HCP-TO-FCC STACKING SWITCH IN THIN COBALT FILMS INDUCED BY CU CAPPING

We report on surface structure analyses by quantitative tow-energy ele ctron diffraction for ultrathin films of 1.5 and 5 ML Co on Cu(111) an d on the structural changes they undergo when additionally covered by 2-3 ML copper. The thin cobalt film is dominated by continuation of th e fee stacking dictated by the substrate whereby a large part of the d omains is capped by copper dissolved from the substrate and possibly s ubstituted by cobalt. Yet, some stacking faults near the interface app ear already at this low coverage in domains uncapped by copper. The 5 ML Co film, on the other hand, is almost fully hexagonally close packe d. 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 dom ains develop. At least one of the cobalt layers undergoes a full regis try shift upon the sandwiching process. This shows that copper deposit ed on top of cobalt not only stabilizes the initial fcc stacking of co balt but also can induce a switch from an existing hcp stacking of a t hicker cobalt film back to fcc.