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Giant antidamping orbit torque originating from the orbital Rashba-Edelstein effect

Current-induced spin-orbit torque in ferromagnetic heterostructures with strong spin-orbit coupling has great potentials for data processing applications in the next-generation spintronic devices. However, the underlying mechanisms for the generation of spin-orbit torque, the spin Hall effect or the interfacial Rashba-Edelstein effect, is still under debate. Moreover, the spin-orbit torque efficiency is reported to be low, leading to a high critical current density for current-induced magnetization switching, which impedes the practical applications of spin-orbit torques. Here, we show that the combination effect of Pt 5d-Co 3d orbital hybridization at Pt/Co interface and Co 3d-O 2p orbital hybridization at Co/SiO2 interface in Pt/Co/SiO2 heterostructures can lead to a giant torque efficiency with magnitude of 2.83, which is several times to an order of magnitude large than previous reported values in Pt-based films. The torque efficiency was found to be strong magnetization direction- and temperature dependent, which cannot be understood in the framework of conventional spin Hall effect and Rashba-Edelstein effect. Based on its interface-originated and anisotropic nature, we proposed an orbital Rashba-Edelstein effect due to current-induced orbital polarization to elucidate our observations. This work highlights the active role of the orbital anisotropy for efficient torque generation and indicates a route for torque efficiency optimization through orbital engineering.  

 

Xi Chen, Yang Liu, Guang Yang, Hui Shi, Chen Hu, Minghua Li & Haibo Zeng“Giant antidamping orbital torque originating from the orbital Rashba-Edelstein effect in ferromagnetic heterostructures”Nature Communications 92569 (2018)

 

Figure a: Variation of antidamping torque efficiency as a function of the angle θ between the magnetization and the film normal direction;

Figure b: Dependence of the anisotropy of antidamping torque (|TAD 2+TAD 4|) and effective magnetic ansiotropy (Keff) on measuring temperature;

Figure c: Pt 5d-Co 3d orbital hybridization at Pt/Co interface and Co 3d-O 2p orbital hybridization at Co/SiO2 interface leads to an enhanced crystal electric field (ECEF) across the Co layer;

Figure d: Illustration of the orbital Rashba-Edelstein effect. The dash circles and the solid circles represent the Fermi surfaces before and after current (j) injection; ΔL is the current-induced orbital polarization.