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-layer 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 value. 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 the deposited film upon annealing. The role of the Co film structure (which depends on the film thickness) is predominant. For 'thick' films (greater than or equal to 6 ML) a sharp transition occurs around 670 K: below this temperature, 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.