Spin-orbit torque engineering via oxygen manipulation

Abstract: Spin transfer torques allow the electrical manipulation of the magnetization at room temperature, which is desirable in spintronic devices such as spin transfer torque memories. When combined with spin-orbit coupling, they give rise to spin-orbit torques which are a more powerful tool for magnetization control and can enrich device functionalities. The engineering of spin-orbit torques, based mostly on the spin Hall effect, is being intensely pursued. Here we report that the oxidation of spin-orbit torque devices triggers a new mechanism of spin-orbit torque, which is about two times stronger than that based on the spin Hall effect. We thus introduce a way to engineer spin-orbit torques via oxygen manipulation. Combined with electrical gating of the oxygen level, our findings may also pave the way towards reconfigurable logic devices.

• The paper introduces a method where oxidation in the CoFeB layer enhances spin-orbit torque, even reversing its direction compared to conventional spin Hall effects.
• The paper demonstrates that tuning the SiO2 capping layer thickness (around 1.5 nm) precisely controls oxygen content and modulates interfacial SOT effects.
• The paper confirms its findings using SIMS, XPS, XAS, and XMCD, linking oxygen-induced changes to altered interfacial spin-orbit coupling and magnetic properties.

Spin-orbit Torque Engineering via Oxygen Manipulation

The paper "Spin-orbit torque engineering via oxygen manipulation" authors propose a novel approach to enhancing spin-orbit torques (SOTs) in magnetic bilayers through the controlled manipulation of oxygen levels. Unlike the conventional methods that rely primarily on the spin Hall effect (SHE) in heavy metals (HMs) to determine the sign and magnitude of SOTs, this work introduces oxidation in the CoFeB layer as a significant factor influencing the behavior of SOT, presenting an alternative mechanism aside from the SHE.

Key Findings

1. Oxygen-Induced Torque Mechanism:
   • The presence of oxidation within the CoFeB layer contributes to a newly identified SOT mechanism that can be twice as powerful as the one predicted by the SHE.
   • Crucially, this mechanism not only enhances the SOT magnitude but also can reverse its direction while maintaining its magnitude, a phenomenon not explicable solely by the SHE.
2. Capping Layer Influence:
   • Modulating the thickness of the SiO2 capping layer allows for effective control of the oxygen content within the CoFeB layer.
   • A crucial threshold is identified at a capping layer thickness of approximately 1.5 nm, beyond which a sudden sign reversal of SOT occurs. This threshold matches the native oxide thickness of silicon, suggesting a precise control parameter for spintronic applications.
3. Experimental Verification:
   • Complementary techniques such as Secondary Ion Mass Spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) confirm the limited oxidation effect on the Pt layer, isolating the effect to the CoFeB layer.
   • X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) provide insight into the altered electronic and magnetic properties due to oxidation.
4. Interfacial Contributions:
   • The findings suggest the role of interfacial spin-orbit coupling at the HM/FM interfaces, likely influenced by the orbital characteristics of the oxidized FM layers.
   • Demonstrates the universality of the oxygen-induced SOT mechanism, as observed in both CoFeB and Co-based systems, extending the applicability of this mechanism across different ferromagnetic materials.

Implications and Future Directions

The discovery that oxygenation can dramatically alter the characteristics of SOT offers a promising path toward enhancing the performance of spintronic devices such as magnetic memory and logic elements. With significant implications for reducing power consumption in magnetization switching and enabling reconfigurable logic elements through electrical gating, this research invites further exploration into the precise nature of interfacial and bulk contributions to SOTs. Future research may focus on:

• Quantifying the contribution of interfacial spin-orbit coupling in varying material systems.
• Developing models to predict and optimize SOT behavior across different oxidation states and material stacks.
• Leveraging the insights gained for applications in low-power, high-speed domain wall motion and nano-oscillator technology.

This paper's findings bridge metal spintronics and oxide electronics, showcasing the potential for hybrid systems that capitalize on the merits of both fields. Therein lies an opportunity for further interdisciplinary research combining material science, surface chemistry, and electronic engineering to harness the full capabilities of spin-orbitronics for the next generation of technology.