All-optical spin injection in silicon investigated by element-specific time-resolved Kerr effect

Understanding how a spin current flows across metal-semiconductor interfaces at pico- and femtosecond time scales is of paramount importance for ultrafast spintronics, data processing, and storage applications. However, the possibility to directly access the propagation of spin currents, within such time scales, has been hampered by the simultaneous lack of both ultrafast element-specific magnetic sensitive probes and tailored well-built and characterized metal-semiconductor interfaces. Here, by means of a novel free-electron laser-based element-sensitive ultrafast time-resolved Kerr spectroscopy, we reveal different magnetodynamics for the Ni M 2 , 3 and Si L 2 , 3 absorption edges. These results are assumed to be the experimental evidence of photoinduced spin currents propagating at a speed of ∼ 0.2 nm/fs across the Ni/Si interface.