  Aiming at studying the mechanism of photo-excited precession of magnetization
in ferromagnetic (Ga,Mn)As, magneto-optical (MO) and differential reflectivity
(DR) temporal profiles are studied at relatively long and ultra-short time
scales for samples with different Mn contents (x=0.01-0.11). As to the
oscillatory MO profiles observed in the long time scale, simulation based on
the LLG equation combined with two different MO effects confirms
photo-inducement of the perpendicular anisotropy component p-Heff. As for the
profiles observed in the ultra-short time scale, they are consistently
explained in terms of the dynamics of photo-generated carriers, but not by the
sudden reduction in magnetization. With those experimental results and
analyses, new mechanism which accounts for the photo-induced p-Heff is
addressed; photo-ionization like excitation of Mn(II). Namely, Mn(II) is
excited into Mn(III) and e . It is discussed that such excitation tips magnetic
anisotropy toward the out-of-plane direction through the inducement of orbital
angular momentum and the gradient dMn(II*)/dz. Validity of the proposed
mechanism is examined by estimating the efficiency of excitation on the basis
of the Lambert-Beer law and the experimental p-Heff values, through which the
efficiency of 1-10 ppm with the nominal optical cross section of around 5 x
10^12 m^2 are obtained.
