Exchange-induced optical spin transitions in semimagnetic semiconductors

Electron-spin flip by electric dipole light absorption in semimagnetic semiconductors (SMSC) is allowed not only by the generally invoked spin-orbit coupling but also due to a possibility of exchange scattering of an electron by a magnetic impurity. The theory of such exchange-induced electric dipole spin resonance (EDSR) in Mn- and Fe-based SMSC is presented. Since exchange scattering changes the electron spin, as well as the impurity state, the bound-electron absorption occurs at the photon energy ħω=ħω_s+ɛ_M, where ħω_s is the electron giant spin splitting and ɛ_M is an excitation energy of the magnetic impurity. In the case of Mn-based SMSC, ɛ_M is simply the Zeeman energy of a Mn ion, while, since an Fe ion may be excited to a set of states, several absorption lines exist in Fe-based SMSC. Exchange-induced EDSR differs from the conventional spin-orbit EDSR in the polarization and magnetic-field dependences of absorption. For free carriers exchange-induced spin-flip absorption has nonresonant character and leads to a rather broad absorption band.