Role of interfacial oxidation in the generation of spin-orbit torques

We report that current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnetic-metal bilayers are strongly altered by the oxidation of the ferromagnetic layer near the interface. We measured damping-like (DL) and field-like (FL) SOTs for Pt/Co and Pt/Ni81Fe19 (Pt/Py) films using spin-torque ferromagnetic resonance. In the Pt/Co film, we found that the oxidation of the Co layer near the interface enhances both DL and FL SOTs in spite of the insulating nature of the CoOx layer. The enhancement of the SOTs disappears by inserting a thin Ti layer at the Pt/CoOx interface, indicating that the dominant source of the SOTs in the Pt/CoOx/Co film is the spin-orbit coupling at the Pt/CoOx interface. In contrast to the Pt/CoOx/Co film, the SOTs in the Pt/PyOx/Py film are dominated by the bulk spin-orbit coupling. Our result shows that the interfacial oxidation of the Pt/Py film suppresses the DL-SOT and reverses the sign of the FL-SOT. The change of the SOTs can be attributed to the change of the real and imaginary parts of the spin mixing conductance induced by the insertion of the insulating PyOx layer. These results show that the interfacial oxidation provides an effective way to manipulate the strength and sign of the SOTs.