Chemically ordered fcc (111) Mn–(FexNi1−x) alloy ultrathin films

Ultrathin Mn deposits (0.25–2.0ML equivalent) on an fcc (111) Fe–Ni substrate layer grown on W(110) have been annealed to form Mn alloys. Low energy electron diffraction (LEED) and directional Auger electron spectroscopy (DAES) were used to assess the long-range and short-range order of the alloy films, respectively. Neither structural nor chemical order was detected before annealing. However, after annealing to 600K for 2min, DAES showed that the Mn, Ni and Fe atoms all occupied fcc (111) structural positions, and a p(2×2) LEED pattern indicated a doubling of the surface unit cell through chemical ordering of the Mn atoms. The dependence of the half-integral LEED spot intensity on the amount of Mn deposited, and the annealing sequences required to observe the p(2×2) pattern are inconsistent with the formation of a surface alloy. Rather, the ordered alloy is formed after diffusion of Mn into the film, and extends to the surface. Since no ordered bulk alloy exists for the Mn–Fe–Ni system, it is argued that the close-packed fcc (111) surface stabilizes its formation in these ultrathin films.