Inverse orbital Hall effect induced terahertz emission enabled by a ferromagnet with quenched orbital moment in Fe/Pt/W trilayers

The inverse orbital Hall effect (IOHE) has recently attracted considerable attention as an emerging mechanism for terahertz (THz) emission based on ultrafast angular-momentum-to-charge conversion. Most experimental studies have focused on materials with strong spin-orbit coupling or pronounced orbital character, where sizable orbital Hall responses are expected. Elemental ferromagnets such as Fe are generally regarded as quenched orbital sources and are not expected to exhibit orbital-dominated THz emission. Here, we report a pronounced enhancement of THz emission in Fe/Pt/W trilayer heterostructures, despite the absence of detectable orbital contributions in the corresponding Fe/Pt and Fe/W bilayers. Thickness-dependent measurements reveal long-distance signal persistence, systematic delay accumulation, and pronounced pulse broadening with increasing W thickness. These features are inconsistent with diffusive spin transport and indicate that orbital angular momentum transport in the W layer, converted into charge current via the IOHE, becomes a dominant channel for THz emission in the trilayer configuration. Our results demonstrate that strong IOHE can emerge in heterostructures incorporating a quenched orbital ferromagnet, providing an effective route to enhance spintronic THz emitters through orbital Hall physics.

💡 Research Summary

The paper investigates terahertz (THz) emission from Fe/Pt/W trilayer heterostructures and demonstrates that the inverse orbital Hall effect (IOHE) can dominate the emission even when the ferromagnetic layer (Fe) has a quenched orbital moment. Conventional spintronic THz emitters rely on ultrafast spin‑to‑charge conversion via the inverse spin Hall effect (ISHE) in heavy metals (HM) such as Pt or W. In Fe/Pt and Fe/W bilayers, the authors confirm that THz signals are governed by ISHE: the amplitude rapidly decays with HM thickness, the temporal position of the pulse remains essentially unchanged, and the pulse width shows no systematic dependence, all consistent with short‑range diffusive spin transport (spin diffusion lengths of only a few nanometers).

In stark contrast, Fe/Pt/W trilayers exhibit several hallmarks of long‑range angular‑momentum transport: (i) the THz amplitude persists up to W thicknesses of 100 nm, far exceeding the spin diffusion length in W; (ii) the pulse delay τ_D increases linearly with W thickness, yielding effective propagation velocities of 0.3–0.6 nm fs⁻¹, considerably slower than the ≈1 nm fs⁻¹ typical for spin currents; (iii) the pulse width broadens markedly with increasing W thickness, indicating dispersive transport. These observations cannot be reconciled with pure spin diffusion and point to orbital angular momentum (OAM) propagation.

The authors propose a three‑step mechanism. First, femtosecond laser excitation of the 2 nm Fe layer generates both a spin current (J_s) and a weak orbital current (J_o) because Fe’s orbital moment is largely quenched. Second, the Pt layer, possessing strong spin‑orbit coupling, converts a substantial fraction of J_s into orbital angular momentum via spin‑to‑orbital conversion (SOC‑mediated). This generated orbital current has a much longer mean free path than the spin current and is injected into the adjacent W layer. Third, W exhibits a large orbital Hall angle; the transmitted orbital current is converted into a transverse charge current by the inverse orbital Hall effect. Importantly, the charge current produced by IOHE in W has the same polarity as the ISHE‑derived current in Pt, allowing constructive interference and a net THz enhancement.

Thickness‑dependent studies reveal an optimal Pt thickness of ~2 nm, where the spin‑to‑orbital conversion is most efficient and the Pt layer remains continuous. At this Pt thickness, the effective decay length of the THz signal in W (λ_eff) reaches a maximum, indicating the slowest attenuation of the orbital current. Varying the W thickness shows a peak THz amplitude at modest W thicknesses (≈0.5 nm for Pt = 2 nm) and a gradual decline thereafter, reflecting a balance between orbital injection and absorption. The authors quantify the cooperative enhancement using an enhancement factor η = (ΔV_trilayer)/(ΔV_bilayer). η exceeds unity over a broad range of Pt thicknesses when W is thin (0.75 nm), but drops below one for thicker W unless Pt is tuned to ≈1.5 nm, underscoring the importance of thickness matching for maximal ISHE + IOHE synergy.

The work establishes that even a ferromagnet with a quenched orbital moment can act as an effective source of orbital angular momentum when paired with a strong spin‑orbit metal (Pt) that mediates spin‑to‑orbital conversion. The resulting orbital current can travel over tens of nanometers in a second heavy metal (W) and be reconverted to charge via IOHE, providing a new pathway to boost THz emission beyond the limits imposed by short spin diffusion lengths. This strategy opens a route to design high‑efficiency, broadband THz emitters by engineering multilayer stacks that exploit both ISHE and IOHE, potentially impacting ultrafast spectroscopy, wireless communications, and on‑chip THz photonics.