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 Ni ₈₀ Fe ₂₀ 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.