The accuracy of the spin sum rule in XMCD

The effective spin sum rule is reviewed with a detailed analysis of the various sources for errors and deviations of this widely used X-ray Magnetic Circular Dichroism (XMCD) tool. The simulations confirm that the final state effects of the core level spin-orbit coupling and the core-valence exchange interactions (multiplet effects) are linearly related with the effective spin sum rule error. Within the ligand field multiplet approach, we have analyzed these effects, in combination with the interactions affecting the magnetic ground state, including the crystal field strength and the 3d spin-orbit coupling. We find that for the late transition metal systems, the error in the effective spin moment is between 5 and 10%. Because of the potentially large value, the spin moment can not reliably be determined for all systems other than Ni. The error for 3d4 systems is very large, implying that, without further information, the derived effective spin sum rule values for 3d4 systems have no meaning.