Mechanism of Spin-Orbit Torques in Platinum Oxide Systems

TL;DR

This study investigates how controlled oxidation of platinum layers affects spin-orbit torques in SOT-MRAM, revealing that intrinsic SOT remains unchanged and previous apparent enhancements are due to systemic effects.

Contribution

It introduces a gradual oxidation method to clarify the effects of platinum oxide on SOT, showing that intrinsic SOT is unaffected by oxidation.

Findings

Oxide inversion layer forms at the FM/HM interface.

Oxygen migrates into the FM layer, not just at the interface.

Intrinsic SOT magnitude remains unchanged despite oxidation.

Abstract

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.