  We report a femtosecond response in photoinduced magnetization rotation in
the ferromagnetic semiconductor GaMnAs, which allows for detection of a
four-state magnetic memory at the femtosecond time scale. The temporal profile
of this cooperative magnetization rotation exhibits a discontinuity that
reveals two distinct temporal regimes, marked by the transition from a highly
non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal,
lattice-heating picosecond regime.
