We developed a dynamical model that successfully explains the observed time evolution of the magnetization in diluted magnetic semiconductor quantum wells after weak laser excitation. Based on a many-particle expansion of the exact p-d exchange interaction, our approach goes beyond the usual mean-field approximation. It includes both the sub-picosecond demagnetization dynamics and the slower relaxation processes which restore the initial ferromagnetic order on a nanosecond timescale. In agreement with experimental results, our numerical simulations show that, depending on the value of the initial lattice temperature, a subsequent enhancement of the total magnetization may be observed on a timescale of few hundreds of picoseconds (see figure). More recently, our model was augmented in order to include the role played by the quantum confinement and the band structure. It was shown that the sample thickness and the background hole density strongly influence the phenomenon of demagnetization. Quantitative results were given for III-V ferromagnetic GaMnAs quantum wells of thickness 4 and 6 nm. Finally, third-order effects in the many-particle expansion of the exact p-d exchange interaction were taken into account. Dynamical RKKY-like interactions and double-exchange mechanism based on the Kondo interaction emerge naturally from our approach. Our analysis reveals that the many-particle expansion is not generally well defined and an infrared Kondo-like divergence can occur. In particular, the bare polarization propagator fails to converge in the presence of a highly confined hole gas and an enhancement of the ion-hole spin correlation is found for low-dimensional systems.
Institut de Physique et de Chimie des Matériaux de Strasbourg