Structure of Ni overlayers on bcc Fe(100)

The structure of Ni layers epitaxially grown on bcc Fe{100} was studied by various techniques. After the growth of up to 3 monolayers (ML), Ni was found to be in a bcc phase with the bccNi100∥bccFe100 epitaxial relationship and with the in-plane lattice parameter close to that of the underlying Fe. When the Ni layer thickness exceeded 3 ML, a structural transition was observed. In the literature it has been reported that the intermediate Ni phase formed upon the transition has an unknown structure which gradually changes to a final fcc Ni{110} structure after the growth of about 200 ML and subsequent annealing. Here we find that this intermediate phase is in fact also fcc Ni{110} with a structure consisting of small domains. It is concluded that the small domain size gives rise to reflection high-energy electron diffraction patterns which can be misinterpreted as being due to a different structure. The intermediate as well as the final Ni structure contains four orientations of fcc Ni domains with the Ni{110} planes parallel to the bcc Fe{100} surface and the Ni〈211〉 crystallographic directions along Fe〈110〉 axes. The origin and consequences of this epitaxial relationship are discussed.