Photoinduced magnetization in dilute magnetic (semimagnetic) semiconductors

Optically induced magnetization was studied in narrow-gap ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te in zero external magnetic field as a function of composition, excitation energy, and temperature. These experimental results, together with previously published data for ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te as well as for ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te [Awschalom et al., Phys. Rev. Lett. 58, 812 (1987)] are analyzed. Two mechanisms of Mn-spin orientation by photoelectrons brought about by the s-d coupling are considered: (i) the static polarization induced by the mean field of spin-polarized electrons, and (ii) the dynamic polarization caused by the s-d spin-flip scattering. The analysis implies the dynamic polarization to be strongly suppressed by interaction among the Mn spins. In the case of ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te the static polarization is calculated to be in quantitative agreement with the measured photomagnetization. In the wide-gap ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te a comparatively small value of the observed photomagnetization implies the presence of an efficient spin relaxation of photoelectrons, presumably related to trapping by bound states. In order to describe the dependence of photomagnetization on excitation energy, the energy loss of photoinjected carriers by LO-phonon emission was considered. In addition to spin relaxation of photoelectrons by magnetic ions, the decay of spin orientation caused by spin-orbit coupling was taken into account. The latter effect is particularly strong in the narrow-gap case. The model correctly reproduces a strong dependence of photomagnetization on excitation energy in narrow-gap ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te and its absence in ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te.