Magneto-optical properties of Co-Pt alloy films were investigated at low and room temperatures. We also investigated atomic and electronic structures of these films. The saturated polar Kerr rotation angles of CoPt and CoPt_(3) alloy films show different temperature dependence. Synchrotron-radiation photoemission spectroscopy was employed to confirm the correlation between the magneto-optical properties and the electronic structures of these films. Changes of a peak at 4.3 eV relevant to the Pt 5d levels hybridized with Co 3d electrons of the CoPt film and of the valence-band shape upon cooling have the same trend as that of the saturated Kerr rotation angle. A structural phase transition of the CoPt film from the tetragonal L1_(0) to orthorhombic phase upon cooling is suggested by the theoretical calculations using linearized-muffin-tin-orbitals methods within the so-called "LDA+U" scheme as well as the experimental observations. X-ray diffraction patterns before and after cooling also support this structural phase transition.
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