Two-dimensional Spin Waves in CrI₃ Atomic and Moiré Crystals

Van der Waals magnets can not only host distinctive spin physics on their own, but also harbor the unprecedented tunability of magnetism and magnetic excitations. Very recent progress has been made to understand the magnetic ground states in atomically thin CrI₃ samples, both in the form of natural few-layers and twisted moiré superlattices. However, the exploration of spin waves­–collective magnetic excitations–has been limited to natural bilayer CrI₃, more specifically, in the ferromagnetic phase above the spin flip transition at B_C = 0.6 T. In this talk, we present a comprehensive polarization-resolved magneto-Raman study of spin waves in natural few-layer (monolayer to four-layer) and twisted double bilayer CrI₃. We will pay particular attention to the spin waves in the antiferromagnetic phase in natural few-layer CrI₃, and compare them to those in the twisted double bilayer CrI₃. We will then move onwards to discuss the magnetic field dependence of spin waves in both types of systems. And finally, we will comment on the spin wave behaviors in twisted double bilayer CrI₃ in the context of their reported magnetic ground states.