Optical spin resonance and transverse spin relaxation in magnetic semiconductor quantum wells

Abstract

Ultrafast optical pulses are used to initiate and measure free-induction decays of coherent conduction electron spins and of embedded magnetic ${\mathrm{Mn}}^{2+}$ ions in a series of magnetic-semiconductor quantum wells. These time-resolved Faraday rotation experiments in transverse applied magnetic fields complement previous studies of spin dynamics in longitudinal fields by unambiguously distinguishing between the spin relaxation of electrons and holes, and by identifying a mechanism by which angular momentum is transferred from spin-polarized carriers to the sublattice of local moments. In transverse fields (Voigt geometry), the precession of the photoexcited spins about the field axis can be measured as an oscillatory induced Faraday rotation signal. We observe the THz free-induction decay of spin-polarized electrons in modest (4 T) magnetic fields and separately identify the more rapid spin relaxation of the holes as functions of field and temperature. The $g$ factors of the electrons and holes are accurately measured as a function of well width. The role of quantum confinement on the stability of the hole spin is discussed, with particular attention given to the observed ability of the transient hole-exchange field to coherently rotate a macroscopic ensemble of local ${\mathrm{Mn}}^{2+}$ moments. This ``tipping pulse'' initiates a free-induction decay in the sublattice of ${\mathrm{Mn}}^{2+}$ spins and enables electron paramagnetic resonance (EPR) studies of the fractional monolayer magnetic planes. These time-domain EPR measurements reveal a significant magnetic field dependence of the Mn transverse spin relaxation time.