Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks.Outstanding examples are excitations with charge e/3 in the fractional quantum Hall effect 1,2 , solitons in one-dimensional conducting polymers 3,4 and Majorana states in topological superconductors 5 . Fractionalization is also predicted to manifest itself in lowdimensional quantum magnets, such as one-dimensional antiferromagnetic S = 1 chains.

We provide a comprehensive computational study of the magneto-optical response of ferromagnetic chromium trihalide monolayers (CrI₃, CrBr₃ and CrCl₃). We find that it is governed by the spin–orbit coupling of the ligand atoms (I, Br and Cl).