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Fe(100) interface formation
Surface Science 402–404 (1998) 609–613
Auger electron diffraction study of V/Fe(100) interface formation Y. Huttel a,*, J. Avila a,b, M.C. Asensio a,b, P. Bencok a,c, C. Richter a,c, V. Ilakovac a,c, O. Heckmann a,c, K. Hricovini a,c a LURE, Baˆtiment 209D, Centre Universitaire Paris Sud, 91405 Orsay, France b Instituto de Ciencia de Materiales, CSIC, 28049 Madrid, Spain c LPMS, Universite´ de Cergy-Pontoise, Neuville/Oise, 95031 Cergy-Pontoise, France Received 6 August 1997; accepted for publication 5 October 1997
Abstract Vanadium atoms present a magnetic moment different to zero when they are part of a thin film deposited on Fe or as a bimetallic Fe–V alloy. The understanding of this phenomenon can only be achieved with a correct structural description of these types of systems. We report an Auger electron diffraction investigation of V films grown on body cubic centred (b.c.c.) Fe(100) substrates. Angular-scanned Auger electron diffraction (AED) patterns of V L M M (473 eV ) and Fe L VV (703 eV ) show the formation 23 23 4 3 of a well-ordered V/Fe interface even at room temperature. The AED patterns of V films in the range of vanadium submonolayer provide evidence of an isotropic Auger emission, indicating the absence of interdiffusion of V atoms into the Fe substrate and absence of cluster growth of the V film. The annealing of these films up to 400°C does not activate the substitution of the topmost Fe surface layers by V atoms. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Auger electron diffraction; Interface formation; V/Fe(100)
1. Introduction In the last few years, magnetism at surfaces and interfaces of transition metals has attracted considerable experimental and theoretical attention. The reduced symmetry, the lower co-ordination number and highly localised surface and interface states offer the possibility of introducing new and exotic phenomena. The understanding of magnetic properties can be achieved only with a correct structural description of these systems. Metallic vanadium has a b.c.c. crystal structure and is known to be non-magnetic. However, the * Corresponding author. Fax: (+33) 1 64 46 41 48; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 00 1 8 -1
isolated vanadium atom in its ground state has a permanent magnetic moment of 3 mB. Calculations indicate that an expansion of the lattice constant [1] changes in the co-ordination number at the surface [2] or the hybridisation between V and Fe orbitals [3] may induce a magnetic ordering. Appearance of the magnetic moment on vanadium atoms in Fe–V alloys, superlattices and V/Fe(100) interfaces has been further demonstrated in several experiments [4–8]. The average V moment is antiparallel to the Fe ones, and its value is determined to vary between 1.5 and 0.3 mB [7,8]. The Fe/V system is interesting because calculations indicate a different magnetic behaviour for V films on Fe compared with V in Fe/V superlattices. Moreover, both Fe and V magnetic moments may depend on epitaxial orientation [9]. From the application
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Y. Huttel et al. / Surface Science 402–404 (1998) 609–613
point of view, it is worth noting that the inverse magnetoresistance has been measured recently in the simple spin-valve system Fe V /Au/Co [10]. 1−x x Both Fe and V possess b.c.c. crystal structures ( lattice mismatch of 6%), and previous studies using RHEED [11] have reported a good epitaxial growth up to six or seven V layers. In this paper, we present an Auger electron diffraction study of the adsorption of vanadium on Fe(100) including vanadium coverage dependence and annealing temperature effects. The room temperature deposition is found to be epitaxial with the absence of any diffusion of V atoms. Also, annealing at 400°C does not activate the diffusion of vanadium atoms as evidenced by angular scanned Auger Electron Diffraction data.
2. Experimental The experiments were performed in the high vacuum French–Spanish (PES 2) experimental station at LURE (Orsay). We used a VSW hemispherical analyzer [acceptance angle ±1°, resolution power 5×10−4 (dE/E)] mounted on a goniometer inside the ultra-high vacuum chamber. The sample was held by a vertical manipulator, providing polar and azimuthal motion. The Fe b.c.c. single crystal samples were pre˚ of pared ex-situ by deposition of about 1000 A pure Fe on MgO substrates. They were cleaned by cycles of Ar+ bombardment at 1 keV and 500 V, followed by annealings at 400°C for a few minutes. Cleanness and structural qualities were checked by Auger electron spectroscopy and Low Energy Electron Diffraction. Vanadium was evaporated by electron bombardment of a pure vanadium wire in a base pressure of 7×10−10 mbar. The V evaporation rate was calibrated by using a thickness monitor, and V coverage was also calculated a posteriori using the Auger spectra. The consistency between the two methods for measuring the vanadium coverages has been checked, and the variation determined by both techniques was found to be less than 10%. The monolayer (ML) coverage is defined as being the amount of V atoms needed to form one atomic plane on the b.c.c. structure, which corresponds to a surface density equal to
1.08×1015 atoms cm−2. The base pressure during data acquisition was in the range of 5×10−10 mbar.
3. Results and discussion We will first focus on the structure of bulk vanadium deposited on the clean Fe(100) surface. For reference, Fig. 1 gives the stereographic projection of the Fe L VV (703 eV ) normalized inten3 sity [(I −background)/background ] as a function 0 of polar (0–60°) and azimuthal (0–360°) angles of the clean Fe(100) surface. The intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity. The short range crystalline order is obvious here by the anisotropic distribution of intensity of forward scattered Auger electrons [medium-range order was checked by LEED, which exhibited a sharp (1×1) pattern]. The b.c.c. structure can be identified by the diffraction spots along the high symmetry [100] and [110] directions, which are partially illustrated by Fig. 2 if substituting the vanadium atoms by the iron ones.
Fig. 1. Stereographic projection of the Fe L VV (703 eV ) nor3 malized intensity as a function of polar (0–60 °) and azimuthal (0–360°) angles of the clean Fe(100) surface. The intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity. The [100] direction corresponds to an azimuthal angle of 0°.
Y. Huttel et al. / Surface Science 402–404 (1998) 609–613
Fig. 2. Auger electron diffraction model for a b.c.c. structure. Side views along [100] and [110] directions.
We now turn to Fig. 3, which displays the corresponding stereographic projections of the V L M M normalized intensity for bulk vanadium 23 23 4 grown on the clean Fe(100) surface. The vanadium bulk was prepared by depositing a thick layer of vanadium until the Fe L VV could not be seen 3 any further and followed by annealing for 5 min at 500°C in order to improve the LEED pattern.
Fig. 3. Stereographic projection of the V L M M (473 eV ) 23 23 4 normalized intensity as a function of polar (0–60 °) and azimuthal (0–360°) angles of the vanadium bulk grown on clean Fe(100) surface. The intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity.
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As can be observed by a simple comparison between Figs. 1 and 3, vanadium is growing, following the b.c.c. structure like the Fe(100) substrate one. It is interesting at this stage of the discussion to note that for the room temperature (RT ) growth, we have observed a slight, but significant and progressive, enhancement of the LEED background after each evaporation of vanadium. Such an increase, which is the straight signature of the diminishing of the medium range order, indicates that vanadium is probably clustering since local order is observed using Auger Electron Diffraction. However, we could improve the LEED patterns by short annealings (5 min) at temperatures between 300°C and 500°C. In order to gain more insight on the structure of V/Fe(100) interface upon annealings, we show in Fig. 4 the azimuthal distribution of V L M M normalized intensity as a function of 23 23 4 vanadium coverage and annealing temperatures [only for 1 ML V/Fe(100)] at a polar angle of 41.5° (41.5° off normal ). Due to the fact that the angular anisotropy in the forward scattering regime presents features that are spatially large, this pattern is essentially the same as that recorded for 45° off normal (not shown here). For the lowest coverage, i.e. 0.3 ML, we observe an isotropic intensity distribution of the Auger forward scattered electrons. Such an isotropic diffraction for a vanadium coverage below the monolayer is clear evidence that the adsorbed vanadium atoms do not diffuse into the Fe crystal bulk. For a vanadium coverage of 1 ML, the angular distribution of forward scattered Auger electrons shows a strong anisotropy indicating either the deposition of more than 1 ML or the formation of V clusters (or islands) that are expected from a thermodynamic point of view. Since the evaporation rate was measured by two different techniques, which give the same results within an error of 10%, we assume that the growth of vanadium films on Fe(100) proceeds through the formation of clusters. As can be seen in Fig. 4, the azimuthal scan for 1 ML (RT ) clearly shows four welldeveloped peaks at 45° (and 135°) and 90° (and 180°) azimuthal angles, which correspond to the diffraction between the first and second layer and between the first and third layer, respectively. A
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in the monolayer range of V/Fe(100) interface formation, vanadium is not growing on the Fe(100) as a perfect atomic layer, but is more likely forming clusters whose height can be slightly reduced by annealing at 400°C, resulting in the formation of a medium-range ordered epitaxial vanadium layer.
4. Conclusions The room-temperature deposition of vanadium atoms on the Fe(100) surface induces the formation of an abrupt interface without any diffusion in the iron substrate. The structure of the grown vanadium films is b.c.c. like that of the substrate. However, in the monolayer range, vanadium is growing by a thicker layer than one simple atomic layer. The similarity between the diffraction patterns corresponding to 1 ML and more than 2 ML suggests the formation of clusters. Annealings of the interface have been shown to improve the LEED patterns, probably due to a reordering of the vanadium layer.
Acknowledgements Fig. 4. Azimuthal distribution of V L M M (473 eV ) nor23 23 4 malized intensity as a function of vanadium coverage and annealing temperatures at a polar angle of 41.5° (41.5° off normal ).
very similar pattern has been recorded for a V film of more than 2.6 ML as well as that corresponding to vanadium bulk (see Fig. 4). Annealing at 300°C has not induced any major changes in these angular distributions; annealing at 400°C produces a slight attenuation of the azimuthal oscillations. Since the diffusion of vanadium atoms into the iron substrate after annealing at 400°C should enhance the forward diffraction of vanadium Auger electrons, it is mandatory, therefore, to conclude that the annealing has not activated the diffusion of V atoms into the iron bulk. However, the attenuation of the azimuthal oscillations and the reduced LEED background are consistent with a flattening or reordering of the surface. Hence, it appears that
This work was financed by DGICYT (Spain) under grant PB-94-0022-C02-01. Part of the access to LURE, Centre Universitaire Paris-Sud, was supported through the Large Scale Facilities program of the European Union.
References [1] T.M. Hattox, J.B. Conklin, Jr., J.C. Slater, S.B. Trickey, J. Chem. Phys. Sol. 34 (1973) 1627. [2] G. Allan, Phys. Rev. B 19 (1979) 4774. [3] A. Vega, L.C. Balba´s, H. Nait-Laziz, C. Demangeat, H. Dreysse´, Phys. Rev. B 48 (1993) 985. [4] T.G. Walker, H. Hopster, Phys. Rev. B 49 (1994) 7687. [5] M. Finazzi, E. Kolb, J. Prieur, Ch. Boeglin, K. Hricovini, G. Krill, C. Chappert, J.-P. Renard, J. Magn. Magn. Mater. 165 (1997) 373. [6 ] P. Fuchs, K. Totland, M. Landolt, Phys. Rev. B 53 (1996) 9123. [7] G.R. Harp, S.S. Parkin, W.L. O’Brien, B.P. Tonner, Phys. Rev. B 51 (1995) 3293.
Y. Huttel et al. / Surface Science 402–404 (1998) 609–613 [8] M.A. Tomaz, W.J. Antel, W.L. O’Brien, G.R. Harp, J. Phys. Cond. Matt. 9 (1997) L179. [9] R. Coehorn, J. Magn. Magn. Mater. 151 (1995) 341. [10] J.-P. Renard, P. Bruno, R. Me´gy, B. Bartelain, P.
613
Beauvillain, C. Chappert, C. Dupas, E. Kolb, M. Mulloy, P. Veillet, E. Ve´lu, Phys. Rev. B 51 (1995) 821. [11] P. Bencok, S. Andrieu, P. Arcade, C. Richter, V. Ilakovac, O. Heckmann, K. Hricovini, Surf. Sci., this issue.
Auger electron diffraction study of V/Fe(100) interface formation Y. Huttel a,*, J. Avila a,b, M.C. Asensio a,b, P. Bencok a,c, C. Richter a,c, V. Ilakovac a,c, O. Heckmann a,c, K. Hricovini a,c a LURE, Baˆtiment 209D, Centre Universitaire Paris Sud, 91405 Orsay, France b Instituto de Ciencia de Materiales, CSIC, 28049 Madrid, Spain c LPMS, Universite´ de Cergy-Pontoise, Neuville/Oise, 95031 Cergy-Pontoise, France Received 6 August 1997; accepted for publication 5 October 1997
Abstract Vanadium atoms present a magnetic moment different to zero when they are part of a thin film deposited on Fe or as a bimetallic Fe–V alloy. The understanding of this phenomenon can only be achieved with a correct structural description of these types of systems. We report an Auger electron diffraction investigation of V films grown on body cubic centred (b.c.c.) Fe(100) substrates. Angular-scanned Auger electron diffraction (AED) patterns of V L M M (473 eV ) and Fe L VV (703 eV ) show the formation 23 23 4 3 of a well-ordered V/Fe interface even at room temperature. The AED patterns of V films in the range of vanadium submonolayer provide evidence of an isotropic Auger emission, indicating the absence of interdiffusion of V atoms into the Fe substrate and absence of cluster growth of the V film. The annealing of these films up to 400°C does not activate the substitution of the topmost Fe surface layers by V atoms. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Auger electron diffraction; Interface formation; V/Fe(100)
1. Introduction In the last few years, magnetism at surfaces and interfaces of transition metals has attracted considerable experimental and theoretical attention. The reduced symmetry, the lower co-ordination number and highly localised surface and interface states offer the possibility of introducing new and exotic phenomena. The understanding of magnetic properties can be achieved only with a correct structural description of these systems. Metallic vanadium has a b.c.c. crystal structure and is known to be non-magnetic. However, the * Corresponding author. Fax: (+33) 1 64 46 41 48; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 00 1 8 -1
isolated vanadium atom in its ground state has a permanent magnetic moment of 3 mB. Calculations indicate that an expansion of the lattice constant [1] changes in the co-ordination number at the surface [2] or the hybridisation between V and Fe orbitals [3] may induce a magnetic ordering. Appearance of the magnetic moment on vanadium atoms in Fe–V alloys, superlattices and V/Fe(100) interfaces has been further demonstrated in several experiments [4–8]. The average V moment is antiparallel to the Fe ones, and its value is determined to vary between 1.5 and 0.3 mB [7,8]. The Fe/V system is interesting because calculations indicate a different magnetic behaviour for V films on Fe compared with V in Fe/V superlattices. Moreover, both Fe and V magnetic moments may depend on epitaxial orientation [9]. From the application
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Y. Huttel et al. / Surface Science 402–404 (1998) 609–613
point of view, it is worth noting that the inverse magnetoresistance has been measured recently in the simple spin-valve system Fe V /Au/Co [10]. 1−x x Both Fe and V possess b.c.c. crystal structures ( lattice mismatch of 6%), and previous studies using RHEED [11] have reported a good epitaxial growth up to six or seven V layers. In this paper, we present an Auger electron diffraction study of the adsorption of vanadium on Fe(100) including vanadium coverage dependence and annealing temperature effects. The room temperature deposition is found to be epitaxial with the absence of any diffusion of V atoms. Also, annealing at 400°C does not activate the diffusion of vanadium atoms as evidenced by angular scanned Auger Electron Diffraction data.
2. Experimental The experiments were performed in the high vacuum French–Spanish (PES 2) experimental station at LURE (Orsay). We used a VSW hemispherical analyzer [acceptance angle ±1°, resolution power 5×10−4 (dE/E)] mounted on a goniometer inside the ultra-high vacuum chamber. The sample was held by a vertical manipulator, providing polar and azimuthal motion. The Fe b.c.c. single crystal samples were pre˚ of pared ex-situ by deposition of about 1000 A pure Fe on MgO substrates. They were cleaned by cycles of Ar+ bombardment at 1 keV and 500 V, followed by annealings at 400°C for a few minutes. Cleanness and structural qualities were checked by Auger electron spectroscopy and Low Energy Electron Diffraction. Vanadium was evaporated by electron bombardment of a pure vanadium wire in a base pressure of 7×10−10 mbar. The V evaporation rate was calibrated by using a thickness monitor, and V coverage was also calculated a posteriori using the Auger spectra. The consistency between the two methods for measuring the vanadium coverages has been checked, and the variation determined by both techniques was found to be less than 10%. The monolayer (ML) coverage is defined as being the amount of V atoms needed to form one atomic plane on the b.c.c. structure, which corresponds to a surface density equal to
1.08×1015 atoms cm−2. The base pressure during data acquisition was in the range of 5×10−10 mbar.
3. Results and discussion We will first focus on the structure of bulk vanadium deposited on the clean Fe(100) surface. For reference, Fig. 1 gives the stereographic projection of the Fe L VV (703 eV ) normalized inten3 sity [(I −background)/background ] as a function 0 of polar (0–60°) and azimuthal (0–360°) angles of the clean Fe(100) surface. The intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity. The short range crystalline order is obvious here by the anisotropic distribution of intensity of forward scattered Auger electrons [medium-range order was checked by LEED, which exhibited a sharp (1×1) pattern]. The b.c.c. structure can be identified by the diffraction spots along the high symmetry [100] and [110] directions, which are partially illustrated by Fig. 2 if substituting the vanadium atoms by the iron ones.
Fig. 1. Stereographic projection of the Fe L VV (703 eV ) nor3 malized intensity as a function of polar (0–60 °) and azimuthal (0–360°) angles of the clean Fe(100) surface. The intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity. The [100] direction corresponds to an azimuthal angle of 0°.
Y. Huttel et al. / Surface Science 402–404 (1998) 609–613
Fig. 2. Auger electron diffraction model for a b.c.c. structure. Side views along [100] and [110] directions.
We now turn to Fig. 3, which displays the corresponding stereographic projections of the V L M M normalized intensity for bulk vanadium 23 23 4 grown on the clean Fe(100) surface. The vanadium bulk was prepared by depositing a thick layer of vanadium until the Fe L VV could not be seen 3 any further and followed by annealing for 5 min at 500°C in order to improve the LEED pattern.
Fig. 3. Stereographic projection of the V L M M (473 eV ) 23 23 4 normalized intensity as a function of polar (0–60 °) and azimuthal (0–360°) angles of the vanadium bulk grown on clean Fe(100) surface. The intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity.
611
As can be observed by a simple comparison between Figs. 1 and 3, vanadium is growing, following the b.c.c. structure like the Fe(100) substrate one. It is interesting at this stage of the discussion to note that for the room temperature (RT ) growth, we have observed a slight, but significant and progressive, enhancement of the LEED background after each evaporation of vanadium. Such an increase, which is the straight signature of the diminishing of the medium range order, indicates that vanadium is probably clustering since local order is observed using Auger Electron Diffraction. However, we could improve the LEED patterns by short annealings (5 min) at temperatures between 300°C and 500°C. In order to gain more insight on the structure of V/Fe(100) interface upon annealings, we show in Fig. 4 the azimuthal distribution of V L M M normalized intensity as a function of 23 23 4 vanadium coverage and annealing temperatures [only for 1 ML V/Fe(100)] at a polar angle of 41.5° (41.5° off normal ). Due to the fact that the angular anisotropy in the forward scattering regime presents features that are spatially large, this pattern is essentially the same as that recorded for 45° off normal (not shown here). For the lowest coverage, i.e. 0.3 ML, we observe an isotropic intensity distribution of the Auger forward scattered electrons. Such an isotropic diffraction for a vanadium coverage below the monolayer is clear evidence that the adsorbed vanadium atoms do not diffuse into the Fe crystal bulk. For a vanadium coverage of 1 ML, the angular distribution of forward scattered Auger electrons shows a strong anisotropy indicating either the deposition of more than 1 ML or the formation of V clusters (or islands) that are expected from a thermodynamic point of view. Since the evaporation rate was measured by two different techniques, which give the same results within an error of 10%, we assume that the growth of vanadium films on Fe(100) proceeds through the formation of clusters. As can be seen in Fig. 4, the azimuthal scan for 1 ML (RT ) clearly shows four welldeveloped peaks at 45° (and 135°) and 90° (and 180°) azimuthal angles, which correspond to the diffraction between the first and second layer and between the first and third layer, respectively. A
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Y. Huttel et al. / Surface Science 402–404 (1998) 609–613
in the monolayer range of V/Fe(100) interface formation, vanadium is not growing on the Fe(100) as a perfect atomic layer, but is more likely forming clusters whose height can be slightly reduced by annealing at 400°C, resulting in the formation of a medium-range ordered epitaxial vanadium layer.
4. Conclusions The room-temperature deposition of vanadium atoms on the Fe(100) surface induces the formation of an abrupt interface without any diffusion in the iron substrate. The structure of the grown vanadium films is b.c.c. like that of the substrate. However, in the monolayer range, vanadium is growing by a thicker layer than one simple atomic layer. The similarity between the diffraction patterns corresponding to 1 ML and more than 2 ML suggests the formation of clusters. Annealings of the interface have been shown to improve the LEED patterns, probably due to a reordering of the vanadium layer.
Acknowledgements Fig. 4. Azimuthal distribution of V L M M (473 eV ) nor23 23 4 malized intensity as a function of vanadium coverage and annealing temperatures at a polar angle of 41.5° (41.5° off normal ).
very similar pattern has been recorded for a V film of more than 2.6 ML as well as that corresponding to vanadium bulk (see Fig. 4). Annealing at 300°C has not induced any major changes in these angular distributions; annealing at 400°C produces a slight attenuation of the azimuthal oscillations. Since the diffusion of vanadium atoms into the iron substrate after annealing at 400°C should enhance the forward diffraction of vanadium Auger electrons, it is mandatory, therefore, to conclude that the annealing has not activated the diffusion of V atoms into the iron bulk. However, the attenuation of the azimuthal oscillations and the reduced LEED background are consistent with a flattening or reordering of the surface. Hence, it appears that
This work was financed by DGICYT (Spain) under grant PB-94-0022-C02-01. Part of the access to LURE, Centre Universitaire Paris-Sud, was supported through the Large Scale Facilities program of the European Union.
References [1] T.M. Hattox, J.B. Conklin, Jr., J.C. Slater, S.B. Trickey, J. Chem. Phys. Sol. 34 (1973) 1627. [2] G. Allan, Phys. Rev. B 19 (1979) 4774. [3] A. Vega, L.C. Balba´s, H. Nait-Laziz, C. Demangeat, H. Dreysse´, Phys. Rev. B 48 (1993) 985. [4] T.G. Walker, H. Hopster, Phys. Rev. B 49 (1994) 7687. [5] M. Finazzi, E. Kolb, J. Prieur, Ch. Boeglin, K. Hricovini, G. Krill, C. Chappert, J.-P. Renard, J. Magn. Magn. Mater. 165 (1997) 373. [6 ] P. Fuchs, K. Totland, M. Landolt, Phys. Rev. B 53 (1996) 9123. [7] G.R. Harp, S.S. Parkin, W.L. O’Brien, B.P. Tonner, Phys. Rev. B 51 (1995) 3293.
Y. Huttel et al. / Surface Science 402–404 (1998) 609–613 [8] M.A. Tomaz, W.J. Antel, W.L. O’Brien, G.R. Harp, J. Phys. Cond. Matt. 9 (1997) L179. [9] R. Coehorn, J. Magn. Magn. Mater. 151 (1995) 341. [10] J.-P. Renard, P. Bruno, R. Me´gy, B. Bartelain, P.
613
Beauvillain, C. Chappert, C. Dupas, E. Kolb, M. Mulloy, P. Veillet, E. Ve´lu, Phys. Rev. B 51 (1995) 821. [11] P. Bencok, S. Andrieu, P. Arcade, C. Richter, V. Ilakovac, O. Heckmann, K. Hricovini, Surf. Sci., this issue.