathin ®lms Co/Au(1 1 1) studied by spin-resolved photoelectron spectroscopy M. Sawadaa,*, K. Hayashia, A. Kakizakib a
Institute for Solid State Physics, University of Tokyo, Tokyo 106-8666, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
b
Received 2 August 1999; accepted 2 November 1999
Abstract We have measured the spin-resolved photoemission spectra of the Co thin ®lms grown epitaxially on Au(1 1 1) substrate in order to investigate their valence band structures. It is proved that the electronic structures of Co thin ®lms are pretty different from that of bulk hcp-Co. It is observed that as the ®lms grow thicker, the electronic structures become closer to those of the bulk Co with the magnetic anisotropy turning into in-plane magnetization from out-of-plane magnetization. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Spin-resolved photoemission; Magnetic thin ®lm; Electoronic structure
1. Introduction Epitaxially grown magnetic thin ®lms in the monolayer regime have attracted many interests for a long time because of the realization of the magnetism with low dimensionality and typical magnetic phenomena such as enhanced magnetic moments and perpendicular magnetic anisotropy [1]. The Co/Au ®lms and multilayers also show their peculiar properties; giant magnetoresistance, oscillatory behavior of interlayer magnetic coupling and the perpendicular magnetic anisotropy [2±4]. Especially, the perpendicular magnetization to the surface are considered to play a prominent role for the development of the magnetic
* Corresponding author. Present address: SRL-ISSP, KEK-PF, Oho 1-1, Tsukuba 305-0081, Japan. Tel.: 81-298-64-2489; fax: 81-298-64-2461. E-mail address: [email protected] (M. Sawada).
data-storage media. Therefore many studies have been performed for the Co/Au(1 1 1) ®lms using various experimental methods which are accessible to their structural or morphological informations and magnetic properties. The epitaxial growth of the system in the early stage have been observed in detail by the scanning tunneling microscopy (STM) [5]. The study shows that the evaporated Co starts to nucleate at the kink of the reconstructed Au(1 1 1) with the zigzag pattern forming ¯at islands with the closed-pack cobalt and then the cobalt islands become co-released and connected with each other forming ®lm domains. The growth mode, caused by the large lattice mismatch between Co and Au, has been supported by the other experiments [6,7] and the hexagonal (hcp) structure of the cobalt has been con®rmed by X-ray diffraction measurement [8]. The magnetic properties of the system also depend on ®lm thickness. Below about 2 ML no magnetization is found at room temperature. The
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 2 6 - 1
M. Sawada et al. / Applied Surface Science 169±170 (2001) 176±179
thicker ®lms have been proved to have out-of-plane magnetization which gradually turns into in-plane magnetization as the ®lm thickness increases [9]. The structure and the magnetic properties of ultrathin ®lms are closely concerned with the electronic structures of the valence states. However, only conventional photoemission experiments with a He discharge lamp have been carried out so far [10], which are not more accessible to the electronic structure with the exchange splitting than spin- and angle-resolved photoemission spectroscopy (SARPES). We have measured the thickness dependence of the spinresolved photoemission spectra for the Co/Au(1 1 1) ®lms in order to demonstrate the spin dependent electronic structures in the monolayers regime and to compare the results with the electronic structures of the bulk hcp-Co [11].
177
3. Results and discussion Fig. 1 shows the spin-resolved spectra and the spin polarization of Co/Au(1 1 1) in relatively low coverage region (3.3 ML) excited by the s-polarized light whose energy is 21.2 eV. The spectrum for the clean Au(1 1 1) is also shown in the ®gure. The spectral features at the binding energies larger than 3 eV correspond to the 5d states of Au and those which range between the Fermi level and the binding energy of 2 eV correspond to the electronic structures of the Co. It has been pointed out that the shoulder near the binding energy of 2.5 eV in the Au(1 1 1) spectrum caused by the surface-derived state [13]. The signal of the Au is still intensive in the spectra for the 3.3 ML thickness. The thickness in this report means the average thickness. This is because the growth mode is not a layer-by-layer one, but a islanding which has
2. Experimental The SARPES measurements were performed at the undulator beamline BL-19A of the Photon Factory, using recently developed spin- and angle-resolved photoelectron spectrometer consisting of a hemispherical electron energy analyzer and a compact retarding type Mott detector [12]. The Au(1 1 1) substrate was cleaned by repeated cycles of ion bombardment and annealing at 5508C. The reconstructed clean Au(1 1 1) surface was con®rmed by the existence of the surface state in the photoemission spectra [13]. The Co was evaporated by an electron bombardment of a high purity Co wire in the vacuum below 4 10ÿ10 Torr keeping the growth rate of 0.1±0.2 ML/min. The cleanliness of the epitaxial Co ®lm was con®rmed by Auger electron spectroscopy (AES) before and after every SARPES measurement. The coverage was assigned by referring to the Auger intensity ratio of Co MVV (53 eV) and the Au NVV(69 eV) and the disappearance thickness of the out-of-plane magnetization was adopted as the coverage standard [9]. The s-polarized incident light was adopted in the SARPES measurements and the photoelectrons emitted normal to the sample surface were collected. The photon energy of 21.2 and 28 eV was selected for the measurements. The energy resolutions of the spectrometer were 0.3 and 0.15 eV for the measurement with the excitation energy of 21.2 and 28 eV, respectively.
Fig. 1. Spin-resolved photoemission spectra and the spin polarizations for a 3.3 ML Co ®lm deposited on Au(1 1 1) substrate. The majority and minority spin spectrum are plotted by the ®lled markers and the un®lled markers, respectively. Photoemission spectrum for a clean surface of Au(1 1 1) is indicated in the solid line. They have been measured in normal emission at the photon energy of 21.2 eV.
178
M. Sawada et al. / Applied Surface Science 169±170 (2001) 176±179
been found by STM investigations [5]. The observation of the Au valence states is due to the relatively large escape depth of the photoelectrons with kinetic energies of 10±17 eV. In the area with thin Co coverage the photoelectrons from the Au substrate may escape into vacuum through Co layers. Small spin polarization in the high binding energy corresponding to the Au structures is shown. Nevertheless, the structure near the binding energy of 2.5 eV have positive polarization and the shoulder which appears in the Au(1 1 1) spectrum collapses in the spectra for 3.3 ML thickness. This modi®cation is considered to indicate the charge transfer from the Au into the Co at the interface and to be consistent with the assignment of the shoulder structure as the surface related state. The positive spin polarization of the spectral feature implies that at the interface, the 5d electrons in the minority spin state are dominantly transferred to the unoccupied Co 3d minority spin states. Above the binding energy of 2 eV the valence electronic structures of Co are shown with the variation of the spin polarization. To analyze the spectral features, we have obtained the second derivative of each energy distribution curve to determine the peak positions. We also assumed that the ®nal state of the photoelectrons was in Bloch states and in the symmetry allowed to transit from the initial state [11]. The excitation energy of 21.2 eV probes the electronic states at the middle point of the A±G line in the hcp Brillouin zone (quarter point of the G±L line in the fcc Brillouin zone), and only the fcc L3 symmetry states are detectable in the s-polarization geometry. In the majority spin spectrum a distinct peak at the binding energy of 0.7 eV and a broad peak at the 1.4 eV are assigned as the upper band and the lower band with L3 symmetry, respectively. The conspicuous peak in the minority spin spectrum corresponds to the lower band, which is counter part of the peak at the 1.4 eV in the majority spectrum. The assignment was con®rmed by the examination with both the polarized light and the nonpolarized light (He I radiation). Fig. 2 shows the thickness dependence of SARPES spectra with the excitation energy of 28 eV corresponding to the G point in the Brillouin zone. The spectra from 2 ML up to the 6 ML were measured in the out-of plane remnant magnetizations and the
Fig. 2. Spin-resolved photoemission spectra for Co/Au(1 1 1) ®lms with various thickness. The majority and minority spin spectra are arranged on the left and right side in the ®gure, respectively. Vertical bars indicate the peak positions. The spectra have been measured in normal emission at the photon energy of 28 eV corresponding to the G point in the Brillouin zone.
in-plane magnetizations were applied to the measurements for thicker samples than 6 ML. We have applied the magnetic ®eld along [0 0 0 1] and [1 0 1 0] direction of the Co ®lm with thickness below 6 ML and above it, respectively. The no remanent magnetization below the thickness of 2 ML and the no perpendicular magnetization above 6 ML were con®rmed. The outof-plane magnetization in the speci®c range of the thickness is comparable with the previous report [9]. In the aspect of the bulk bands, there is only two electronic states in each spin spectrum, connecting to the states presented in the Fig. 1 with the band dispersions along the sample normal direction. One of them is the upper band corresponding to the eg state and the other one is the lower band corresponding to the t2g state. In the majority spin spectra, it is proved that a distinct peak derived from the upper band and a
M. Sawada et al. / Applied Surface Science 169±170 (2001) 176±179
weak peak derived from the lower band appear for each coverage. The binding energies of the upper band peaks do not change much from the 0.5 eV for the thinnest ®lm to the 0.65 eV for the thickest ®lm. On the other hands, the remarkable thickness dependence is found in the lower band ranging from 0.9 to 1.7 eV as the thickness increases from 2.1 to 11.5 ML. In the minority spin spectra, the peaks are proved to correspond to the lower band. The lower peaks range from the 0.4 to 0.65 eV with the increasing thickness. The structure of the minority upper band appears near the Fermi level. It is found that the spectral intensities just below the Fermi level increase as the coverage grows thicker and the minority upper state comes down to the occupied state above the thickness of the 6 ML, where the magnetic anisotropy turns into the in-plane magnetization. This observation implies a strong relationship between the change of the lattice constant of the Co ®lm and the electronic structures responsible to the perpendicular maagnetizaiton anisotropy. According to the previous spin-integrated photoemission experiment to investigate the bulk Co bands [11], the binding energies of the upper bands are 0.05 and 0.9 eV for the minority and majority part, respectively, which lead to the exchange splitting of the 0:85 0:2 eV. In the same way, the 0.8 and 2.0 eV for the lower bands with the split of 1:2 0:3 eV can be referred. Concerning the low coverage spectra in the present study, the spectral features seem to be shifted by about 0.5 eV toward higher binding energy than those in the bulk spectra. As the thickness increases, the peaks move gradually towards the lower binding energy. It is proved that the electronic structures shown in the spectra of thick ®lms are closer to that
179
of the bulk Co. However, the discrepancies still remains between the electronic structures of the ®lms and those of the bulk. Most remarkable point is that the obtained exchange energies of 0.35 (the upper bands) and 1.05 eV (the lower bands) are conspicuously small compared with those reported for the bulk or the Co/ Cu(1 1 1) ®lm [14]. This is considered to be caused by the con®ned islands on the substrate [5] or the lattice strain originates from the large lattice mismatch. References [1] J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures I, Springer, Berlin, 1994. [2] C. Dupas, P. Beauvillain, J. Appl. Phys. 67 (1990) 5680. [3] A. Bounouh, P. Beauvillain, P. Bruno, C. Chappert, R. MeÂgy, P. Veillet, Europhys. Lett. 33 (1996) 315. [4] C.H. Lee, Hui He, F.J. Lamelas, W. Vavra, C. Uher, R. Clarke, Phys. Rev. B 42 (1990) 1066. [5] B. VoigtlaÈnder, G. Meyer, N.M. Amer, Phys. Rev. B 44 (1991) 10354. [6] C. ToÈlkes, P. Zeppenfelt, M.A. Krzyzowski, R. David, G. Comsa, Phys. Rev. B 55 (1972) 13932. [7] N. Marsot, R. Belkhou, F. Scheurer, B. Bartenlian, N. Barrett, M.A. Delaunay, C. Guillot, Surf. Sci. 377±379 (1997) 225. [8] D. Renard, G. Nihoul, Philos. Mag. B 55 (1987) 75. [9] R. Allenspach, M. Stampanoni, A. Bischof, Phys. Rev. Lett. 65 (1990) 3344. [10] R. Mamy, B. Carricaburu, J. Phys. Condens. Matter 5 (1993) 6537. [11] F.J. Himpsel, D.E. Eastman, Phys. Rev. B 21 (1980) 3207. [12] S. Qiao, A. Kimura, A. Harasawa, M. Sawada, J.-G. Chung, A. Kakizaki, Rev. Sci. Instrum. 68 (1997) 4390. [13] H.-G. Zimmer, A. Goldmann, Surf. Sci. 176 (1986) 115. [14] U. Alkemper, C. Carbone, E. Vescovo, W. Eberhardt, O. Rader, W. Gudat, Phys. Rev. B 50 (1994) 17496.
Electronic structures of magnetic ultrathin ®lms Co/Au(1 1 1) studied by spin-resolved photoelectron spectroscopy M. Sawadaa,*, K. Hayashia, A. Kakizakib a
Institute for Solid State Physics, University of Tokyo, Tokyo 106-8666, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
b
Received 2 August 1999; accepted 2 November 1999
Abstract We have measured the spin-resolved photoemission spectra of the Co thin ®lms grown epitaxially on Au(1 1 1) substrate in order to investigate their valence band structures. It is proved that the electronic structures of Co thin ®lms are pretty different from that of bulk hcp-Co. It is observed that as the ®lms grow thicker, the electronic structures become closer to those of the bulk Co with the magnetic anisotropy turning into in-plane magnetization from out-of-plane magnetization. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Spin-resolved photoemission; Magnetic thin ®lm; Electoronic structure
1. Introduction Epitaxially grown magnetic thin ®lms in the monolayer regime have attracted many interests for a long time because of the realization of the magnetism with low dimensionality and typical magnetic phenomena such as enhanced magnetic moments and perpendicular magnetic anisotropy [1]. The Co/Au ®lms and multilayers also show their peculiar properties; giant magnetoresistance, oscillatory behavior of interlayer magnetic coupling and the perpendicular magnetic anisotropy [2±4]. Especially, the perpendicular magnetization to the surface are considered to play a prominent role for the development of the magnetic
* Corresponding author. Present address: SRL-ISSP, KEK-PF, Oho 1-1, Tsukuba 305-0081, Japan. Tel.: 81-298-64-2489; fax: 81-298-64-2461. E-mail address: [email protected] (M. Sawada).
data-storage media. Therefore many studies have been performed for the Co/Au(1 1 1) ®lms using various experimental methods which are accessible to their structural or morphological informations and magnetic properties. The epitaxial growth of the system in the early stage have been observed in detail by the scanning tunneling microscopy (STM) [5]. The study shows that the evaporated Co starts to nucleate at the kink of the reconstructed Au(1 1 1) with the zigzag pattern forming ¯at islands with the closed-pack cobalt and then the cobalt islands become co-released and connected with each other forming ®lm domains. The growth mode, caused by the large lattice mismatch between Co and Au, has been supported by the other experiments [6,7] and the hexagonal (hcp) structure of the cobalt has been con®rmed by X-ray diffraction measurement [8]. The magnetic properties of the system also depend on ®lm thickness. Below about 2 ML no magnetization is found at room temperature. The
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 2 6 - 1
M. Sawada et al. / Applied Surface Science 169±170 (2001) 176±179
thicker ®lms have been proved to have out-of-plane magnetization which gradually turns into in-plane magnetization as the ®lm thickness increases [9]. The structure and the magnetic properties of ultrathin ®lms are closely concerned with the electronic structures of the valence states. However, only conventional photoemission experiments with a He discharge lamp have been carried out so far [10], which are not more accessible to the electronic structure with the exchange splitting than spin- and angle-resolved photoemission spectroscopy (SARPES). We have measured the thickness dependence of the spinresolved photoemission spectra for the Co/Au(1 1 1) ®lms in order to demonstrate the spin dependent electronic structures in the monolayers regime and to compare the results with the electronic structures of the bulk hcp-Co [11].
177
3. Results and discussion Fig. 1 shows the spin-resolved spectra and the spin polarization of Co/Au(1 1 1) in relatively low coverage region (3.3 ML) excited by the s-polarized light whose energy is 21.2 eV. The spectrum for the clean Au(1 1 1) is also shown in the ®gure. The spectral features at the binding energies larger than 3 eV correspond to the 5d states of Au and those which range between the Fermi level and the binding energy of 2 eV correspond to the electronic structures of the Co. It has been pointed out that the shoulder near the binding energy of 2.5 eV in the Au(1 1 1) spectrum caused by the surface-derived state [13]. The signal of the Au is still intensive in the spectra for the 3.3 ML thickness. The thickness in this report means the average thickness. This is because the growth mode is not a layer-by-layer one, but a islanding which has
2. Experimental The SARPES measurements were performed at the undulator beamline BL-19A of the Photon Factory, using recently developed spin- and angle-resolved photoelectron spectrometer consisting of a hemispherical electron energy analyzer and a compact retarding type Mott detector [12]. The Au(1 1 1) substrate was cleaned by repeated cycles of ion bombardment and annealing at 5508C. The reconstructed clean Au(1 1 1) surface was con®rmed by the existence of the surface state in the photoemission spectra [13]. The Co was evaporated by an electron bombardment of a high purity Co wire in the vacuum below 4 10ÿ10 Torr keeping the growth rate of 0.1±0.2 ML/min. The cleanliness of the epitaxial Co ®lm was con®rmed by Auger electron spectroscopy (AES) before and after every SARPES measurement. The coverage was assigned by referring to the Auger intensity ratio of Co MVV (53 eV) and the Au NVV(69 eV) and the disappearance thickness of the out-of-plane magnetization was adopted as the coverage standard [9]. The s-polarized incident light was adopted in the SARPES measurements and the photoelectrons emitted normal to the sample surface were collected. The photon energy of 21.2 and 28 eV was selected for the measurements. The energy resolutions of the spectrometer were 0.3 and 0.15 eV for the measurement with the excitation energy of 21.2 and 28 eV, respectively.
Fig. 1. Spin-resolved photoemission spectra and the spin polarizations for a 3.3 ML Co ®lm deposited on Au(1 1 1) substrate. The majority and minority spin spectrum are plotted by the ®lled markers and the un®lled markers, respectively. Photoemission spectrum for a clean surface of Au(1 1 1) is indicated in the solid line. They have been measured in normal emission at the photon energy of 21.2 eV.
178
M. Sawada et al. / Applied Surface Science 169±170 (2001) 176±179
been found by STM investigations [5]. The observation of the Au valence states is due to the relatively large escape depth of the photoelectrons with kinetic energies of 10±17 eV. In the area with thin Co coverage the photoelectrons from the Au substrate may escape into vacuum through Co layers. Small spin polarization in the high binding energy corresponding to the Au structures is shown. Nevertheless, the structure near the binding energy of 2.5 eV have positive polarization and the shoulder which appears in the Au(1 1 1) spectrum collapses in the spectra for 3.3 ML thickness. This modi®cation is considered to indicate the charge transfer from the Au into the Co at the interface and to be consistent with the assignment of the shoulder structure as the surface related state. The positive spin polarization of the spectral feature implies that at the interface, the 5d electrons in the minority spin state are dominantly transferred to the unoccupied Co 3d minority spin states. Above the binding energy of 2 eV the valence electronic structures of Co are shown with the variation of the spin polarization. To analyze the spectral features, we have obtained the second derivative of each energy distribution curve to determine the peak positions. We also assumed that the ®nal state of the photoelectrons was in Bloch states and in the symmetry allowed to transit from the initial state [11]. The excitation energy of 21.2 eV probes the electronic states at the middle point of the A±G line in the hcp Brillouin zone (quarter point of the G±L line in the fcc Brillouin zone), and only the fcc L3 symmetry states are detectable in the s-polarization geometry. In the majority spin spectrum a distinct peak at the binding energy of 0.7 eV and a broad peak at the 1.4 eV are assigned as the upper band and the lower band with L3 symmetry, respectively. The conspicuous peak in the minority spin spectrum corresponds to the lower band, which is counter part of the peak at the 1.4 eV in the majority spectrum. The assignment was con®rmed by the examination with both the polarized light and the nonpolarized light (He I radiation). Fig. 2 shows the thickness dependence of SARPES spectra with the excitation energy of 28 eV corresponding to the G point in the Brillouin zone. The spectra from 2 ML up to the 6 ML were measured in the out-of plane remnant magnetizations and the
Fig. 2. Spin-resolved photoemission spectra for Co/Au(1 1 1) ®lms with various thickness. The majority and minority spin spectra are arranged on the left and right side in the ®gure, respectively. Vertical bars indicate the peak positions. The spectra have been measured in normal emission at the photon energy of 28 eV corresponding to the G point in the Brillouin zone.
in-plane magnetizations were applied to the measurements for thicker samples than 6 ML. We have applied the magnetic ®eld along [0 0 0 1] and [1 0 1 0] direction of the Co ®lm with thickness below 6 ML and above it, respectively. The no remanent magnetization below the thickness of 2 ML and the no perpendicular magnetization above 6 ML were con®rmed. The outof-plane magnetization in the speci®c range of the thickness is comparable with the previous report [9]. In the aspect of the bulk bands, there is only two electronic states in each spin spectrum, connecting to the states presented in the Fig. 1 with the band dispersions along the sample normal direction. One of them is the upper band corresponding to the eg state and the other one is the lower band corresponding to the t2g state. In the majority spin spectra, it is proved that a distinct peak derived from the upper band and a
M. Sawada et al. / Applied Surface Science 169±170 (2001) 176±179
weak peak derived from the lower band appear for each coverage. The binding energies of the upper band peaks do not change much from the 0.5 eV for the thinnest ®lm to the 0.65 eV for the thickest ®lm. On the other hands, the remarkable thickness dependence is found in the lower band ranging from 0.9 to 1.7 eV as the thickness increases from 2.1 to 11.5 ML. In the minority spin spectra, the peaks are proved to correspond to the lower band. The lower peaks range from the 0.4 to 0.65 eV with the increasing thickness. The structure of the minority upper band appears near the Fermi level. It is found that the spectral intensities just below the Fermi level increase as the coverage grows thicker and the minority upper state comes down to the occupied state above the thickness of the 6 ML, where the magnetic anisotropy turns into the in-plane magnetization. This observation implies a strong relationship between the change of the lattice constant of the Co ®lm and the electronic structures responsible to the perpendicular maagnetizaiton anisotropy. According to the previous spin-integrated photoemission experiment to investigate the bulk Co bands [11], the binding energies of the upper bands are 0.05 and 0.9 eV for the minority and majority part, respectively, which lead to the exchange splitting of the 0:85 0:2 eV. In the same way, the 0.8 and 2.0 eV for the lower bands with the split of 1:2 0:3 eV can be referred. Concerning the low coverage spectra in the present study, the spectral features seem to be shifted by about 0.5 eV toward higher binding energy than those in the bulk spectra. As the thickness increases, the peaks move gradually towards the lower binding energy. It is proved that the electronic structures shown in the spectra of thick ®lms are closer to that
179
of the bulk Co. However, the discrepancies still remains between the electronic structures of the ®lms and those of the bulk. Most remarkable point is that the obtained exchange energies of 0.35 (the upper bands) and 1.05 eV (the lower bands) are conspicuously small compared with those reported for the bulk or the Co/ Cu(1 1 1) ®lm [14]. This is considered to be caused by the con®ned islands on the substrate [5] or the lattice strain originates from the large lattice mismatch. References [1] J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures I, Springer, Berlin, 1994. [2] C. Dupas, P. Beauvillain, J. Appl. Phys. 67 (1990) 5680. [3] A. Bounouh, P. Beauvillain, P. Bruno, C. Chappert, R. MeÂgy, P. Veillet, Europhys. Lett. 33 (1996) 315. [4] C.H. Lee, Hui He, F.J. Lamelas, W. Vavra, C. Uher, R. Clarke, Phys. Rev. B 42 (1990) 1066. [5] B. VoigtlaÈnder, G. Meyer, N.M. Amer, Phys. Rev. B 44 (1991) 10354. [6] C. ToÈlkes, P. Zeppenfelt, M.A. Krzyzowski, R. David, G. Comsa, Phys. Rev. B 55 (1972) 13932. [7] N. Marsot, R. Belkhou, F. Scheurer, B. Bartenlian, N. Barrett, M.A. Delaunay, C. Guillot, Surf. Sci. 377±379 (1997) 225. [8] D. Renard, G. Nihoul, Philos. Mag. B 55 (1987) 75. [9] R. Allenspach, M. Stampanoni, A. Bischof, Phys. Rev. Lett. 65 (1990) 3344. [10] R. Mamy, B. Carricaburu, J. Phys. Condens. Matter 5 (1993) 6537. [11] F.J. Himpsel, D.E. Eastman, Phys. Rev. B 21 (1980) 3207. [12] S. Qiao, A. Kimura, A. Harasawa, M. Sawada, J.-G. Chung, A. Kakizaki, Rev. Sci. Instrum. 68 (1997) 4390. [13] H.-G. Zimmer, A. Goldmann, Surf. Sci. 176 (1986) 115. [14] U. Alkemper, C. Carbone, E. Vescovo, W. Eberhardt, O. Rader, W. Gudat, Phys. Rev. B 50 (1994) 17496.