Dominant spin Hall torque and negligible orbital Hall torque in α-W/ferromagnet heterostructures with artifacts-free angular momentum detectors

Abstract

α-phased W was theoretically predicted to have a negative spin Hall conductivity and a positive orbital Hall conductivity at the same time, leaving the physical origin of the current-induced torque a critical open question. Here, we develop two angular momentum detectors of Cu/Ni/Cu and Cu/FeCoB/Cu that are free of artifacts torques (e.g., self-induced torque and spin-vorticity torque) and clarify that the spin-orbit torque contributed by W remains negative and predominantly from the spin Hall effect in the entire thickness regime. With both detectors, the damping-like torque exhibits a monotonic decay as the W thickness increases above 5 nm, which results from the structural phase transition from \b{eta}-W to α-W. The negative torque in the entire thickness regime suggests negligible orbital Hall torque and orbital current from the α-W. This result is consistent with the theory that the orbital Hall effect from simplified band structure calculations is not a non-local angular momentum source. These findings suggest poor generality or even universal absence of orbital current.