Orbital to charge current conversion in copper oxide heterostructures

We investigate the orbital-to-charge current conversion in CoFeB|CuO bilayers as a function of CuO thickness, employing orbital pumping via ferromagnetic resonance. The dynamic injection of orbital angular momentum into the CuO layer generates a transverse voltage through the Inverse Orbital Hall Effect (IOHE). By systematically varying the CuO thickness from 2 nm to 30 nm, we observe a pronounced dependence of the IOHE-induced voltage on the CuO layer thickness, indicating efficient orbital-to-charge conversion. These results highlight the key role of the orbital degree of freedom in orbitronics and provide insights into the potential of transition-metal oxides for next-generation orbitronic devices.

💡 Research Summary

In this work the authors investigate the conversion of orbital angular momentum (OAM) into charge current via the inverse orbital Hall effect (IOHE) in CoFeB|CuO bilayers. Using ferromagnetic resonance (FMR) to pump orbital angular momentum from a 15 nm Co₄₀Fe₄₀B₂₀ layer into an adjacent CuO layer of varying thickness (2–30 nm), they detect a transverse dc voltage that they attribute to the IOHE. The samples were fabricated by sputtering on mica substrates under high vacuum, and their structural and chemical quality was verified by atomic‑force microscopy and X‑ray photoelectron spectroscopy. XPS revealed the expected Cu²⁺/Cu⁺ mixed valence and the evolution of O 1s components with film thickness, confirming the formation of stoichiometric CuO.

The experimental configuration placed the bilayer at the centre of an X‑band (9.8 GHz) cavity, where the microwave magnetic field excites FMR in the CoFeB while the electric field is minimized to suppress spurious galvanic effects. The dc voltage V_dc measured across the sample consists of a symmetric component V_Sym and a much smaller antisymmetric component V_Asym. For CuO layers thicker than 4 nm, V_Sym dominates, shows a clear sign reversal upon 180° rotation of the sample (changing the orbital polarization direction), and scales linearly with microwave power—hallmarks of a pure IOHE signal.

To interpret the data the authors adopt a one‑channel orbital diffusion model. The orbital chemical potential μ_L(z) obeys d²μ_L/dz² = μ_L/λ_L², where λ_L is the orbital diffusion length. Boundary conditions enforce continuity of the orbital current at the FM/NM interface and zero current at the outer surface. Solving these equations yields expressions for the orbital accumulation, the back‑flow current, and ultimately the transverse charge current J_c(z) = θ_OH (2e/ħ) J_L(z). Integration across the CuO thickness gives the IOHE voltage V_IOHE, which depends on the orbital Hall angle θ_OH, λ_L, the longitudinal conductivity σ_NM, and the pumped orbital current density J_L(0).

The pumped orbital current J_L(0) is linked to the Gilbert damping enhancement observed in FMR. By measuring the peak‑to‑peak linewidth ΔH_pp as a function of frequency (6–16 GHz) the authors extract the damping parameter α for each CuO thickness. α increases modestly with CuO thickness, indicating that CuO acts as an orbital angular‑momentum sink without significant spin absorption. Using the relation α = (γħ g_eff^L)/(4πM_s t_FM) they determine the effective orbital mixing conductance g_eff^L, which together with the measured conductivity (four‑probe van der Pauw) allows quantitative fitting of V_Sym versus CuO thickness.

The fitting yields an orbital diffusion length λ_L ≈ 6 nm and an orbital Hall angle θ_OH ≈ 2 %. The diffusion length is remarkably long for an oxide, comparable to values reported for light metals such as Ti and Al, where efficient orbital transport has been demonstrated despite weak spin‑orbit coupling. The orbital Hall angle indicates that CuO converts OAM to charge current with an efficiency similar to that predicted for materials with moderate spin‑orbit coupling. The thickness dependence of V_Sym—rapid increase for thin CuO followed by saturation beyond λ_L—matches the theoretical expectation from the diffusion model.

Overall, the study provides compelling experimental evidence that transition‑metal oxides can support robust, diffusive orbital transport and efficient orbital‑to‑charge conversion. The modest increase in Gilbert damping with CuO thickness, together with the dominance of the symmetric voltage component, confirms that the observed effect originates from pure orbital pumping rather than conventional spin pumping. These findings broaden the material palette for “orbitronics” and suggest that CuO and similar oxides could be employed in low‑power, high‑efficiency orbital‑based spintronic devices, offering an alternative to heavy‑metal systems traditionally used for spin Hall phenomena.