  Doped ZnO is a promising material for spintronics applications. For such
applications, it is important to understand the spin dynamics and particularly
the spin coherence of this II-VI semiconductor. The spin lifetime $\tau_{s}$
has been measured by optical orientation experiments, and it shows a surprising
non-monotonic behavior with temperature. We explain this behavior by invoking
spin exchange between localized and extended states. Interestingly, the effects
of spin-orbit coupling are by no means negligible, in spite of the relatively
small valence band splitting. This is due to the wurtzite crystal structure of
ZnO. Detailed analysis allows us to characterize the impurity binding energies
and densities, showing that optical orientation experiments can be used as a
characterization tool for semiconductor samples.
