Cascading Spin-Orbitronic Terahertz Emission in Ferromagnet/Nonmagnet Bilayers

Abstract

Spintronic terahertz (THz) emitters based on ferromagnet/nonmagnet (FM/NM) bilayer thin films provide a powerful time-domain optical platform for probing ultrafast angular-momentum transport on sub-picosecond timescales. Although THz emission is generally attributed to spin-to-charge conversion, the contribution and microscopic origin of orbital angular momentum remain unclear. Here, we clarify the generation pathway of optical-driven ultrafast orbital currents by engineering the spin-orbit coupling (SOC) strength of the FM layer and disentangling spin- and orbital-to-charge conversion in the NM layer. Using weak-SOC Ni80Fe20 and strong-SOC Ni combined with Pt and W-materials exhibiting opposite spin Hall but identical orbital Hall polarities-we uncover a pronounced thickness-dependent crossover from spin-dominated to orbital-dominated THz emission in Ni/W bilayers. Time-resolved THz spectroscopy reveals that the orbital contribution arises from the inverse orbital Rashba-Edelstein effect, enabled by efficient spin-to-orbital conversion within the strong-SOC ferromagnet. These results establish a cascading spin-orbitronic THz emission mechanism and demonstrate SOC engineering as an effective strategy for controlling ultrafast charge and angular-momentum transport in functional spin-orbitronic heterostructures.