Mechanism of photo-excited precession of magnetization in (Ga,Mn)As on the basis of time-resolved spectroscopy

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.