Seminars Archive

Chemical order in ultra-thin Pt_xCo_1-x/Pt(111) films: an in situ study by surface X-ray diffraction

Abstract
Recently hcp Co-rich films, with an easy axis of magnetization perpendicular to the surface, were obtained by MBE codeposition on a heated substrate1. These films show long range chemical order along the (0001) growing direction. Pt rich alloys elaborated in a similar manner exhibit also peculiar magnetic properties correlated to Co clustering2, with preference for homoatomic pairs in planes parallel to the surface. By mean of surface X-ray diffraction, we compared the structure of ultra-thin PtxCo1-x/Pt(111) films obtained either by annealing Co deposits 3, 6 and 10 ML thick, or by codeposition. We found that the first method gives always a bulk type phase. In codeposited films instead Pt segregate at the surface in the first stages of the growth, and this seems to be the driving force for chemical ordering. The new phases are obtained therefore by trapping surface equilibrium states during evaporation, and there is not a magnetically driven miscibility gap in bulk phase diagram at temperatures lower than 400C. Structure factors of the bulk type oredered phase were found thanks to anomalous diffraction. The experiments were carried out on the beam line installed by the French "Collaborating Research Group on Interfaces" at ESRF. The UHV chamber is mounted on a Z-axis diffractometer and allows for in-situ sample preparation including metal deposition or codeposition and annealing during X-ray measurement. 1) G. R. Harp, D. Weller, T, A. Rabedeau, R. F. Farrow and M. F. Toney, Phys. Rev. Lett. 71, 2493 (1993). 2) P. W. Rooney, A. L. Shapiro, M. Q. Tran and F. Hellerman, Phys. Rev. Lett. 75, 184 (1995).