Acoustic orbital Hall effect and orbital pumping in light-metal-ferromagnet bilayers

TL;DR

This paper demonstrates that surface acoustic waves in light metals can generate and manipulate orbital currents via phonon-orbital coupling, revealing a new pathway for low-dissipation orbitronic devices driven by lattice dynamics.

Contribution

It introduces the acoustic orbital Hall effect in light metals and shows how surface acoustic waves induce orbital currents, a novel mechanism for orbitronics involving lattice dynamics.

Findings

Ti exhibits higher efficiency and longer diffusion length of orbital currents than Pt.

Rectified voltages depend on orbital-to-spin conversion and magnetoelastic coupling.

Evidence of acoustic orbital pumping from ferromagnetic resonance excitation.

Abstract

Orbital currents provide a new degree of freedom for controlling magnetism, yet their interaction with lattice dynamics remains largely unexplored. Here we report a systematic investigation of the acoustic orbital Hall effect in light metals such as Ti and Cr, where surface acoustic waves generate orbital currents through phonon-orbital coupling. The acoustic orbital current in Ti exhibits higher efficiency and longer diffusion length compared to the acoustic spin current in Pt. The sign and magnitude of the rectified acoustic voltages in nonmagnetic (Ti, Cr)/ferromagnetic (Ni, Co, Fe$_x$Co$_{1-x}$) bilayers are determined by the product of orbital-to-spin conversion and magnetoelastic coupling efficiencies of the ferromagnet. Additionally, we find evidence for acoustic orbital pumping, whereby the excitation of ferromagnetic resonance by surface acoustic waves injects an orbital current from the ferromagnet into the nonmagnet. These results establish lattice dynamics as an efficient driver of orbital transport, opening opportunities for low-dissipation orbitronic devices that harness and sense phonons.