Abstract: Van der Waals magnets provide a fertile setting in which reduced dimensionality, enhanced fluctuations, topology, and strong correlations reshape familiar forms of magnetism. Much of this physics is encoded in subtle changes to the magnetic order parameter — its symmetry, spatial texture, and coupling to other degrees of freedom — that are difficult to resolve with conventional magnetic probes. Here, I will show how symmetry-sensitive nonlinear optical microscopy can unveil these hidden orders and follow their dynamics.
I will first discuss NiPS3, where we use second-harmonic generation microscopy to show how reduced dimensionality transforms the material’s bulk antiferromagnetism, giving rise to a six-state clock order emerging from a BKT phase in the monolayer limit. I will then turn to the van der Waals multiferroic NiI2, where chiral magnetism is intimately coupled to ferroelectric polarization. By combining complementary electric- and magnetic-dipole-sensitive nonequilibrium probes, we identify terahertz collective modes with an exceptionally large chiral dynamical magnetoelectric response. Beyond their fundamental implications, these experiments establish new avenues for engineering and manipulating correlated spin systems in atomically thin magnets, with potential applications in spintronics, quantum sensing, and light-driven quantum technologies.