Relativistic tight-binding calculations of x-ray absorption and magnetic circular dichroism at theL2andL3edges of nickel and iron

The soft-x-ray absorption and magnetic circular dichroism (MCD) cross sections at the ${\mathit{L}}_2$ and ${\mathit{L}}_3$ core-level edges of Ni and Fe have been calculated using a one-electron tight-binding band-structure approach. The tight-binding scheme is taken from the work of Papaconstantopoulos, supplemented with the inclusion of spin-orbit coupling in the d bands and fully relativistic dipole selection rules. In Ni, a fit to the various ${\mathit{L}}_2$ and ${\mathit{L}}_3$ intensity ratios can be achieved, but only with values of the d-band spin-orbit parameter ξ and exchange parameter ${\mathrm{\Delta }}_{\mathrm{ex}}$ at variance with the ground-state band-structure values. For Fe, there is no plausible value of ξ capable of explaining the intensity ratios; also, the predicted substructure within the ${\mathit{L}}_2$ and ${\mathit{L}}_3$ white lines is not seen in experiment. These failures of the one-electron approach are qualitatively consistent with expected many-body electronic rearrangements associated with core-hole creation. Some discussion is offered on sum rules and on orbital versus spin magnetic moments.