  Spin-Orbit Torque (SOT) Magnetic Random-Access Memories (MRAM) have shown
promising results towards the realization of fast, non-volatile memory systems.
Oxidation of the heavy-metal (HM) layer of the SOT-MRAM has been proposed as a
method to increase its energy efficiency. But the results are widely divergent
due to the difficulty in controlling the HM oxidation because of its low
enthalpy of formation. Here, we reconcile these differences by performing a
gradual oxidation procedure, which allows correlating the chemical structure to
the physical properties of the stack. As an HM layer, we chose Pt because of
the strong SOT and the low enthalpy of formation of its oxides. We find
evidence of an oxide inversion layer at the FM/HM interface: the oxygen is
drawn into the FM, while the HM remains metallic near the interface. We further
demonstrate that the oxygen migrates in the volume of the FM layer rather than
being concentrated at the interface. Consequently, we find that the intrinsic
magnitude of the SOT is unchanged compared to the fully metallic structure. The
previously reported apparent increase of SOTs is not intrinsic to platinum
oxide and instead arises from systemic changes produced by oxidation.
