Collective Excitations in Two-Dimensional Magnetic Atomic Crystals and Moiré Superlattices Probed by Magneto-Raman Spectroscopy

Two-dimensional (2D) magnetism has been long sought-after since its theoretical investigations in the 1960s–1970s. It is only very recently that 2D magnetism is realized in magnetic atomic crystals and further tailored in magnetic moiré superlattices. Due to the small sample sizes of atomic and moiré crystals, traditional diffraction-based techniques become extremely challenging, and advanced optical spectroscopy techniques turn out to be uniquely suitable for probing magnetic properties in 2D magnets – both static and dynamic ones. In this tutorial, we will focus on collective excitations of 2D magnetism probed by magneto-Raman spectroscopy. Using CrI₃ atomic crystals as an example, we will discuss four types of collective excitations in it: phonon for lattice, magnetism-coupled phonon for static magnetic order, magnon for dynamic magnetic excitations, and polaron for phonon-dressed excitons. Then applying such acquired knowledge into twisted double bilayer CrI₃ moiré superlattices, we will reveal unexpected net magnetization and potentially noncollinear spin textures that are absent in the layered antiferromagnetic CrI₃ bilayer and four-layer films.