Colossal magneto-excitonic effects in 2D van der Waals magnetic semiconductor CrSBr

2D magnetic semiconductors, which intrinsically couple a rich landscape of magnetic orders with tightly bound electron-hole pairs (excitons), present an exciting platform to investigate the interplay between optical and magnetic phenomena at the atomic scale. In such systems, the strength of magneto-optical effects determines how deeply the magnetic properties can be revealed. Here, we report the observation of remarkably strong and unusual magneto-excitonic effects in the 2D magnetic semiconductor CrSBr that allow probing its magnetic order with unprecedented sensitivity. By investigating optical transitions above the fundamental exciton energy, we discover massive spectral changes of the energy of those transitions (both red- and blue-shifts) approaching 100 meV under applied magnetic fields. Such massive shifts are an order of magnitude larger than previously observed magneto-excitonic responses. Our comprehensive magneto-optical experiments, accompanied by detailed DFT calculations, indicate that the likely origin of such intriguing behavior is a field-induced change in the CrSBr band structure. These findings open avenues for exploiting magneto-excitonic phenomena in newly accessible regimes, enabling novel opto-spintronic applications previously limited by weak magnetic responses.