  We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be
modulated by picosecond acoustic pulses. In this approach a picosecond strain
pulse injected into the structure induces a tilt of the magnetization vector M,
followed by the precession of M around its equilibrium orientation. This effect
can be understood in terms of changes in magneto-crystalline anisotropy induced
by the pulse. A model where only one anisotropy constant is affected by the
strain pulse provides a good description of the observed time-dependent
response.
