  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.
