Magneto-optical Kerr effects in perovskite-type transition-metal oxides: ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$

Magneto-optical Kerr effects have been investigated for perovskite-type hole-doped oxides ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3\hspace{0.5em}(0\mathrm{}<~x<~0.3)$ and ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3\hspace{0.5em}(0\mathrm{}<~x<~0.3)$ in the photon energy range between 0.9 and 5.3 eV at room temperature. In ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3,$ charge-transfer-type Kerr spectra are observed for the lowest $\left(\sim 1.2\hspace{0.5em}\mathrm{eV}\right)$ and the second-lowest $\left(\sim 3.5\hspace{0.5em}\mathrm{eV}\right)$ optical transitions that are likely to arise from electron excitations from the $\mathrm{O}\hspace{0.5em}2p$ to the majority-spin $e_g$ band and to the minority-spin $t_{2g}$ band, respectively. In ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3,$ a crystal-field-transition-type Kerr spectrum is observed that is assigned to the electron excitation from the bonding $t_{2g}$ state to the antibonding $e_g$ state allowed by the strong hybridization effect between $\mathrm{Co}\mathrm{}\hspace{0.5em}3d$ and $\mathrm{O}\hspace{0.5em}2p$ states. Different characters in the Hund’s-rule coupling and covalency seem to cause such a difference in magneto-optical properties between ferromagnetic Mn and Co oxides.