Abstract:

Electron spin dephasing dynamics in ferromagnetic (Ga,Mn)As with Mn concentration x spanning the wide range from 0.5% to 15% across the insulator-to-metal transition, is systematically studied by varying temperature and optical pumping density with time-resolved magneto-optical Kerr effect measurements (TR-MOKE). For (Ga,Mn)As with different Mn doping concentration, the electron spin dephasing time shows quite different temperature and photo-excitation density dependent behavior, implying different spin relaxation mechanisms are involved. In the metallic and near insulating (Ga,Mn)As, the electron spin relaxation is suggested to be dominated by Dyakonov-Perel (DP) mechanism considering screening effect together with p-d exchange coupling. In the insulating (Ga,Mn)As, BAP mechanism is suggested to be important. However, deeply into the insulating and heavily doped regimes, where abundant Mn ions don¡¯t participate in the long-ranged magnetic order, the s-d exchange scattering is discussed to play an important role for electron spin relaxation process.
I will also present our experimental study of the nonlinear second harmonic spin wave excitation in the 10-nm epitaxially grown Fe thin film on GaAs by double femtosecond laser pulses pumping with TR-MOKE. Under single beam pumping, we observed the uniform spin-wave oscillation, however, when we applied the double laser beams with their polarization axial symmetrical to the [100] axis, a clear secondary oscillation with double frequency of the uniform mode was observed. The origin of this second harmonic spin wave mode is unknown and needs theoretical collaboration.
 
 
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