Optically induced spin currents have proven to be useful in spintronics applications, allowing for sub-ps all-optical control of magnetization. However, the mechanism responsible for their generation is still heavily debated. Here we use the excitation of spin-current induced THz spin-waves in noncollinear bilayer structures to directly study optical spin-currents in the time domain. We measure a significant laser-fluence dependence of the spin-wave phase, which can quantitatively be explained assuming the spin current is proportional to the time derivative of the magnetization. Measurements of the absolute spin-wave phase, supported by theoretical calculations and micromagnetic simulations, suggest that a simple ballistic transport picture is sufficient to properly explain spin transport in our experiments and that the damping-like optical STT dominates THz spin-wave generation. Our findings suggest laser-induced demagnetization and spin-current generation share the same microscopic origin.
@article{arxiv.2103.06029,
title = {Probing optical spin-currents using THz spin-waves in noncollinear magnetic bilayers},
author = {Tom Lichtenberg and Maarten Beens and Menno H. Jansen and Rembert A. Duine and Bert Koopmans},
journal= {arXiv preprint arXiv:2103.06029},
year = {2022}
}