Orbital torque in magnetic bilayers

TLDR

The orbital Hall effect generates an orbital current perpendicular to an applied electric field, and injecting this current into a ferromagnet can produce a torque that allows its detection. The study investigates current‑induced spin‑orbit torques in ferromagnet/heavy‑metal bilayers through theoretical modeling and experimental measurements. The authors analyze spin‑orbit torques in these bilayers using both theory and experiment to quantify the contributions from orbital and spin Hall effects. Analysis reveals that the orbital Hall effect contributes to the torque, with Ni/Ta bilayers exhibiting a torque opposite to spin‑Hall predictions, confirming orbital torque generation and indicating potential for spintronic device applications.

Abstract

The orbital Hall effect describes the generation of the orbital current flowing in a perpendicular direction to an external electric field, analogous to the spin Hall effect. As the orbital current carries the angular momentum as the spin current does, injection of the orbital current into a ferromagnet can result in torque on the magnetization, which provides a way to detect the orbital Hall effect. With this motivation, we examine the current-induced spin-orbit torques in various ferromagnet/heavy metal bilayers by theory and experiment. Analysis of the magnetic torque reveals the presence of the contribution from the orbital Hall effect in the heavy metal, which competes with the contribution from the spin Hall effect. In particular, we find that the net torque in Ni/Ta bilayers is opposite in sign to the spin Hall theory prediction but instead consistent with the orbital Hall theory, which unambiguously confirms the orbital torque generated by the orbital Hall effect. Our finding opens a possibility of utilizing the orbital current for spintronic device applications, and it will invigorate researches on spin-orbit-coupled phenomena based on orbital engineering.