Coherent Spin Waves in Thin-Film GaMnAs

We report on the observation of coherent oscillations associated with standing spin waves in thin films of the ferromagnetic semiconductor GaMnAs. The oscillations were observed in differential magnetic Kerr measurements using a standard pump-probe setup. Subpicosecond pump pulses from a Ti: sapphire laser induce a coherent precession of the magnetization which is detected by measuring the rotation of the polarization of the delayed probe pulses. The magnetic anisotropy and spin stiffness constants ($D)$ were determined from the magnetic-field dependence of the spin-wave frequencies. We obtain $D=0.7\pm 0.1\times 10^{-13}T\cdot cm^2$ for two as-grown samples ($T_{C~}$= 65 K) and $D=3.5\pm 0.5\times 10^{-13}T\cdot cm^2$ for an annealed sample with $T_{C~}$= 80 K.