The importance of the magnetic dipole term in magneto-circular x-ray absorption dichroism for 3d transition metal compounds

The application of the magneto-circular x-ray absorption dichroism (MCXD) spin sum rule for 3d transition metal compounds faces two problems: the unknown value of the magnetic dipole operator T-z and the division between the L(2) and L(3) edges. A systematic study of the order of magnitude of the T-z-operator for 3d(n) ions is presented. The variation of the T-z-values with temperature is described and analysed, for all cases from d(1) to d(9) cations in two different situations. Firstly the perfect octahedral case is considered. It is shown that T-z is non-zero for low temperature; but, as it originates only from d-electron spin-orbit splitting, it is washed out at room temperature. Secondly, a model of the surface situation is considered. In this case T-z originates mainly from the crystal-field splitting. It then exhibits quite large values at any temperature and can by no means be neglected when applying the sum rule. The error introduced in the sum rule due to the mixing of L(2) and L(3) edges has been estimated.