Segregation and ordering at the (1×2) reconstructed Pt80Fe20(110) surface determined by low-energy electron diffraction

The surface of an ordered Pt80Fe20(110) crystal exhibits (1×2) and (1×3) reconstructions depending on the annealing treatment after ion bombardment. The (1×3) structure occurs after annealing in the range 750 to 900 K. Annealing above 1000 K leads to the (1×2) structure, which is, from the present result, unambiguously attributed to the same geometrical reconstruction as Pt(110) but with smaller relaxation amplitudes: a detailed low-energy electron-diffraction analysis concludes to a missing-row structure with row pairing in layers 2 and 4 accompanied by a buckling in layers 3 and 5. The top layer spacing is contracted by 13%, and further relaxations are detectable down to the fifth layer. The specific diffraction spots associated with the bulk chemical ordering along the dense [1¯10] rows are very weak: The I(V) analysis shows that this chemical ordering is absent in the outermost ‘‘visible’’ rows but gradually recovers over five to six layers deep. General Pt enrichment is found in the surface ‘‘visible’’ rows (in layers 1–3), but segregation and order yield a subtle redistribution of Pt and Fe atoms in deeper rows: For example, in layer 2, the visible row is Pt rich, whereas the other row (buried under layer 1) is enriched with Fe. Because of the many parameters considered, a fit procedure was applied to a large data basis to solve the structure; the results were confirmed and illustrated subsequently by a standard I(V) analysis for the most relevant parameters. The final r factors are RDE=0.36, RP=0.34, and RZJ=0.14 for two beam sets at normal and oblique incidence consisting of 26 and 21 beams, respectively

FAUT-IL UNE THÉORIE DYNAMIQUE DE LA D. E. L. POUR ACCÉDER AUX VIBRATIONS DES ATOMES DE SURFACE ?

Les premières expériences faites par Germer et Mac Rae sur le Nickel, montrent une anisotropie de la composante u// de l'amplitude de vibration atomique en surface, ainsi qu'une variation avec l'énergie des électrons incidents de la température de Debye effective E eff(V). Ces résultats ont été prévus qualitativement par des calculs théoriques ; la D. E. L. est-elle donc un outil idéal pour l'étude de la dynamique des surfaces des monocristaux ? Dans ce cadre, nous avons repris une étude faite par Jones, McKinney et Webb sur la face (111) de l'Argent, mais avec une technique plus précise. Nos résultats montrent qu'une théorie pseudo-cinématique comme celle qu'ont employée Jones et alii ne peut expliquer les courbes expérimentales Eeff(V), si l'on veut conserver des valeurs raisonnables pour le mouvement des atomes proches de la surface.First experiments, by Germer and Mac Rae on Nickel, show an anisotropic component u// of the root mean square displacements of the surface atoms, and a dependence of the effective Debye temperature Eeff(V) on the energy of the incident electrons. These results being qualitatively predicted by theoretical calculations, is L. E. E. D. an ideal tool for studying the dynamics of monocrystal surfaces ? On this basis, we have undertaken a similar study as that done by Jones, McKinney and Webb on the (111) face of Silver, but with a more precise technique. Our results show that such a pseudokinematical theory as that used by Jones and alii cannot explain the experimental Eeff(V) curves, if reasonable values of the atomic displacement near the surface are to be used

The (110) surface of the disordered Pt25Co75 alloy: a sandwich of almost pure monoatomic layers

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Surface core level spectroscopy of the Pt25Co75 (110) and (100) alloy surfaces

Core level photoemission spectroscopy of the Pt 4f7/2 level is used to investigate the surface and bulk Pt environments in the disordered alloy Pt25Co75. For the (110) face no Pt is observed at the surface, and only a single Pt bulk site is identified with a chemical shift of 250meV. For the denser (100) face the same bulk site is observed together with a surface peak, corresponding to a surface layer with Pt enrichment. The chemical shift at the surface is weaker than in the bulk