Fe-Ni surface alloy formation on Ni(111) investigated by scanning tunneling microscopy

The structure of Fe films grown on Ni(111) with an Fe coverage of 3.2 monolayers and its conversions during heating in ultrahigh vacuum have been investigated by scanning tunneling microscopy (STM). The Fe films consist of elongated domains stacked with four to six monatomic layers of Fe, forming ridgelike structures. A phase transition from fcc(111) to bcc(110) with the Nishiyama-Wassermann orientation relationship occurs from at least the third monatomic layer. A triangular network structure and large accumulated bcc-Fe islands are formed on the surface after heating the Fe films at 550 K. The network is considered to be misfit dislocation loops caused by the lattice misfit between the fcc-Fe films and the Ni substrate. A Surface structure consisting of two distinct randomly distributed spots and a well-ordered (5 x 5) superstructure are observed after further heating at 620 and 700 K, respectively, and they are Caused by the formation of the Fe-Ni surface alloy on the Ni(111) surface.