First principles investigation of the validity and range of applicability of the x-ray magnetic circular dichroism sum rule

X-ray magnetic circular dichroism (MCD) spectra and orbital angular momentum, 〈${\mathit{L}}_{\mathit{z}}$〉, for transition metal bulk and surfaces were studied for both ground state and core hole excitations using a highly precise local density approach. For Fe(001), we predict a double peak structure in both the MCD and total absorption spectra and a strong enhancement of 〈${\mathit{I}}_{\mathit{z}}$〉. Surprisingly, the MCD orbital sum rule is found to be valid to within (5–10)%. Finally, the results suggest possible solutions to several problems faced in applying the MCD sum rule to measure 〈${\mathit{L}}_{\mathit{z}}$〉.