Optically probing low energy magnetic excitations in bilayer CrI₃

Study of spin waves, collective magnetic excitations, in two-dimensional (2D) magnets emerges as one key yet new topic in the research of 2D magnetism. Among all the 2D magnets studied so far, bilayer CrI₃ stands out because of the realization of the layered antiferromagnetism, in which the spins order ferromagnetically within each layer and antiferromagnetically between the two layers. So far, the magnetic excitations in natural bilayer CrI₃ have been rarely studied, with the exception of inelastic magneto-tunneling spectroscopy results. A further question of how such magnetic excitations depends on the angular alignment between the two layers and the resulted long-range Moiré superlattice remains totally unexplored. Here, I will first present our ultra-low frequency Raman spectroscopy studies on the spin waves in natural bilayer CrI₃ and discuss its comparison to the inelastic magneto-tunneling spectroscopy. I will further show the magnetic field dependence of the spin waves in natural bilayer CrI₃. Furthermore, I will explore how these low energy spin waves depend on the twist angle between bilayer CrI₃.