Optical Spin Sensing and Metamagnetic Phase Control in the 2D Van der Waals Magnet Yb³⁺-Doped CrPS₄

The emergence of two-dimensional magnets within the van der Waals toolkit has introduced extraordinary opportunities to develop ultrathin spintronic technologies. Strong coupling between spin and optical properties in such materials can further enable powerful spin-photonic capabilities of both fundamental and technological interest. Here, we investigate the optical and spin properties of the air-stable, layered A-type antiferromagnet chromium thiophosphate (CrPS₄) when doped with Yb³⁺. We show that the collective spin properties of CrPS₄ are encoded in the sharp f-f luminescence of isolated Yb³⁺ dopants via strong magnetic superexchange coupling between the two, and that spontaneous magnetic ordering in CrPS₄ induces large exchange splittings in the narrow Yb³⁺ f-f photoluminescence features below T_N. Spin reorientation in CrPS₄ via a "spin-flop" metamagnetic transition modulates the Yb³⁺ f-f luminescence energies and exchange splittings. This pronounced link between spin and optical properties enables the demonstration of optically driven spin-flop transitions in CrPS₄.