Role of interfacial oxidation in generation of spin-orbit torques

Abstract

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/Ni${}_{81}$Fe${}_{19}$ (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 CoO${}_x$ layer. The enhancement of the SOTs disappears by inserting a thin Ti layer at the Pt/CoO${}_x$ interface, indicating that the dominant source of the SOTs in the Pt/CoO${}_x$/Co film is the spin-orbit coupling at the Pt/CoO${}_x$ interface. In contrast to the Pt/CoO${}_x$/Co film, the SOTs in the Pt/PyO${}_x$/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 PyO${}_x$ layer. These results show that the interfacial oxidation provides an effective way to manipulate the strength and sign of the SOTs.

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