Abstract
Oxygen incorporation has been reported to increase the current-induced spin-orbit torque in ferromagnetic heterostructures, but the underlying mechanism is still under active debate. Here, we report on an in-situ study of the oxygen exposure effect on spin-orbit torque in Pt/Co bilayers via controlled oxygen exposure, Co and Mg deposition, and electrical measurements in ultrahigh vacuum. We show that the oxygen exposure on Pt/Co indeed leads to an increase of spin-orbit torque, but the enhancement is not as large as those reported previously. Similar enhancement of spin-orbit torque is also observed after the deposition of an MgO capping layer. The results of ab initio calculations on the Rashba splitting of Pt/Co and Pt/Co/O suggest that the enhancement is due to enhanced Rashba-Edelstein effect by surface-adsorbed oxygen. Our findings shed some light on the varying roles of oxygen in modifying the spin torque efficiency reported previously.
Highlights
Oxygen incorporation has been reported to increase the current-induced spin-orbit torque in ferromagnetic heterostructures, but the underlying mechanism is still under active debate
This is important because in our previous studies we have unveiled that the effects of oxygen and oxide are different when it comes to the influence on perpendicular magnetic anisotropy (PMA)[30]; oxide capping layer (e.g., MgO) enhances PMA of Co whereas oxygen adsorbed on the surface of Co brings about the different effect, i.e., decreasing the magnetic moment
As we reported previously[30], Pt (2)/Co (t) exhibits in-plane magnetic anisotropy (IMP) when t = 0.2–0.5 nm and clear perpendicular magnetic anisotropy (PMA) when t is in the range of 0.6–1.2 nm beyond which it goes back to IMP
Summary
Oxygen incorporation has been reported to increase the current-induced spin-orbit torque in ferromagnetic heterostructures, but the underlying mechanism is still under active debate. In all the experiments reported so far, oxygen is incorporated in the form of oxide, and it is difficult to ascertain if the effects observed are due to the oxide layer in direct contact with the ferromagnet or atomic/molecular oxygen This is important because in our previous studies we have unveiled that the effects of oxygen and oxide are different when it comes to the influence on perpendicular magnetic anisotropy (PMA)[30]; oxide capping layer (e.g., MgO) enhances PMA of Co whereas oxygen adsorbed on the surface of Co brings about the different effect, i.e., decreasing the magnetic moment. Thermal annealing can change the oxygen distribution inside the sample within a certain range, it is difficult to control the oxygen dose and distribution in a quantitative manner In this context, in this work, we conducted a well-controlled oxygen exposure study of spin-orbit torque in Pt/Co heterostructures in an in-situ setup. These results demonstrate the importance of oxygen in modifying the surface and interface states, thereby enhancing the SOT
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