Phonon Anharmonicity and Spin–Phonon Coupling in CrI[sub.3]

We report on the far-infrared, temperature-dependent optical properties of a CrI[sub.3] transition metal halide single crystal, a van der Waals ferromagnet (FM) with a Curie temperature of 61 K. In addition to the expected phonon modes determined by the crystalline symmetry, the optical reflectance and transmittance spectra of CrI[sub.3] single crystals show many other excitations as a function of temperature as a consequence of the combination of a strong lattice anharmonicity and spin–phonon coupling. This complex vibrational spectrum highlights the presence of entangled interactions among the different degrees of freedom in CrI[sub.3].