Chiral-phonon generation of orbital currents in light transition metals

TL;DR

This paper demonstrates that chiral surface acoustic waves can generate significant orbital currents in light-metal/ferromagnet bilayers, revealing a new phonon-driven pathway for orbitronic applications and orbital magnetism control.

Contribution

It introduces a novel method using chiral surface acoustic waves to coherently generate and detect orbital currents in light metals, advancing orbitronics.

Findings

Strong orbital currents observed in Ni/Cr and Ni/Ti bilayers.

Negligible responses in Ni/Al and cobalt-based bilayers.

Phonon excitation more efficiently generates orbital currents than spin-torque methods.

Abstract

Orbital angular momentum offers a new channel for information transport in a vast set of materials. Its coherent generation and detection remain, however, largely unexplored. Here, we demonstrate that chiral surface acoustic waves (SAWs) generate sizable orbital currents in light-metal/ferromagnet bilayers through both the acoustic orbital Hall effect and acoustic orbital pumping. Using symmetry analysis of SAW-driven voltages, we disentangle vorticity-sensitive orbital currents arising from lattice rotation in the non-magnetic layer from angular-momentum pumping from the ferromagnet. Strong signals are observed only in nickel/chromium and nickel/titanium, while nickel/aluminum and all cobalt-based bilayers show negligible responses, revealing the critical roles of orbital Hall conductivity, phonon-orbital coupling, and interfacial orbital transparency. Comparison with spin-torque ferromagnetic resonance and second-harmonic measurements -- where electrically driven orbital angular momentum are weaker -- demonstrates that phonon excitation generates orbital currents more efficiently. These results establish chiral SAWs as an effective route for orbitronic functionality and open pathways toward phonon-controlled orbital magnetism.