Epitaxial alloyed films out of the bulk stability domain: surface structure and composition of Ni3Al and NiAl films on a stepped Ni(111) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPALEED) and Auger Electron Spectroscopy (AES) Ni-Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(111) surface. The surface structure and composition of the thin epitaxial Ni3Al and NiAl films, obtained respectively below and above a critical Al initial coverage c θ , differ markedly from those of corresponding bulk alloys. The Ni3Al ordered films form in a concentration range larger than the stability domain of the L12 Ni3Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases. It also appears that the value of c θ depends on the morphology of the Ni(111) substrate, increasing from c θ =4.5 ML for a flat surface to c θ =10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni-Al interdiffusion. * Present address : Centre Interdisciplinaire de Nanoscience de Marseille, CNRS UPR 3118, Aix-Marseille Université Campus de Luminy, Case 913, 13288 Marseille Cedex 09, France