Giant spontaneous valley polarization in two-dimensional ferromagnetic heterostructures

Abstract

Screening two-dimensional (2D) materials with inherent out-of-plane magnetization is the key to spontaneous valley polarization. Based on first-principles calculations, the thermodynamic stability, magnetic orders and electronic band structures of 2D ScX2 (X = Cl, Br and I) monolayers and their van der Waals junctions are studied to identify potential valley materials, while their monolayers and homo-structural bilayers exhibit intrinsic in-plane magnetization. Particularly, ScI2 is found to have a strongest valley polarization effect when its magnetization direction is shifted to the z direction. A strategy is proposed to achieve out-of-plane magnetization by creating hetero-structures with monolayer MSe2 (M=Zr, Hf and Sn). All these constructed heterostructures display out-of-plane magnetization with enhanced valley splitting. The predicted strongest valley splitting reaches about 121 meV in the heterostructure ScI2/ZrSe2, which is much larger than that in the pristine ScI2 monolayer, demonstrating enhanced valley polarization that results from both the compressed ScI2 lattice and the interlayer interaction with MSe2. It is noted that the hybridization of px and py orbitals of I atoms is increased in heterostructures and is responsible for magnetization variation. Our study not only extends the family of 2D spontaneous valley polarization, but also provides in-depth insights for the fundamental investigations of 2D valleytronics.