Ultrafast magnetic polaron dynamics in Cd_1-xMn_xTe/ZnTe multiple quantum wells

Optical pump-probe experiments were carried out in Cd_1-xMn_xTe/ZnTe multiple quantum wells to study the dynamics of the excitons and the exciton magnetic polarons in these materials. In the pump-probe spectra, three signals were detected. The main signal is negative and monitors the energy relaxation of the excitons created by the pump pulse. The two other pump-probe signals are positive and their intensity grows during the time when the excitons created by the pump pulse recombine. The low-energy positive pump-probe signal is found to track the average Mn spin polarization in the sample. Experiments performed with circular polarization show that long-range order is induced by a sufficiently intense pump beam. But in our samples, the spins of the excitons are found to be oriented mainly antiparallel to the long-range magnetization generated after the pump excitation. Furthermore, we found that, at high excitation densities, the excitons couple antiferromagnetically to the long-range 'background' magnetization of the Mn spins.