Tunable Magneto-Excitonic Coupling in Alloyed van der Waals Antiferromagnet

The unique coupling between magnetic order and photo-generated excitons, electron-hole pairs bound by Coulomb interaction, in layered magnetic semiconductors offers a powerful mechanism for controlling light-matter interactions. In the van der Waals antiferromagnet CrSBr, this coupling is exceptionally strong and manifests distinctly between two coexisting excitonic states: the localised, Frenkel-like XA exciton and the more delocalised, Wannier-Mott-like XB exciton, providing a unique playground for the optical control of magnetism. Here, we reveal how chlorine incorporation reshapes the magneto-optical interplay in CrSBr1-xClx by simultaneously modifying its electronic structure, excitonic properties, and magnetic interactions. Combining magneto-optical spectroscopy up to 85 T with state-of-the-art quasiparticle self-consistent GW (QSGW) calculations on alloy supercells, we show that Cl insertion progressively localises the excitonic wavefunctions and drives both states toward a more Frenkel-like regime. This evolution is accompanied by a systematic reduction of the magnetic-field-induced energy renormalisation, most prominently for the XB exciton. Our work connects exciton character directly to magneto-excitonic coupling. Furthermore, it establishes compositional alloying as an effective strategy for engineering the coupling between magnetic and optical properties in van der Waals magnetic semiconductors.