Using Spin Waves to Probe Ultrafast Spin Current Generation in Rare Earth Ferromagnets

Reinoud LAVRIJSEN

All-optical switching (AOS) of ferrimagnetic rare earth-transition metal compounds with femtosecond laser pulses shows great promise for technological applications [1]. However, the possibly essential role of spin transport has scarcely been addressed. While it has been claimed that Gd can produce large spin currents [2], these are notoriously difficult to probe, impeding a full understanding of the physics at play. We demonstrate the use of spin waves to probe spin currents generated by ferromagnetic rare earth films. Upon fs laser pulse excitation, spin waves are excited in an in-plane Co layer via an out-of-plane spin current [3, 4] originating from a ferrimagnetic Co/Gd bilayer. Here, Co stabilizes the antiparallel Gd magnetization, and provides a spin current to compare the effect of Gd to. For increasing Gd thickness, the spin current is expected to shift from Coto Gd-dominated, reversing its polarization. Using time-resolved MOKE, we find that the homogeneous (FMR) mode experiences a phase rotation of nearly 180° over a small Gd thickness range, which confirms a large contribution of Gd to the overall spin current. Qualitative modeling supports this interpretation, with efforts underway to better quantify the Gd contribution. Substituting Tb for Gd strongly decreases the amplitude of the FMR mode, implying weaker spin current generation. This might also partly explain the apparent difficulty in achieving AOS in Tb-containing systems. The same spin currents can excite THz frequency standing spin waves in the in-plane layer [5, 6], which appear to be strongly suppressed with increasing rare earth thickness. This is consistent with the relatively slow magnetization dynamics in these materials [7] leading to longer lasting spin currents, which excite high frequency modes less efficiently. This approach for probing optically generated spin currents can elucidate the processes at work in AOS, giving valuable insight for implementation in data storage devices of the future.

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[7] B. Frietsch et al., Science Advances, 6.39, eabb1601 (2020)