It was found in 2005 that optical pulse excitation of ferromagnetic semiconductor (Ga,Mn)As induces precession of magnetization (M) [1]. In 2008, a mechanism of the photo-induced precession has been discussed, on the basis of new data taken at 10 K without an external field, in terms of a torque that is produced by a slight tilt of a p-d-exchange-induced magnetic anisotropy field with photo-generated holes of the order of 10~(17) - 10~(17) cm~(-3) [2], the first example showing non-thermal influence of optical excitation in the system with free carners. Coherent control of magnetization precession has been demonstrated soon after this work; the amplitude of precession of M has either be enhanced or suppressed by tuning the interval between two pump pulses [3]. These phenomena can be viewed as the temporal imprint of an ultra-short light pulse on an ordered spin system, and may have interesting applications in the field of optical signal processing. Examples are buffer memories [4] and signal repeaters.
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