Stacking dependent 2D magnetism

In van der Waals layered materials, the symmetries and functionalities could be controlled by modifying the stacking order through rotation and translation between the layers. Whereas most previous work has focused on the electronic and optical properties associated with the van der Waals stacking, the recent discovery of magnetism in 2D materials, achieved through both mechanical exfoliation and molecular beam epitaxy (MBE), provides an exciting opportunity to explore the effects of stacking order on a material's magnetic properties. In this talk, I will present two of our recent studies in this direction. 1) By discovering a giant nonreciprocal second harmonic generation from layered antiferromagnetism in mechanically exfoliated bilayer CrI3, we revealed the underlying C2h symmetry, and thus monoclinic stacking order in bilayer CrI3, providing crucial structural information for the microscopic origin of layered antiferromagnetism. 2) By using the molecular beam epitaxy to grow monolayer and bilayer CrBr3 and in situ characterization with a spin-polarized scanning tunneling microscopy and spectroscopy, we observed that while individual CrBr3 monolayer is ferromagnetic, the interlayer coupling in bilayer depends strongly on the stacking structure and can be either ferromagnetic or antiferromagnetic. Thus, the direct correlation between stacking order and interlayer magnetism pave the way for manipulating 2D magnetism.