- The paper presents a novel methodology using harmonic voltage measurements to separate SOT and thermoelectric contributions in FM/NM bilayers.
- The analysis shows that Joule heating-induced thermal gradients can significantly skew antidamping SOT estimates, particularly in Ta/Co systems.
- The findings provide actionable insights for optimizing spintronic device design by accurately decoupling intertwined physical phenomena.
The paper presents a detailed investigation into the interplay of spin-orbit torque (SOT) and thermoelectric effects in ferromagnet/normal metal (FM/NM) bilayers. Through harmonic transverse voltage measurements, the paper aims to quantify and separate the complex interactions between Joule heating-induced thermal gradients and SOTs, which coexist and produce alternating current (AC) transverse signals with comparable symmetry and magnitude.
Theoretical and Methodological Advances
The authors establish a methodology to distinguish between thermoelectric and spin-orbit torque contributions in FM/NM bilayers, particularly in systems with different spin-orbit coupling strengths. The proposed method relies on leveraging the symmetry and field-dependence of the transverse resistance measurements. This is achieved by performing harmonic analysis of the transverse voltage signals under the application of in-plane current and external magnetic fields.
Experimental Results
A broad set of FM/NM bilayers are investigated, including those with negligible spin-orbit coupling (such as Ti/Co) and those with strong spin-orbit coupling (such as Pt/Co and Ta/Co). The studies in bilayers with negligible spin-orbit coupling confirm that vertical thermal gradients can be induced by an in-plane current flow, dictated by the resistivity difference and layer order within the stack. The authors provide evidence of significant thermoelectric effects in bilayers like Ta(6nm)/Co(2.5nm) and almost negligible effects in Pt(6nm)/Co(2.5nm).
Quantitative Insights
One of the critical quantitative insights of the paper is the observation that second harmonic thermoelectric contributions can lead to a considerable overestimation of the antidamping SOT, especially in systems such as Ta/Co bilayers. After accurately accounting for these effects, the paper finds that normalized antidamping SOT fields are comparable to those found in thinner Co layers with perpendicular magnetization, while field-like SOTs are an order of magnitude smaller.
Implications and Future Directions
The ability to accurately separate and measure SOT and thermoelectric effects is particularly important in the design and optimization of spintronic devices, where precise control over magnetic states is required. The approaches demonstrated in this paper could pave the way for more nuanced experimental setups that can decouple intertwined physical phenomena, facilitating advanced device architectures and novel functionalities leveraging thermoelectric as well as spin-orbit interactions.
Future exploration in this domain could focus on the impact of different materials and configurations, potentially offering new insights into the interplay between spin, charge, and thermal transport phenomena. Enhancing the comprehension of these interdependent systems could stimulate the design of more efficient thermoelectric materials and spintronic applications, possibly influencing fields such as energy conversion and magneto-transport device engineering.
In conclusion, this paper contributes significantly to the understanding of complex interactive phenomena in FM/NM bilayers, providing a foundational methodological framework that can enhance the analysis and practical utilization of SOT and thermoelectric effects in advanced materials.