Orbital torque efficiency in NixFe1-x/Pt bilayers

Abstract

Distinguished from spin torque dominantly determined by spin Hall angle of nonmagnetic metal, the generation and modulation of orbital torque strongly relies on the electronic structure, orbital magnetic moment, and spin–orbit coupling strength of the ferromagnetic material (FM). Here, the dependence of orbital torque generation on the FM is investigated by varying the composition in NixFe1-x/Pt bilayer systems. It is found that the torque efficiency varies non-monotonically with Ni concentration and peaks in pure Fe. Notably, Fe/Pt exhibits an order of magnitude higher orbital torque efficiency than other systems, which is enhanced by the specific bilayer combination. The long-range propagation observed in FM underscores a significant role of orbital Hall effect in torque generation. Using a diffusion model, we disentangle the spin and orbital contributions, and extract the characteristic diffusion lengths of Pt. Our work provides a viable material-engineering strategy for substantially improving orbital torque efficiency, contributing to the development of low-power orbitronic devices.