Observation of nontrivial spin-orbit torque in single-layer ferromagnetic metals

Abstract

We report the experimental observation of net spin-orbit torques (SOT) in single conductive ferromagnets (FMs), e.g., ${\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}(\mathrm{Py})$, Co, and Ni, despite the system symmetry constraint. Unlike the traditional FM/heavy metal (HM) bilayers, where only the in-plane (IP) damping-like SOT is presented, an unexpected out-of-plane (OOP) damping-like SOT is also unequivocally observed in the single FMs. Using the field angular-dependent, spin-torque ferromagnetic resonance technique, we quantify the IP and OOP SOT efficiencies of 0.018 \ifmmode\pm\else\textpm\fi{} 0.003 and 0.047 \ifmmode\pm\else\textpm\fi{} 0.002 in the pure Py, respectively. We argue that the IP SOT is primarily related to the Py bulk spin Hall effect, while the unconventional SOT is ascribed to the out-of-plane polarized spin current generated from the nonequilibrium spin swapping effect. Additionally, we confirm that such self-induced SOTs also generally exist in the most studied HM/FM multilayer systems, especially for the HMs with low conductivity. Hence, our findings provide a new perspective on understanding SOT control of magnetization reversal and dynamics by an in-plane current in thin, single FM and FM/HM bilayer systems.