Circular Magnetic X-Ray Dichroism at Fe K-Edges in Rare-Earth Iron Garnets

We propose a mechanism for circular magnetic X-ray dichroism (CMXD) in a pre-edge region of the Fe K-edge in rare-earth iron garnets, within a cluster approximation. The pre-edge absorption is assumed to be dominated by the electric-dipole transition from 1s states to 4p states mixed with 3d states of Fe ions through the hybridization between these orbitals and 2p orbitals of nearest-neighbor O ions. Considering the different symmetries and the different spin orientations for Fe ions at the tetrahedral (Td) site and at the octahedral (Oh) site, we calculate CMXD for both cases based on Td and Oh iron-oxygen clusters. It is found that CMXD is a manifestation of the combined effect of the spin-orbit coupling and the spin polarization in the 3d orbitals. The result is discussed in connection with experimental observations.