Electron spin relaxation in semiconductors and semiconductor structures

We suggest an approach to the problem of a free electron spin evolution in a semiconductor with arbitrary anisotropy or quantum structure in a magnetic field. The developed approach utilizes quantum kinetic equations for average spin components. These equations represent the relaxation in terms of correlation functions for fluctuating effective fields responsible for spin relaxation. In a particular case when autocorrelation functions are dominant, the kinetic equations reduce to the Bloch equations. The developed formalism is applied to the problem of electron spin relaxation due to exchange scattering in a semimagnetic quantum well (QW) as well as to the spin relaxation in a QW due to the Dyakonov-Perel mechanism. The results permit us to separate the longitudinal $T_1$ and transversal $T_2$ relaxation times in a strong enough magnetic field and to trace the cases of undistinguished parameters $T_1$ and $T_2$ in zero and small magnetic fields. Some new predictions of the developed theory are discussed. Namely, we discuss the nonmonotonic behavior of spin relaxation caused by exchange scattering under an external magnetic field and new peculiarities of electron spin evolution caused by the presence of three relaxation times (rather than two) for the Dyakonov-Perel mechanism in a quantum well.