Absence of orbital current torque in Ta/ferromagnet bilayers

It has become a heated debate as to whether the orbital Hall effect of a material could generate a non-local orbital current and a non-zero spin-orbit torque on an adjacent magnetic layer. Here, we report unambiguous evidence that, regardless of the ferromagnets (FMs) (e.g., Ni, Ni81Fe19, Fe, Fe60Co20B20, and Fe50Pt50), the spin-orbit torque generated by an adjacent Ta, which is predicted to have a 50 times greater positive orbital Hall conductivity than the negative spin Hall conductivity, has essentially the same, negative efficiency. This agrees with the spin Hall effect of Ta being the only source of the interfacial torque. We identify that the constant, positive estimate of the torque of the Ta/FM samples from spin-torque ferromagnetic resonance (ST-FMR) analysis in a specific FM thickness range (≥2 nm for Ni) results from the overlook of a significant thick-dependent self-induced ST-FMR signal of the FM. These results indicate the absence of orbital current torque in Ta/ferromagnet systems, regardless of the type, the spin-orbit coupling, and the layer thickness of the ferromagnets.