Microscopic origin of the magneto-optical properties of CoPt alloys

The microscopic origin of the magneto-optical (MO) properties of Co_xPt_1-x alloys is studied on the basis of Kerr effect measurements and first-principles band-structure calculations in a wide range of photon energy (ħω=0.75–5.8 eV) and Co content (x=0.03–0.5). Spin-polarized relativistic linear-muffin-tin-orbital (LMTO) calculations performed within the local density approximation and supercell approach reproduce well the spectral shape of the measured off-diagonal optical conductivity tensor components. Using the ab initio LMTO band-structure calculations, the band-by-band and k-space decompositions as well as the analysis of the magneto-optical transitions between electronic states localized in different energy regions are performed. It is found that for the alloys studied the uv part of the MO spectra with a pronounced peak at 4 eV photon energy comes from optical transitions between initial and final states located within well-defined energy intervals that are very close to those obtained for fcc Pt metal in external magnetic field. The correlation between the band structure of the alloys and the energy dependence of their MO spectra is investigated. The importance of the spin-orbit interaction for the transitions in different energy regions is demonstrated by calculating the state- and site-projected density of the expectation value of the orbital moment. The evolution of the electronic structure of Co-Pt alloys with the increase of Co content and the influence of the hybridization between the Co and Pt electronic states are analyzed and discussed.