Understanding how spin currents flow across metal-semiconductor interfaces at femtosecond timescales has impacts on ultrafast spintronics, data processing and storage applications. However, the possibility to directly access the propagation of spin currents on these time scales has been hampered by the simultaneous lack of both ultrafast site selective magnetic sensitive probes and tailored metal-semiconductor interfaces. By means of FEL-based core-resonant Kerr spectroscopy, we report direct experimental evidence of spin currents across a Ni/Si interface in terms of different magnetodynamics at the Ni M<sub>2,3</sub> and Si L<sub>2,3</sub> absorption edges. This further allows us to extract the propagation velocity of the spin current in Si as 0.2 nm/fs.

![](https://assets.underline.io/uploads/markdown_image/1/image/ba4eb9360a1db64b8e33f4c491757403.jpg)

Fig. 1: a) The sample stack consists of a Si substrate (blue), a thin Si3N4 passivation layer (grey), 7 nm of Ni film (red) and 2 nm of Ag capping layer (yellow). The white arrow represents the in-plane magnetization of the Ni film. The FEL pulses at the Si L2,3 edge (blue arrow) and at the Ni M2,3 edge (red arrow) are also depicted. The IR laser pulses at 1.55 eV hit the sample at a time delay t-t0, before the FEL pulses arrival. The penetration depths of the pump and the probes are represented by the length of the arrows. The fraction of the IR laser energy absorbed by each layer is reported above the probe arrow. b) Scheme of the FEL RMOKE set-up at MagneDyn. RMOKE was probed in longitudinal configuration at an angle of incidence of 45°. The polarization ε of the incoming FEL pulses (blue arrow) is in the scattering plane, while the Kerr rotation of the polarization of the reflected pulse is indicated as θ.

![](https://assets.underline.io/uploads/markdown_image/1/image/388e11ba365f236752de3629d9784f15.jpg)

Fig. 2: RMOKE magnetic hysteresis in degrees of Kerr rotation at the Ni M2,3 edge (a) and at the Si L2,3 edge (b). Empty and filled circles curves represent the unpumped and pumped hysteresis after 0.5 ps (Ni) and 0.3 ps (Si) (colored star markers in c). c) relative change of the site resolved magnetization M (Ni - red and Si - blue dots) as a function of time delay measured in saturation at 550 mT. The solid lines represent the tting results, from which we extract the characteristic times for demagnetization and recovery. The difference (ΔM/M)js is also shown (gray pentagons).

All-Optical Spin Injection in Silicon Revealed by Element Specific Time-Resolved Kerr Effect

**Session chairs:** Hélène Béa, Université Grenoble Alpes, Spintec and Emilie Jué, National Institute of Standards and Technology


GOQ - Spin Injection and Spin Transfer Torques

technical paper

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All-Optical Spin Injection in Silicon Revealed by Element Specific Time-Resolved Kerr Effect

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