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Directional Dichroism of X-Ray Absorption in a Polar Ferrimagnet GaFeO_3

We study the directional dichroic absorption spectra in the x-ray region in a polar ferrimagnet GaFeO_3. The directional dichroism on the absorption spectra at the Fe pre-K-edge arises from the E1-E2 interference process through the hybridization between the 4p and 3d states in the noncentrosymmetric environment of Fe atoms. We perform a microscopic calculation of the spectra on a model of FeO_6 with reasonable parameter values for Coulomb interaction and hybridizations. We obtain the difference in the absorption coefficients when the magnetic field is applied parallel and antiparallel to the c axis. The spectra shus obtained have similar shapes to the experimental curves as a function of photon energy in the Fe pre-K-edge region, although they have opposite signs.