Out-of-plane carrier spin in transition-metal dichalcogenides under electric current.

Absence of spatial inversion symmetry allows a nonequilibrium spin polarization to be induced by electric currents, which, in two-dimensional systems, is conventionally analyzed using the Rashba model, leading to in-plane spin polarization. Given that the material realizations of out-of-plane current-induced spin polarization (CISP) are relatively fewer than that of in-plane CISP, but important for perpendicular-magnetization switching and electronic structure evolution, it is highly desirable to search for new prototypical materials and mechanisms to generate the out-of-plane carrier spin and promote the study of CISP. Here, we propose that an out-of-plane CISP can emerge in ferromagnetic transition-metal dichalcogenide monolayers. Taking monolayer [Formula: see text] and [Formula: see text] as examples, we calculate the out-of-plane CISP based on linear-response theory and first-principles methods. We deduce a general low-energy model for easy-plane ferromagnetic transition-metal dichalcogenide monolayers and find that the out-of-plane CISP is due to an in-plane magnetization together with intrinsic spin-orbit coupling inducing an anisotropic out-of-plane spin splitting in the momentum space. The CISP paves the way for magnetization rotation and electric control of the valley quantum number.