Annealing effects on surface structure and magnetic properties of ultrathin films Co on Pt(111)

Annealing effects on magnetic properties, surface structure, and alloy formation for 1 and 2 ML of CoPt (111) were studied. The tools used in the study were in situ magneto-optical Kerr effect, low energy electron diffraction, and Auger electron spectroscopy. The Curie temperatures of both samples decrease with increase of the annealing temperature, which is associated with concentration change of Co-Pt surface alloy. A structural phase transition from CoPt to Co Pt3 of an ordered surface alloy was observed when the annealing temperature was between 780 and 810 K. The value of critical exponent Β near the Curie point turns from a two-dimensional-like to a three-dimensional- like magnetic phase. The curves of coercivity versus sample temperature reveal thermal hysteresis with two plateaus during the cooling process. The mechanism of the thermal hysteresis was discussed. The Curie temperature and the coercivity of the CoPt (111) system can be adjusted by controlling the annealing temperature, which alters the concentrations of the alloy formation.