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 ($h_{\mathrm{DL}})$ 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 $h_{\mathrm{DL}}$, 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 $h_{\mathrm{DL}}$ 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 ($h_{\mathrm{FL}})$ 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.