Strong magneto-optical effects in ACr2 O4 (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 $A{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ ($A=\text{Fe},\text{Co}$) spinel oxides with noncollinear spin orders in their ground states. We found large magneto-optical Kerr rotation and ellipticity at the on-site $d\ensuremath{-}d$ transitions of the ${A}^{2+}$ ions located within the charge gap. The magneto-optical Kerr rotation of ${\ensuremath{\vartheta}}_{\mathrm{Kerr}}\ensuremath{\approx}{12}^{\ensuremath{\circ}}$ observed in ${\mathrm{CoCr}}_{2}{\mathrm{O}}_{4}$ is unprecedentedly large among magnetic semiconductors and points toward the uniqueness of tetrahedrally coordinated ${\mathrm{Co}}^{2+}$ ions in generating strong magneto-optical response. Criteria of strong magneto-optical effects emerging at on-site $d\ensuremath{-}d$ transitions of transition metal ions are discussed.