Terahertz Time-Domain Spectroscopy of the van der Waals Ferromagnet CrI₃

CrI₃ is a ferromagnetic van der Waals insulator with Ising-type long-range ordering below the Curie temperature of 61 K. We have studied the low-frequency electrodynamics of bulk CrI₃ single crystals by using terahertz time-domain spectroscopy at various magnetic fields and temperatures. We identified a zero-field magnon mode at 2.5 cm^-1 at 1.5 K and modeled the magnetic Hamiltonian based on the behavior of the magnon under the external magnetic field along each crystal axis. Also, the evidence of coupling between the crystal structure and magnetism was discovered from the temperature and magnetic field dependence of certain E_u in-plane optical phonon modes. Our results suggest a strong the Kitaev-type interaction and present the correct structure-field phase diagram of bulk CrI₃.