Recently, Professor Xiao Li, from the School of Physics and Technology at Nanjing Normal University (NNU), has made important research progress in current-induced spin polarization. The paper, titled “Out-of-plane carrier spin in transition-metal dichalcogenides under electric current”, was published in Proceedings of the National Academy of Sciences of the United States of America (PNAS). NNU Professor Xiao Li is the first author of this paper, while Hua Chen, a professor from the Colorado State University, US is the co-corresponding author and Professor Qian Niu from the University of Texas at Austin, US is the co-author of this paper.
In the current digital age, magnetic electronic components are an indispensable part of various smart devices. The high-speed transmission of information depends on the fast and reliable recording and reading of information by these magnetic devices, which means effective control of the magnetic properties of materials is conducive to the development of the information age. Compared with the magnetic field, the energy-efficient electrical methods make magnetic properties more controllable. And among those methods, the current-induced spin polarization (CISP) and the associated spin-orbit torque are effective ways to adjust the magnetic properties by electrical means. Non-equilibrium carrier spin and associated spin-orbit torque are induced by an applied electric current in a strong spin-orbit coupled electron system, which has practical use in magnetization switching and domain-wall motion for novel spintronic devices. Currently, the research of CISP focuses more on Rashba-type spin-orbit coupling and heterostructure. Only in-plane induced spin emerges with the redistribution of carriers on the Fermi surface under an electric current, in contrast with a vanishing normal component. However, it is highly desirable to realize out-of-plane carrier spins and magnetization reorientation. The multi-directional control of magnetic properties and the controlled evolution of electronic properties require the exploration of new out-of-plane CISP and corresponding prototype materials.
In this work, the research team proposed that ferromagnetic transition-metal dichalcogenides can achieve vertical CISP. Transition-metal dichalcogenides having attracted much attention are the kinds of valley electronic materials with energy valley degree of freedom and their energy valley structures show different degenerate states depending on the magnetization direction. By symmetry analysis, first-principles calculations and linear response theory, the research team took VSe2 monolayer and VTe2 monolayer for examples and found that the out-of-plane carrier spin can emerge in ferromagnetic transition-metal dichalcogenides monolayer. This out-of-plane CISP comes from the destruction of the rotational symmetry of the electronic structure caused by the intrinsic spin-orbit coupling of the material and the in-plane magnetic sequence. They constructed an energy-efficient model distinct from widely discussed Rashba systems, explaining the related numerical findings. Their proposed out-of-plane CISP is expected to achieve electrical regulation of magnetic properties and energy valley structures, providing a new strategy for designing new spin and energy valley devices.
This work was mainly supported by the National Natural Science Foundation Youth Project and the Jiangsu Distinguished Professor Program.