Giant excitonic and magneto-optical responses in two-dimensional ferromagnets*

The magneto-optical (MO) effects, such as the magneto-optical Kerr effect (MOKE) and the Faraday effect, have been intensively investigated in a variety of magnetic materials serving as a highly sensitive probe for electronic and magnetic properties. Recent experiments using MOKE have discovered a few two-dimensional (2D) magnets, and demonstrated their rich magnetic behaviors. In particular, a giant Kerr response has been measured in monolayer and few-layer CrI₃. However, by far, the microscopic origin of such MO signals is still unknown, because the essential spin-orbit coupling and excitonic effects are beyond the capability of existing first-principles methods. With newly developed GW and GW-BSE methods, we show that the exceedingly large optical and MO responses in ferromagnetic monolayer CrI₃ arise from the strongly bound exciton states consisting of spin-polarized electron-hole states. With a realistic experimental setup, we find that the substrate configuration and excitation frequency strongly shape the MO signals. Our first-principles results are in good agreement with recent experiments on CrI₃.