Bulk and Interface Spin-Orbit Torques in Pt/Co/MgO Thin Film Structures

We investigate the origin of spin-orbit torques (SOTs) in archetypical Pt/Co/MgO thin films structures by performing harmonic Hall measurements. The behavior of the dampinglike (DL) effective field (hDL) with varying the Pt layer thickness and the Co layer thickness indicates that bulk spin-Hall effect (SHE) in Pt is mainly responsible for DL-SOT. The insertion of a Pd ultrathin layer at the Pt/Co interface leads to a step decrease in hDL, attributed to the modification of interfacial spin transparency. Further increase in Pd thickness leads to a reduction of the interfacial spin-orbit coupling (iSOC) quantified by the decrease in the surface magnetic anisotropy. The consistent insensitivity of hDL to variations in iSOC at the bottom Pt/Co interface and oxidation at the top Co/MgO interface provides additional evidence for the bulk SHE origin of DL-SOT. The strong reduction in the fieldlike (FL) torque effective field (hFL) with decreasing iSOC at the Pt/Co interface points to the interfacial nature of FL-SOT, either due to iSOC-induced interfacial spin currents or to the Rashba-Edelstein effect at the Pt/Co interface. Furthermore, we demonstrate that a FL-SOT develops at the top Co/MgO interface opposing the one generated at the bottom Pt/Co interface, the strength of which increases with Co/MgO interfacial oxidation, and is attributed to the Rashba-Edelstein effect.