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 (CrPS 4 ) when doped with Yb 3+ . We show that the collective spin properties of CrPS 4 are encoded in the sharp f – f luminescence of isolated Yb 3+ dopants via strong magnetic superexchange coupling between the two, and that spontaneous magnetic ordering in CrPS 4 induces large exchange splittings in the narrow Yb 3+ f – f photoluminescence features below T N . Spin reorientation in CrPS 4 via a “spin-flop” metamagnetic transition modulates the Yb 3+ 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 CrPS 4 .