Probing optical spin-currents using THz spin-waves in noncollinear magnetic bilayers

TL;DR

This study uses THz spin-waves in noncollinear magnetic bilayers to directly investigate optically induced spin currents, revealing their dependence on magnetization dynamics and supporting a ballistic transport model.

Contribution

It provides direct time-domain measurements of optical spin-currents and demonstrates that a simple ballistic transport model explains spin transport in these structures.

Findings

Spin-wave phase depends on laser fluence.

Spin current is proportional to the time derivative of magnetization.

Ballistic transport explains observed spin transport behavior.

Abstract

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.