Ultrafast laser-induced coherent spin dynamics in ferromagnetic Ga1-xMnxAs/GaAs structures

Ultrafast pump-probe magneto-optical spectroscopy is used to study coherent spin dynamics in ferromagnetic semiconductor Ga1-xMnxAs systems at excitation photon energies E-ph both above and below the band gap E-g of GaAs. Above E-g, the temporal Kerr rotation signal is found to be strongly dependent on pump photon polarization. This polarization dependence, persisting to room temperature, is attributed to spins of electrons photoexcited to the conduction band, and disappears for E-ph < E-g. Below the Curie temperature T-C of the Ga1-xMnxAs samples, the temporal Kerr rotation acquires an additional oscillatory component with a period of the order of 100 ps, attributed to the precession of the ferromagnetically coupled Mn spins. This precession is observed for excitation both above and below E-g, regardless of the pump polarization states. The detailed characteristics of this ferromagnetic precession are discussed in terms of the Landau-Lifshitz-Gilbert model. In discussing the observed results, special attention is given to the process of the magnetization precession due to excitation of the pump, to its dependence on the pump intensity, and ambient temperature, and to the relationship between the damping of the magnetization precession and the defects characteristic of ferromagnetic GaMnAs.