  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.
