Strong magneto-optical effects in ACr2O4 A=Fe, Co spinel oxides generated by tetrahedrally coordinated transition metal ions

Magneto-optical effects have been investigated over the infrared{visible spectral range in ACr2O4 (A = Fe, Co) spinel oxides with non-collinear spin orders in their ground states. We found large magneto-optical Kerr rotation and ellipticity at the on-site d-d transitions of the A2+ ions located within the charge gap. The magneto-optical Kerr rotation of 12 deg observed in CoCr2O4 is unprecedentedly large among magnetic semiconductors and points towards the uniqueness of tetrahedrally coordinated Co2+ ions in generating a strong magneto-optical response. Criteria of strong magneto-optical effects emerging at on-site d-d transitions of transition metal ions are discussed.