Strong Rashba-Edelstein Effect-Induced Spin-Orbit Torques in Monolayer Transition-Metal Dichalcogenide/Ferromagnet Bilayers (1811.08583v1)

Abstract: The electronic and optoelectronic properties of two dimensional materials have been extensively explored in graphene and layered transition metal dichalcogenides (TMDs). Spintronics in these two-dimensional materials could provide novel opportunities for future electronics, for example, efficient generation of spin current, which should enable the efficient manipulation of magnetic elements. So far, the quantitative determination of charge current induced spin current and spin-orbit torques (SOTs) on the magnetic layer adjacent to two-dimensional materials is still lacking. Here, we report a large SOT generated by current-induced spin accumulation through the Rashba-Edelstein effect in the composites of monolayer TMD (MoS$_2$ or WSe$_2$)/CoFeB bilayer. The effective spin conductivity corresponding to the SOT turns out to be almost temperature-independent. Our results suggest that the charge-spin conversion in the chemical vapor deposition-grown large-scale monolayer TMDs could potentially lead to high energy efficiency for magnetization reversal and convenient device integration for future spintronics based on two-dimensional materials.

• The paper demonstrates that the Rashba-Edelstein effect produces significant field-like spin-orbit torques in TMD/CoFeB bilayers.
• It employs CVD-grown monolayer MoS2 and WSe2 with a second-harmonic method to quantify effective spin conductivities, revealing temperature-independent in-plane torque behavior.
• The research confirms negligible damping-like torques, highlighting the potential for advanced spintronic device engineering using adaptive charge-spin conversion.

Overview of a Study on Spin-Orbit Torques in TMD/Ferromagnet Bilayers

The paper "Strong Rashba-Edelstein Effect-Induced Spin-Orbit Torques in Monolayer Transition Metal Dichalcogenides/Ferromagnet Bilayers" focuses on elucidating the spin-orbit torques (SOTs) induced via the Rashba-Edelstein effect in bilayers of transition metal dichalcogenides (TMDs) and ferromagnetic layers. The investigation centers around monolayer TMDs, specifically MoS$_2$ and WSe$_2$, interfaced with CoFeB, revealing a substantial field-like torque contribution while uncovering negligible damping-like torque effects.

Highlights and Methodology

The primary objective of this research is to provide quantitative insight into the charge-spin conversion in two-dimensional (2D) TMD materials. This conversion is examined through the effective spin conductivity corresponding to SOTs. The researchers employed chemical vapor deposition (CVD) to grow large-scale monolayers of MoS$_2$ and WSe$_2$, followed by the deposition of a CoFeB ferromagnetic layer. The system was scrutinized using a second-harmonic method to quantify both field-like and damping-like torques.

Key observations include:

1. Field-Like Torque Dominance: The paper reports a significant in-plane spin-orbit effective field in MoS$_2$/CoFeB and WSe$_2$/CoFeB bilayers. Results indicate values of $H_{\parallel}$, Oe/mA of 0.13 and 0.19 for MoS$_2$ and WSe$_2$ respectively.
2. Temperature Independence: Spin conductivities were analyzed over different temperatures, revealing almost temperature-independent behavior for the in-plane effective spin conductivity. This suggests that the Rashba spin-splitting characteristics and the Fermi level position remain stable against temperature changes.
3. Negligible Damping-Like Torque: Contrary to materials known for strong damping-like torques, such as heavy metals, the paper finds negligible values in the TMD/CoFeB bilayer system, reinforcing the Rashba-Edelstein effect as the dominant mechanism.

Implications and Future Prospects

This research significantly contributes to the understanding of SOTs in TMD/ferromagnet bilayers. The Rashba-Edelstein effect observed in 2D materials like MoS$_2$ and WSe$_2$ expands potential applications in spintronic devices, which are increasingly reliant on efficient spin-current generation for lower power consumption and further miniaturization.

The paper proposes prospective developments that may enhance the charge-spin conversion efficiency, such as incorporating magnetic insulators to prevent shunting and further leveraging inverse processes. There is promise for refined device engineering by utilizing metallic TMD phases like 1T' WTe$_2$, known for producing unique torque characteristics.

Furthermore, the possibility of externally tuning the Rashba-Edelstein effect through gate voltages presents a compelling avenue for crafting devices with adaptive functionalities, thereby broadening the application horizon of spintronic devices in data storage, sensing, and beyond.

Conclusion

This paper provides a detailed paper into the interplay of the Rashba-Edelstein effect and spin-orbit torques in TMD/ferromagnet bilayer structures, offering a foundation for advanced 2D spintronic devices. It emphasizes the potential and challenges of integrating these materials in practical applications and outlines avenues for future exploration, especially concerning material synthesis and device architecture. The findings underscore the importance of exploring and understanding SOT phenomena in 2D materials for the future of efficient and scalable electronics.