Erbium Probes of Magnetic Order in a Layered van der Waals Material

There is growing interest in characterizing magnetic order and dynamics in two-dimensional magnets, yet most efforts to date rely on external probes that interrogate the sample from tens of nanometers away and inevitably average over that length scale. Here we use internal, lattice-embedded Er3+ defects in CrSBr as atomic-scale probes, accessing their telecom-band photoluminescence with spectroscopy and temperature-dependent confocal imaging to read out magnetism from within the material. At room temperature we observe narrow, long-lived photoluminescence (PL) lines in the telecom band, characteristic of erbium emitters. Upon cooling to 3 K and reheating, the Er3+ PL intensity and excited-state lifetime display pronounced thermal hysteresis with a minimum near 132 K, at the reported antiferromagnetic (AFM) transition of CrSBr. Remarkably, we observe magnetic signatures persisting over a broader temperature range than expected from bulk benchmarks, suggesting nanoscale magnetic order that locally survives beyond the nominal phase boundary. Further, a moderate in-plane field of 0.3 T shifts the PL minimum by +8 K, which we tentatively associate to field-biased ferromagnetic correlations.