  We propose a model to explore the dynamics of spin-systems coupled by
exchange interaction to the conduction band electrons of a semiconductor
material that forms the channel in a ferromagnet/semiconductor/ferromagnet
spin-valve structure. We show that recent observation of the novel transient
transport signature in a MnAs/GaAs/MnAs spin-valve structure with paramagnetic
Mn impurities [D. Saha et al., Phys. Rev. Lett., 100, 196603 (2008)] can be
quantitatively understood in terms of current driven dynamical polarization of
Mn spins. Using our model of spin polarized transport through Schottky barriers
at the two ferromagnet/semiconductor junctions in a spin-valve structure and a
dynamical equation describing the paramagnetic impurities coupled to conduction
band electrons we explain the scaling behaviour of observed transient features
such as the magnitude and time-scale with temperature.
