Accessing the orbital degree of freedom through chiral phonons in ferromagnetic/light-metal bilayers

Abstract

Orbital angular momentum provides a promising channel for information transport across a wide range of materials. However, its coherent generation and detection remain largely unexplored. In this talk we will review our recent experiments demonstrating the phonon-induced generation of orbital currents in ferromagnet/non-magnet heterostructures driven by surface acoustic waves (SAWs). The chiral phonons associated with the SAW excite orbital currents in the non-magnetic layer that interact with the magnetization dynamics of the ferromagnet and convert into dc voltages via the inverse orbital Hall effect. We compare metal caps with different orbital-to-charge conversion efficiencies: strong (Chromium, Titanium) and weak (Aluminum), and contrast ferromagnets with distinct emission behaviors: Nickel, which can effectively generate orbital currents, and Cobalt, which does not. We observe large signals in Ni/Cr and Ni/Ti bilayers, while almost no response appears in Ni/Al. On the other hand, Co-based bilayers show negligible voltages, confirming their weak coupling to orbital angular momentum.