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

Understanding howa spin current flows across metal-semiconductor interfaces at pico- and femtosecond time scales is ofparamount importance for ultrafast spintronics, data processing, and storage applications. However, the possibility todirectly access the propagation of spin currents, within such time scales, has been hampered by the simultaneous lack ofboth ultrafast element-specific magnetic sensitive probes and tailoredwell-built and characterized metal-semiconductorinterfaces. Here, by means of a novel free-electron laser-based element-sensitive ultrafast time-resolved Kerr spectroscopy,we reveal different magnetodynamics for the Ni M2;3 and Si L2;3 absorption edges. These results are assumed tobe the experimental evidence of photoinduced spin currents propagating at a speed of 0.2 nm/fs across the Ni/Siinterface.