First principles investigations of MCD spectra and sum rules for 3d transition metal surfaces

Results of precise calculations of the X-ray magnetic-circular-dichroism (MCD) spectra and orbital magnetic moment, 〈L_z〉, based on our thin film full potential linearized augmented plane wave energy band method and a tight binding analysis carried out for Fe(001), Co(0001) and Ni(001) surfaces are shown to provide clear physical insight for this new and exciting phenomenon. In addition to the spin magnetic moment enhancement, the orbital magnetic moment is also found to be enhanced at the surface due to band narrowing. A double peak structure of the MCD spectra for Fe(001) and Co(001) is predicted. For Ni(001), the absorption side peak 6 eV above the main peak is attributed to contributions from the d component in a high-lying state. The MCD orbital sum rule is found to be valid to within 5-10% even in the band approach. However, for the spin sum rule, large errors are found for Ni systems and, in addition, the magnetic dipole term is also found to be very important in this sum rule for atoms without cubic symmetry. This raises a question about the applicability of the spin sum rule to obtain the spin magnetic moment from MCD spectra. Finally, the effects of a photo-induced core hole are found to be strongly screened, and to induce an enhancement of 〈L_z〉.