  We report on the experimental evidence for a nanosecond time-scale spin
memory based on nonradiative excitons. The effect manifests itself in
magnetic-field-induced oscillations of the energy of the optically active
(radiative) excitons. The oscillations detected by a spectrally-resolved
pump-probe technique applied to a GaAs/AlGaAs quantum well structure in a
transverse magnetic field persist over a time scale, which is orders of
magnitude longer than the characteristic decoherence time in the system. The
effect is attributed to the spin-dependent electron-electron exchange
interaction of the optically active and inactive excitons. The spin relaxation
time of the electrons belonging to nonradiative excitons appears to be much
longer than the hole spin relaxation time.
