  We report on both experimental and theoretical study of conduction-electron
spin polarization dynamics achieved by pulsed optical pumping at room
temperature in GaAs(1-x)N(x) alloys with a small nitrogen content (x = 2.1,
2.7, 3.4%). It is found that the photoluminescence circular polarization
determined by the mean spin of free electrons reaches 40-45% and this giant
value persists within 2 ns. Simultaneously, the total free-electron spin decays
rapidly with the characteristic time ~150 ps. The results are explained by
spin-dependent capture of free conduction electrons on deep paramagnetic
centers resulting in dynamical polarization of bound electrons. We have
developed a nonlinear theory of spin dynamics in the coupled system of
spin-polarized free and localized carriers which describes the experimental
dependencies, in particular, electron spin quantum beats observed in a
transverse magnetic field.
