Gigantic Kerr rotation induced by a d-d transition resonance in MCr₂S₄ (M=Mn,Fe)

The magneto-optical Kerr effect (MOKE) has been investigated for ferrimagnetic spinel compounds MCr₂S₄ (M=Mn and Fe). In FeCr₂S₄, the gigantic magneto-optical response, reaching up to 4.3° in Kerr rotation, is observed at the energy of the intra-atomic d-d transition of Fe²⁺, ⁵E→⁵T₂. By analyzing the resonance feature in the framework of the ligand field theory, we have clarified that the large oscillator strength enhanced by the strong covalency of the ligand sulfur as well as the local breakdown of inversion symmetry at the Fe²⁺ site is responsible for the gigantic signal. In MnCr₂S₄, the spin-forbidden d-d transition (⁶A₁→⁴T₁) of Mn²⁺ is revealed in the MOKE spectra, which is not discernible in the optical reflectivity spectra.