Erbium Probes of Magnetic Order in a van der Waals Material

Embedded atomic emitters offer a complementary approach to characterizing magnetism in two-dimensional materials by accessing the magnetic environment from within. Here, we demonstrate that ion-implanted Er3+ centers act as internal optical probes in the van der Waals antiferromagnet CrSBr. Temperature-dependent telecom photoluminescence reveals signatures of the magnetic transition, persistent local magnetic behavior above the bulk Néel temperature, pronounced thermal hysteresis, and redistribution of optical spectral weight near the transition. Upon cooling, the Er resonances sharpen to the instrumental limit, pointing to remarkably homogeneous local environments despite implantation. Measurements spanning a 20-fold range of implantation fluence, structural characterization, and first-principles calculations suggest that the detected emission originates predominantly from an optically active subset of Er centers occupying well-defined environments and minimally perturbing the host magnetism. These results establish rare-earth defects as embedded probes of local magnetism and open opportunities for quantum-optical interrogation at individual lattice sites.