Emergence of 2D-XY antiferromagnetism in monolayer NiPS₃

The realization of stable magnetic orderings in two-dimensional (2D) crystals beyond the Mermin-Wagner theorem offers a unique lens into the rich physics inherent to reduced dimensionality. Though the stabilization of 2D magnetic order with out-of-plane anisotropy has been unveiled in many 2D van der Waals systems, the demonstration of a 2D magnet susceptible to other types of persistent symmetry breaking has long been sought but has remained elusive. Here, we combine advanced optical techniques to unravel signatures of a Berezinskii–Kosterlitz–Thouless transition in NiPS₃ and track its instability towards an antiferromagnetic long-range-ordered state. Analysis of the critical scaling of the order parameter reveals excellent agreement with the 2D XY universality class below the critical temperature. These results are interpreted via a six-clock model and supported by theoretical simulations. Our findings shed light on the intricate interplay between topology and symmetry breaking in two dimensions and establish a protocol to discover novel magnetic textures for spintronics.