Evidence for 3d n to 3d n-1 4s transitions in magnetite and in lithium and magnesium ferrites

Optical reflectivity and magneto-optical polar Kerr effect measurements in the photon energy ranges from 0.006 to 12 eV and 0.5 to 5 eV, respectively, are reported for magnetite and for Li and Mg ferrites. The diagonal and off-diagonal elements of the dielectric tensor are derived. From a comparison of the experimental results with the cation distribution formulae of the materials the excitations occurring below 2 eV can be assigned to transitions from Fe 2+ in octahedral sites and the excitation at about 3.9 eV to Fe 3+ in tetrahedral sites. Energy level schemes are deduced and are supported by a model describing the shape and the sign of the various magneto-optical transitions. In this model 3d n to 3d n-1 4s orbital promotion processes for the three kinds of iron ions are considered. Above 5 eV oxygen 2 p band to iron 4 s band charge-transfer transitions are shown to be responsible for the very strong optical absorption.