“Extraordinary” Phase Transition Revealed in a van der Waals Antiferromagnet

An uncommon and even counterintuitive situation in surface phase transitions is that the surface order emerges at a higher temperature than the bulk one, despite the two-dimensional (2D) Mermin-Wagner fluctuations. Such a phase transition, where the bulk order sets in after the surface order, has already been theoretically established and dubbed “extraordinary”. They can only happen if the surface interactions are much stronger than the bulk ones. While theoretically possible, they have been hardly realized in any materials so far. Here, we demonstrate the presence of an extraordinary phase transition in bulk CrSBr, a van der Waals (vdW) antiferromagnet (AFM). Using a combination of various second harmonic generation (SHG) techniques, we capture the surface and bulk magnetic phase transitions, spin correlations, as well as distinguish the two degenerate AFM domain states. Density functional theory calculations further identify key factors contributing to the enhanced surface magnetism.