  Photo-excited precession of magnetization in (Ga,Mn)As is investigated by
measuring time-resolved magneto-optical response and transient differential
reflectivity with pump-and-probe technique. In the time region less than 1 ps,
rapidly oscillating and spike-like signals are observed, respectively, with
excitation of below and above the GaAs band gap. Analysis with gyromagnetic
model and autocorrelation function concludes that those signals are not
attributed to ultrafast demagnetization but due to interference between pump
and probe pulses incorporating sub-ps carrier dynamics characteristic of
low-temperature grown semiconductors. Photo-ionization of Mn ions (Mn2+ ->
Mn3+) is proposed as a mechanism which dynamically induces orbital angular
momentum and affects hole-mediated magnetic anisotropy in (Ga,Mn)As.
