  We investigate the origin of spin-orbit torques (SOTs) in archetypical
Pt/Co/MgO thin films structures by performing harmonic Hall measurements. The
behaviour of the damping like (DL) effective field ($h_{DL}$) 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 $h_{DL}$,
attributed to the modification of interfacial spin transparency. Further
increase in Pd thickness led to a reduction of the interfacial spin-orbit
coupling (iSOC) quantified by the decrease in the surface magnetic anisotropy.
The consistent insensitivity of $h_{DL}$ 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
field-like (FL) torque effective field ($h_{FL}$) 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,
whose strength increases with Co/MgO interfacial oxidation, and attributed to
the Rashba-Edelstein effect.
