Co ultra-thin films on Pt(111) and Co-Pt alloying: a LEED, Auger and synchrotron x-ray diffraction study

Co ultra-thin Alms deposited on Pt(111) are analysed in detail, using LEED, Auger spectroscopy and x-ray diffraction. The growth is quasi layer-by-lay er up to about 3 ML. Co grows as islands, in incoherent epitaxy with the Pt substrate; the corresponding satellites are observed even for coverages as low as 0.12 ML. The Co in-plane parameter is very close to its own bulk va lue. For thicker films, deposited at room temperature, face centerd cubic ( fcc), twin fee and hexagonal close packed (hcp) fractions are present. Using x-ray diffraction, we followed in real-time the transformations of th e deposited film upon annealing. The role of the Co film structure (which d epends on the film thickness) is predominant. For 'thick' films (greater th an or equal to 6 ML) a sharp transition occurs around 670 K: below this tem perature, the film is mainly hcp so that it allows little Pt incorporation, while above it turns to a very homogeneous fee alloy of composition close to Pt60Co40. Segregation phenomena were analysed and found similar to those occurring at the surface of the corresponding bulk alloys.