Orbital to charge current conversion in copper oxide heterostructures

Abstract

We investigate the orbital-to-charge current conversion in Co$_{40}$Fe$_{40}$B$_{20}$\textbar 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 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.

Figures (4)

• Figure 1: High-resolution XPS spectra of CuO thin films with thicknesses of 5, 15, and 30 nm. (a–c) Cu 2$p$ core-level spectra for $t_{\mathrm{CuO}} = 5$, 15, and 30 nm, respectively, displaying the Cu 2$p_{3/2}$ and Cu 2$p_{1/2}$ components together with their satellite features. (d–f) Corresponding O 1$s$ core-level spectra for the same thicknesses, showing the deconvoluted oxygen components and their evolution with film thickness.
• Figure 2: (a) Schematic illustration of the CoFeB$|$CuO bilayer used for orbital pumping and $V_{\mathrm{IOHE}}$ measurements. A microwave field excites ferromagnetic resonance (FMR) in the CoFeB layer, leading to the injection of orbital angular momentum into the adjacent CuO layer. The resulting orbital-to-charge conversion generates a transverse voltage $V_{\mathrm{dc}}$, measured across the sample. (b) Derivative of the FMR absorption signal as a function of the applied magnetic field, measured at a frequency of 9.8 GHz and a microwave power of 197 mW. (c) Measured inverse orbital Hall voltage $V_{\mathrm{dc}}$ for the complete set of CoFeB$|$CuO samples with CuO thicknesses ranging from 2 to 30 nm.
• Figure 3: (a) Peak-to-peak linewidth $\Delta H_{\mathrm{pp}}$ as a function of frequency for the complete set of samples. The solid lines represent fits using Eq. (\ref{['Gilbert']}). (b) Extracted damping parameter $\alpha$ as a function of CuO thickness.
• Figure 4: Blue circles correspond to the symmetric voltage component $V_{\mathrm{Sym}}$ as a function of the CuO thickness $t_{\mathrm{CuO}}$, showing an increasing trend from 2 to 30 nm. Red circles correspond to the values obtained for the voltage using the orbital diffusion model.