hcp-to-fcc stacking switch in thin cobalt films induced by Cu capping

We report on surface structure analyses by quantitative tow-energy electron diffraction for ultrathin films of 1.5 and 5 ML Co on Cu(111) and on the structural changes they undergo when additionally covered by 2-3 ML copper. The thin cobalt film is dominated by continuation of the fee stacking dictated by the substrate whereby a large part of the domains is capped by copper dissolved from the substrate and possibly substituted by cobalt. Yet, some stacking faults near the interface appear already at this low coverage in domains uncapped by copper. The 5 ML Co film, on the other hand, is almost fully hexagonally close packed. While the stacking of the thinnest film is practically stable upon further copper deposition, the sandwiching of the thicker film induces a structural switch from hcp to fcc stacking, whereby twinned fcc domains develop. At least one of the cobalt layers undergoes a full registry shift upon the sandwiching process. This shows that copper deposited on top of cobalt not only stabilizes the initial fcc stacking of cobalt but also can induce a switch from an existing hcp stacking of a thicker cobalt film back to fcc.