Origin of the polar Kerr rotation in ordered and disordered FePt multilayers

The electronic structure and the magneto-optical properties of ordered and disordered FePt multilayers have been calculated by means of the spin-polarized relativistic linear muffin-tin orbital (SPR-LMTO) method within both the local spin-density approximation (LSDA) and generalized gradient approximation (GGA). Both approximations lead to the same magneto-optical results. The ordered FePt magneto-optical properties have also been calculated within the linear augmented plane wave method and the results are in good agreement with the SPR-LMTO calculations. The complex Kerr angle for ordered and disordered FePt has been calculated for photon energies of up to 6 eV and is found to be in a good agreement with experiment. Different structures in the optical conductivity and Kerr rotation as a function of the photon energy are analyzed and discussed. To show the microscopic origin of the strong Kerr rotation at some particular photon energies the symmetry character of the bands contributing to interband transitions together with with the interband electric dipole momentum matrix elements are analyzed in the whole Brillouin zone (BZ). This analysis showed that the assignment of the peaks is complex and cannot only be attributed to interband transitions along high symmetry BZ directions.