Direct observation of non-thermal influence in the process of photo-induced ferromagnetic resonance in (Ga,Mn)As

摘要

Excitation dynamics of photo-induced ferromagnetic resonance (phi-FMR) in (Ga,Mn)As is investigated at both ps and ns time regions with time-resolved magneto-optical (MO) and reflectivity measurements by using pump-probe technique with a fs-laser source in the regime of weak excitation (0.34 - 3.4 μJ/cm~2 per pulse). Magnetization precession is observed as oscillating MO signals in ns timescale for the excitation energy hv extending from 1.41 eV to 1.65 eV. Rapidly oscillating and spike-like signals appearing at the onset of phi-FMR within 1ps are analyzed with Landau-Lifshitz-Bloch (LLB) equation and convoluted autocorrelation function, from which those signals are identified as the optical effects due to autocorrelation between pump and probe, but not due to ultrafast demagnetization. Our results, in particular, the occurrence of phi-FMR with the photon energy smaller than the GaAs bandgap energy (hv= 1.51 eV at 10 K) without ultrafast demagnetization, suggests a new mechanism of controlling magnetic anisotropy with Mn-induced states in the band gap of (Ga,Mn)As.