Probing Laser-Induced Spin-Current Generation in Synthetic Ferrimagnets Using Spin Waves

TL;DR

This study investigates laser-induced spin-current generation in synthetic ferrimagnets, revealing the roles of Gd and Tb in spin transport and the frequency-dependent excitation of spin waves, with implications for opto-magnetic data storage.

Contribution

It isolates the contribution of rare-earth materials to spin currents using spin wave dynamics and models the temporal profile of these currents in synthetic ferrimagnets.

Findings

Gd significantly contributes to spin currents, unlike Tb.

Rare-earth generated spin currents cannot excite sub-ps dynamics.

Modeling with the s-d model explains the temporal behavior of spin currents.

Abstract

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future opto-magnetic data storage applications, the role of non-local spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring THz standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current, but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the sub-ps timescale. We present modelling efforts using the $s$-$d$ model, which effectively reproduce our results and allow us to explain the behavior in terms of the temporal profile of the spin current.