  Long before the recent fascination with two-dimensional materials, the
critical behaviour and universality scaling of phase transitions in
low-dimensional systems has been a topic of great interest. Particularly
intriguing is the case of long-range magnetic order in two dimensions, once
considered to be excluded in systems with continuous symmetry by the
Hohenberg-Mermin-Wagner theorem. While an out-of-plane anisotropy has been
shown to stabilize 2D magnetic order, this proof has remained elusive for a 2D
magnet with in-plane rotational symmetry. Here, we construct a nearly ideal
easy-plane system, a CrCl3 monolayer grown on Graphene/6H-SiC (0001), and
unambiguously demonstrate robust in-plane ferromagnetic ordering with a
critical scaling behaviour characteristic of a 2D-XY system. These observations
suggest the first realization of a finite-size Berezinskii-Kosterlitz-Thouless
(BKT) phase transition in a large-area, quasi-freestanding, van der Waals
monolayer magnet with a XY universality class; and further constitute an ideal
platform to study exotic phenomena like superfluid spin transport or 2D
topological in-plane spin textures -- such as merons.
