Wannier excitons in the ferromagnetic semiconductors EuO and EuS

Fine structures in the thermoreflectance (TR) spectra of EuO and EuS in the ultraviolet region are ascribed for the first time to Wannier excitons in magnetic semiconductors. The characteristic temperature-induced splittings of the spectra of EuO and EuS at the ferromagnetic spin-ordering temperatures are successfully analyzed in terms of the exciton and the spin-polarization effect of the relevant one-electron bands, an $s$-like conduction and $p$-like valence bands, at the $\Gamma$ point. The magnetic-spin-exchange energies, $J_c$ and $J_v$, between the localized $\mathrm{Eu}-4f$ electrons and the band electrons, the spin-orbit interaction energy $\lambda$ for the valence band, and the electron-hole spin-exchange energy $\Delta$ for the exciton are estimated. It is demonstrated that the TR measurements open a new aspect of the optical study of magnetic-semiconductor excitons which become an ideal probe for directly detecting the interaction between the localized-magnetic-spin system and the nonlocalized-Bloch-electron system in the magnetic semiconductors.