The in situ magneto-optic Kerr effect was used to study the magnetic properties of Co
ultrathin films grown on a flat and a sputtered Pt(111) surface. The perpendicular magnetic
anisotropy appears for Co on the flat Pt(111) surface with a thickness of less than 4.0 ML
and it changes to in-plane after the coverage of Co is more than 6.5 ML, whereas both the
out-of-plane and in-plane anisotropy occur on the sputtered Pt(111) surface. The coercive
force decreases as the Co coverage increases for the flat Pt(111) surface, but it is independent
of the thickness for the sputtered one. A surface in island growth and surface roughness seem
beneficial to the reversal of a magnetic domain wall. Thermal energy also helps the domain
wall motion. The perpendicular Kerr intensity increases in a linear relation with thickness of
Co before the magnetization turns to in-plane for the flat surface, and it is a smooth curve with
a maximum at 5.5 atomic monolayers of Co for the sputtered surface. The Curie temperature
of Co thin films on the flat Pt(111) was measured. The Curie temperature increases as
the thickness of Co increases. An enhancement of perpendicular magnetic anisotropy was
observed when a Co-Pt alloy is developed. The hybridization of electronic states in the
formation of a Co-Pt surface alloy and the large spin-orbit coupling in this system are possible
mechanisms for the enhancement.
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