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

Atomic-scale defects in van der Waals (vdW) materials are emerging building blocks for quantum technologies and precision sensing. The layered magnetic semiconductor CrSBr, which combines a direct bandgap with a rich magnetic phase diagram, is a compelling host for optically active dopants. Here we implant Er³⁺ ions into exfoliated CrSBr flakes and use confocal microscopy, temperature-dependent optical spectroscopy to examine their emission. 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—close to the reported antiferromagnetic (AFM) transition of CrSBr. Notably, the temperature dependence indicates AFM-like signatures persisting over a broader range than expected from bulk benchmarks, consistent with nanoscale (≈nm) magnetic order that locally survives beyond the nominal phase boundary. These magneto-optical responses establish telecom-compatible Er³⁺ defects as sensitive local probes of magnetic textures and dynamics in a 2D vdW semiconductor, and point toward fiber-networked spin–photon interfaces in layered magnets.