  We study electron spin dynamics in diluted magnetic quantum wells. The
electrons are coupled by exchange interaction with randomly distributed
magnetic ions polarized by magnetic field. This coupling leads to both spin
relaxation and spin decoherence. We demonstrate that even very small spatial
fluctuations of quantum well width dramatically increase rate of decoherence.
Depending on the strength of exchange interaction and the amplitude of the
fluctuations the decoherence can be homogeneous or inhomogeneous. In the
homogeneous regime, the transverse (with respect to magnetic field) component
of the electron spin decays on the short time scale exponentialy, while the
long-time spin dynamics is non-exponential demonstrating long-lived power law
tail. In the inhomogeneous case, the transverse spin component decays
exponentially with the exponent quadratic in time.
