Ultrafast all-optical spin injection in silicon revealed at FERMI

Understanding how a spin current flows across metal-semiconductor interfaces at pico- and femtosecond timescales has implications for ultrafast spintronics, data processing and storage applications.

However, the possibility to directly access the propagation of spin currents on such time scales has been hampered by the simultaneous lack of both ultrafast element specific magnetic sensitive probes and tailored metal-semiconductor interfaces. Here, by means of free electron laser-based element sensitive Kerr spectroscopy, we report experimental evidence of spin currents across a Ni/Si interface in the form of different magnetodynamics at the Ni M2,3 and Si L2,3 absorption edges. This further allows us to calculate the propagation velocity of the spin current in silicon, which is on the order of 0.2 nm/fs.

RMOKE magnetic hysteresis in degrees of Kerr rotation at the Ni M2,3 edge (panel a) and at the Si L2,3 edge (panel b). The empty and filled circles curves represent the unpumped (label up) and pumped (label pp) hysteresis measured at a delay time of 0.5 ps (for the Ni edge) and 0.3 ps (for the Si edge) (colored star markers in panel c). Panel c) relative change of the site resolved magnetization M (Ni - red dots and Si - blue dots) as a function of the time delay measured in saturation with an applied magnetic field of 550 mT. The Ni demagnetization curve was rescaled to account for the different pump fluence applied. The solid lines represent the fitting results, from which we extract the two characteristic times for demagnetization and recovery. The difference of the two magnetization dynamics is also shown (gray pentagons)