Long range order in ultrathin Pt-Co(111) alloys

Long range order (LRO) in ultrathin Pt-Co films was studied by surface X-ra y 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) s ingle crystal. Superstructure reflections were observed, which agreed with the extinction rules of either the L1(2) or the L1(0) chemically ordered bu lk phases. The relationship between their structure factor and the atomic s cattering factors was found with anomalous scattering performed near both P t L-III and Co K edges. This method is very promising for studies of surfac e alloying. The films were also studied in real time during annealing. They evolved in a quite different way, depending on the initial Co thickness, b ut LRO always occurred by heating above 400 degrees C. At this temperature the films became Pt-rich, with a stoichiometry close to Pt60Co40 We did not succeed in obtaining long range chemical order in a Co-rich alloy by annea ling a Co/Pt(111) deposit, contrary to what happens in films grown by codep osition.