  We investigate theoretically the influence of an external magnetic field on
the spin dynamics of excitons in diluted magnetic semiconductor quantum wells.
To this end, we apply a quantum kinetic theory beyond the Markov approximation
which reveals that non-Markovian effects can significantly influence the
exciton spin dynamics. If the magnetic field is oriented parallel to the growth
direction of the well, the Markovian spin-transfer rate decreases monotonically
with increasing field as predicted by Fermi's golden rule. The quantum kinetic
theory follows this result qualitatively but predicts pronounced quantitative
differences in the spin-transfer rate as well as in the long-time spin
polarization. However, for an in-plane magnetic field, where the Markovian
spin-transfer rate first drops and then increases again, quantum kinetic
effects become so pronounced that the Markovian trend is completely reversed.
This is made evident by a distinct maximum of the rate followed by a monotonic
decrease. The deviations can be traced back to a redistribution of carriers in
energy space caused by correlations between excitons and magnetic dopants. The
same effect leads to a finite electron-spin polarization at long times in
longitudinal as well as transverse fields which is much larger than the
corresponding Markovian prediction.
