  The optical spin orientation effect in a GaAs/(Ga,Al)As quantum well
containing a high-mobility 2D electron gas was found to be due to
spin-polarized minority carriers, the holes. The observed oscillations of both
the intensity and polarization of the photoluminescence in a magnetic field are
well described in a model whose main elements are resonant absorption of the
exciting light by the Landau levels and mixing of the heavy- and light-hole
subbands. After subtraction of these effects, the observed influence of
magnetic fields on the spin polarization can be well interpreted by a standard
approach of the optical orientation method. The spin relaxation of holes is
controlled by the Dyakonov-Perel' mechanism. Deceleration of the spin
relaxation by the magnetic field occurs through the Ivchenko mechanism - due to
the cyclotron motion of holes. Mobility of holes was found to be two orders of
magnitude smaller than that of electrons, being determined by the scattering of
holes upon the electron gas.
