Excitonic spin torque in a magnetic semiconductor

Excitons are electronic excitations in semiconductors that mediate the interaction between light and matter. They underpin a wide range of phenomena from optoelectronics to nonlinear optics and sensors. Here we show that a bound exciton reservoir in the two-dimensional magnetic semiconductor CrSBr exerts a spin torque that coherently acts as damping-like and anti-damping-like torques. Crucially, this excitonic spin torque drives a non-trivial trajectory on the magnetic potential energy surface, enabling rapid and repeated switching between antiferromagnetic configurations from a single optical pulse. Unlike traditional spin torques that are fixed by heterostructure symmetry and interface design, this excitonic spin torque is an intrinsic material property. Our results establish excitons as a practical control knob for optospintronic devices, bidirectional quantum transducers, spintronic memory and studies of non-equilibrium magnetic phase transitions. Distinct magnon dynamics are observed at high excitation pump power in the two-dimensional antiferromagnetic semiconductor CrSBr, and attributed to an interplay between excitons and magnetization.