Optically resolved exchange splittings in the doped van der Waals ferromagnet CrBr3:Yb3+

The chromium-trihalides (CrX3; X = Cl, Br, I) have long served as model systems for understanding magnetism and magneto-optics in ionic crystals and they have recently also emerged as archetypes of magnetic two-dimensional (2D) van der Waals materials. Although the physical properties of CrX3 compounds have now been explored in great depth, the use of impurity doping to generate new properties remains surprisingly underexplored. Here we report on the magnetic and magneto-optical properties of lanthanide-doped CrBr3, in which Yb3+ is introduced as a spin-bearing optical point defect. Narrow-line near-infrared dopant photoluminescence is efficiently sensitized by CrBr3-to-Yb3+ energy transfer. Magneto-optical measurements show facile spin manipulation of the paramagnetic Yb3+ impurities by the surrounding CrBr3 lattice. Photoluminescence data reveal that the Kramers spin degeneracies of the ground- and excited-state Yb3+ doublets are spontaneously lifted by strong magnetic exchange with neighboring Cr3+ ions, such that individual Yb3+ spin transitions are easily resolved even at zero external magnetic field. The temperature and field dependence of these splittings can be used to probe both local and long-range spin correlations. These results highlight the use of extrinsic optical impurities to add new spin-photonic functionality to this classic 2D van der Waals magnet.