Magneto-optical properties of CrI₃, CrBr₃ and CrCl₃ monolayers: chemical trends and excitonic effects

Chromium trihalides (CrI₃, CrBr₃ and CrCl₃) are a prominent family of isostructural insulating layered materials in which ferromagnetic order has been observed down to the monolayer. Here we provide a comprehensive computational study of the magneto-optical properties useful to probe ferromagnetic order of monolayers, magnetic circular dichroism and magneto-optic Kerr effect. We combine density functional theory calculations and many-body perturbation theory. We solve the Bethe-Salpeter to obtain the excitonic states and to calculate the optical absorption and Kerr angle spectra. We compare the magneto-optical response of the chromium trihalides series. We find strong excitonic effects on the optical response. Moreover, the Kerr angle spectrum is governed by the spin-orbit coupling of the ligand atoms (I, Br or Cl) and is a fingerprint of the degree of magnetic anisotropy of these compounds.