Raman spectroscopy studies of spin waves in two-dimensional CrI₃

The long-range magnetic order in the two-dimensional (2D) limit at finite temperatures has recently been demonstrated in a honeycomb Ising ferromagnet CrI₃. It has triggered tremendous interest in 2D magnet-based applications. To achieve a complete understanding of the 2D magnetic phase, a direct measurement of the magnetic excitations, i.e., spin waves, or magnons, is pressingly in need. In this talk, we will present the experimental evidence of spin waves in the 2D honeycomb ferromagnet, CrI₃. Using polarized micro-Raman spectroscopy, we have identified two branches of zero-momentum spin waves in 2D CrI₃ with exceptionally high frequencies in the THz range, as compared to those of the conventional ferromagnets. By tracking the layer thickness dependence of both spin waves, we further show that both are surface magnons, and their lifetimes remain an order of magnitude longer than their temporal period down to the monolayer limit.