Direct experimental identification of the structure of ultrathin films of bcc iron and metastable bcc and fcc cobalt

Abstract

X-ray-excited photoemission and Auger-electron angular distributions from epitaxial transition-metal films are shown to be characteristic of crystal structure but insensitive to elemental species. This feature is used to identify the structures of ultrathin films of iron and cobalt, grown at room temperature on single-crystal metal substrates. It is found that cobalt grows in the metastable bcc Co(001) structure on bcc Fe(001) thin films, and fcc Co(001) grows on fcc Cu(001). While bcc Fe(001) can be stabilized on fcc Ag(001) substrates, this is not the case for cobalt films. Epitaxy of cobalt on Ag(001) results in a complex structure that is neither fcc nor bcc but is consistent with a body-centered tetragonal structure. These results are used to determine an upper bound on the strain that bcc cobalt films can sustain in room-temperature growth.