Roles of 4f-5d Exchange Interactions in Magnetic Circular X-Ray Dichroism at the Rare-Earth L_2,3 Edges

The systematic trend in magnetic circular X-ray dichroism (MCXD) at the L_2,3 edges for various rare-earth elements is studied by taking into account the effects of the 4f-5d exchange interaction on the electric-dipole transition from the 2p core state to the outermost 5d state. The exchange interaction gives rise to, in addition to the usual polarization effect on the 5d ground state, a novel effect of contraction in the radial part of the 5d orbits concerned. It is pointed out that, especially the latter effect, which leads to the enhancement of the transition dipole matrix element depending on the spin and the orbital momenta of the 5d state, plays a crucial role in successfully interpreting the experimental results observed. Quadrupole contributions just below the edges are also discussed.