Exploring moiré magnetism in twisted two-dimensional magnets

Moiré superlattice develops upon the interference between two mismatched periodic lattices and has been realized in two-dimensional (2D) hetero- and homo-structures made of two vertically stacked atomic crystals. The introduction of moiré superlattice has shown tremendous impact on modifying the electronic and excitonic band structures and hence realizing novel phases of matter. In parallel with controlling charge degree of freedom (DoF), it has been recently predicted that moiré superlattice also has the power of modifying spin DoF and designing new magnetic phases. In this presentation, I will show our recent experimental effort in studying moiré magnetism in an archetypical system, twisted double bilayer CrI₃ whose building block, bilayer CrI₃, is a collinear A-type antiferromagnet with zero magnetization and strong easy-axis anisotropy. I will show how the competition between interlayer ferromagnetic (FM) and antiferromagnetic (AFM) exchange coupling within moiré supercells leads to a new magnetic state with nonzero magnetization and noncollinear spins. Given such a new magnetic ground state, I will then discuss how the spin wave excitation spectra are modified and whether any new magnetic excitations are captured. Finally, I will comment on other 2D magnets that we are working on to study moiré magnetism.