Control of Coherent Magnetization Precession in GaMnAs by Ultrafast Optical Excitation

We report comprehensive temperature and photoexcitation intensity dependent studies of the photoinduced magnetization precession in Ga 1-x Mn x As ( x = 0.035) by time-resolved Kerr rotation measurements. We observe coherent oscillations of local Mn spins triggered by an ultrafast photo-induced reorientation of the easy axis due to changes in the magnetic anisotropy. The amplitude saturation of these oscillations above certain pump intensity is indicative of stabilization of the magnetic easy axis orientation on temperatures above ~Tc/2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent of ambient temperature.