  Non-thermal laser induced spin excitations, recently discovered in
conventional oxide and metal ferromagnets, open unprecedented opportunities for
research and applications of ultrafast optical manipulation of magnetic
systems. Ferromagnetic semiconductors, and (Ga,Mn)As in particular, should
represent ideal systems for exploring this new field. Remarkably, the presence
of non-thermal effects has remained one of the outstanding unresolved problems
in the research of ferromagnetic semiconductors to date. Here we demonstrate
that coherent magnetization dynamics can be excited in (Ga,Mn)As non-thermally
by a transfer of angular momentum from circularly polarized femtosecond laser
pulses and by a combination of non-thermal and thermal effects due to a
transfer of energy from laser pulses. The thermal effects can be completely
suppressed in piezo-electrically controlled samples. Our work is based on
pump-and-probe measurements in a large set of (Ga,Mn)As epilayers and on
systematic analysis of circular and linear magneto-optical coefficients. We
provide microscopic theoretical interpretation of the experimental results.
