Nonlinear Optical Probe Thickness Dependent Surface and Extraordinary Magnetic Phase Transitions

Understanding how reduced dimensionality reshapes magnetic orders is central to both fundamental condensed-matter physics and the design of next-generation spintronic devices. Bulk CrSBr exhibits both surface and bulk magnetic phase transitions, whereas few-layer CrSBr hosts only a single magnetic transition. However, the nature of the crossover from the three-dimensional to the two-dimensional limit remains elusive. Here, we employ a combination of nonlinear optical probes including rotation anisotropy (RA) second harmonic generation (SHG), third-harmonic generation (THG), and photoluminescence (PL), to selectively probe the surface and bulk magnetic phase transitions in CrSBr. By tracking the evolution of these transitions with decreasing CrSBr thickness, we aim to construct a dimensional crossover phase diagram that captures the interplay between the surface and extraordinary bulk phase transitions.