Orbital torque effect in Mo-based magnetic heterostructures

Abstract

Utilizing the orbital current effect represents an effective strategy for enhancing current-induced torque in magnetic heterostructures. Molybdenum (Mo) is theoretically predicted to exhibit a large orbital Hall conductivity; however, experimental verification of current-induced orbital torque in Mo-based heterostructures remains sparse. In this work, we report a sizable torque efficiency in such structures, as quantified by the harmonic Hall voltage technique in Ni/Mo structures and confirmed through current-induced magnetization switching in MgO/CoFeB/Mo structures with perpendicular magnetic anisotropy. It is found that the torque efficiency strongly depends on the material of the ferromagnetic layer: the torque in MgO/CoFeB/Mo and Ni/Mo systems exhibits not only opposite signs but also distinct magnitudes. The observed long-range dependence of the torque on Mo layer thickness, combined with its correlation with the materials of the ferromagnetic layer, allows us to attribute its origin primarily to the orbital Hall effect of Mo. Furthermore, by inserting a Pt interlayer into MgO/CoFeB/Mo stacks, we demonstrate both a sign reversal and a significant enhancement of the torque efficiency at an optimized Pt thickness. Collectively, these findings not only establish Mo as an efficient source of orbital-mediated torque but also provide viable strategies for its effective control through interface engineering.