  The Van der Waals semiconductor metamagnet CrSBr offers an ideal platform for
studying the interplay between optical and magnetic properties in the
two-dimensional limit. Here, we carried out an exhaustive optical
characterization of this material by means of temperature and magnetic field
dependent photoluminescence (PL) on flakes of different thicknesses down to the
monolayer. We found a characteristic emission peak that is quenched upon
switching the ferromagnetic layers from an antiparallel to a parallel
configuration and exhibits a different temperature dependence from that of the
peaks commonly ascribed to excitons. The contribution of this peak to the PL is
boosted around 30-40 K, coinciding with the hidden order magnetic transition
temperature. Our findings reveal the connection between the optical and
magnetic properties via the ionization of magnetic donor vacancies. This
behavior enables a useful tool for the optical reading of the magnetic states
in atomically thin layers of CrSBr and shows the potential of the design of
two-dimensional heterostructures with magnetic and excitonic properties.
