LONG RANGE ORDER IN ULTRATHIN Pt–Co(111) ALLOYS

Long range order (LRO) in ultrathin Pt-Co films was studied by surface X-ray diffraction (XRD). Several fee alloys of nanometric thickness were grown in. situ by annealing at 460 degrees C Co layers deposited onto a Pt(lll) single crystal. Superstructure reflections were observed, which agreed with the extinction rules of either the L1(2) or the L1(0) chemically ordered bulk phases. The relationship between their structure factor and the atomic scattering factors was found with anomalous scattering performed near both Pt L(III) and Co K edges. This method is very promising for studies of surface alloying. The films were also studied in real time during annealing. They evolved in a quite different way, depending on the initial Co thickness, but LRO always occurred by heating above 400 degrees C. At this temperature the films became Pt-rich, with a stoichiometry close to Pt(60)Co(40) We did not succeed in obtaining long range chemical order in a Co-rich alloy by annealing a Co/Pt(111) deposit, contrary to what happens in films grown by codeposition.