  We report on large modifications of current-induced spin-orbit torques in a
gated Pt/Co/Gd-oxide microstrip due to voltage-driven O$^{2-}$ migration. The
Slonczewski-like and field-like torques are quantified using a low-frequency
harmonic technique based on the polar magneto-optical Kerr effect.
Voltage-induced oxidation of Co enhances the Slonczewski-like torque by as much
as an order of magnitude, and simultaneously reduces the anisotropy energy
barrier by a factor of ~5. Such magneto-ionic tuning of interfacial spin-orbit
effects may significantly enhance the efficiency of magnetization switching and
provide additional degrees of freedom in spintronic devices.
