Chemically ordered fcc (1 1 1) Mn-(FexNi1−x) alloy ultrathin films

Ultrathin Mn deposits (0.25-2.0 ML equivalent) on an fcc (1 1 1) Fe-Ni substrate layer grown on W(1 1 0) 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 600 K for 2 min, DAES showed that the Mn, Ni and Fe atoms all occupied fcc (1 1 1) 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 (1 1 1) surface stabilizes its formation in these ultrathin films.