Optically Resolved Exchange Splittings in the Doped van der Waals Ferromagnet CrBr₃:Yb³⁺

The chromium trihalides (CrX₃) have been widely studied for their magnetic properties that are maintained down to the monolayer. Despite their promising magnetic capabilities, integration into photonic devices has relied on pairing with other optically active 2D Van der Waals (VdW) materials due to the broad luminescence features and poor optical coherence of the chromium trihalides themselves. The spectral characteristics of CrX₃ can be enhanced through the introduction of optical dopant impurities, but doping of layered 2D VdW materials has thus far received little attention. Here, we investigate the magneto-optical properties of Yb³⁺-doped CrBr₃, in which Yb³⁺ serves as a spin-bearing optical point defect that is magnetically pinned to the Cr³⁺ magnetic lattice.
Photoluminescence excitation spectroscopy shows efficient energy transfer from Cr³⁺ to Yb³⁺, leading to full conversion of the broad Cr³⁺ d-d luminescence into narrow-line Yb³⁺ f-f emission. Magneto-optical measurements show that Yb³⁺ closely follows CrBr₃ magnetization, leading to full magnetic saturation of the paramagnetic Yb³⁺ at an external field of only 0.3 T. Consequently, the CrBr₃ magnetization can be optically monitored via Yb³⁺ photoluminescence (PL). Inversely, magnetic saturation of Yb³⁺ can be achieved at significantly lower fields than are typically required.
The highly resolved Yb³⁺ peaks allow for detailed analysis of the electronic structure of Yb³⁺ in CrX₃. Variable-temperature PL measurements reveal that magnetic-exchange interactions between Yb³⁺ and nearest-neighbor Cr³⁺ ions lead to energetic splitting of the Yb³⁺ spin-up and -down states, in the absence of an external magnetic field. From the magnitude of the exchange splitting, the Yb³⁺-Cr³⁺ exchange parameter is deduced to be half as large as the Cr³⁺-Cr³⁺ exchange parameter, indicating atypically strong Yb³⁺-Cr³⁺ interactions. Variable-field PL measurements demonstrate that Yb³⁺ is aligned antiferromagnetically to the CrBr₃ lattice in the ground state, but ferromagnetically in the excited state. These findings indicate that Yb³⁺ undergoes an optically-induced local spin reorientation upon photoexcitation. The large and resolved exchange splittings observed in CrBr₃:Yb³⁺ present unique opportunities for optical spin manipulation of point defects in the absence of external magnetic fields.