Investigation of Optical Spin Transfer Torque in Ferromagnetic Semiconductor GaMnAs by Magneto-Optical Pump-and-Probe Method

We report on magnetization precession induced in (Ga,Mn)As by an optical spin transfer torque (OSTT). This phenomenon, which corresponds to a transfer of angular momentum from optically-injected spin-polarized electrons to magnetization, was predicted theoretically in 2004 and observed experimentally by our group in 2012. In this paper we provide experimental details about the observation of OSTT by a time-resolved pump-and-probe magneto-optical technique. In particular, we show that the precession of magnetization due to OSTT can be experimentally separated from that induced by the well known magnetic-anisotropy-related mechanism in a hybrid structure piezo-stressor/(Ga,Mn)As with an in situ electrical control of the magnetic anisotropy. We also illustrate that OSTT is clearly apparent in the measured dynamical magneto-optical signal in a large variety of (Ga,Mn)As samples with a Mn concentration from 3% to 9%.