Effect Of Site Disorder On The Electronic, Magnetic, And Ferroelectric Properties Of Gallium Ferrite

The electronic and magnetic properties of the magnetoelectric solid GaFeO3 are investigated within the generalized gradient approximation including the Hubbard interaction (U) on the localized d orbitals of iron. It was found that using an onsite U = 8 eV describes consistently the experimental results. The origin of ferrimagnetism was attributed to the cationic site disorders. The density of states at the iron sites, in a octahedral geometry, show that the occupied e(g) states are below the t(2g) states in contradiction with the crystal-field splitting obtained by a point-charge model. However, for the unoccupied states the ab initio data agree qualitatively with the model, showing the complexity of the electron-electron interaction in GaFeO3. The computed electric polarization of the system as a function of the temperature in the linear regime shows a monotonic decreasing trend. To determine the nature of the magnetoelectric coupling, the computed electric polarization as a function of the rotation of the magnetization axis indicates that the magnetoelectric effect observed experimentally could not have been due to a direct coupling between the electric and magnetic order parameters. Finally, the calculated x-ray absorption and x-ray magnetic circular dichroism spectra for the disordered system are shown to be in good agreement with experiment.