Giant spontaneous valley polarization in two-dimensional ferromagnetic heterostructures

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 p_x and p_y 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.