Bulk versus Interface Contributions to the Spin-Orbit Torque in Ferromagnetic Heterostructures*

We present an ab initio-based theoretical framework which elucidates the origin of the bulk versus interface contributions to the spin-orbit torque (SOT) in Heavy-Metal(HM)/Ferromagnet(FM) heterostructures. We find that
(i) the Field-Like (FL) SOT is dominated by the interface component and (ii) both components contribute to the Damping-Like (DL) SOT on equal footing. We demonstrate that the FL-SOT can be expressed in terms of the non-equilibrium spin-resolved orbital moment accumulation.
The calculations reveal that the experimentally reported oxygen-induced sign-reversal of the FL-SOT in Pt/Co bilayers is due to the significant reduction of the majority-spin orbital moment accumulation on the interfacial HM atoms.