Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer

TL;DR

This paper reports the discovery of intrinsic in-plane ferromagnetism in a monolayer CrCl3, demonstrating a 2D-XY phase transition and opening avenues for exploring exotic 2D magnetic phenomena.

Contribution

It provides the first experimental realization of a 2D-XY ferromagnet with in-plane symmetry in a van der Waals monolayer, confirming a BKT transition.

Findings

Robust in-plane ferromagnetic order observed in CrCl3 monolayer.

Evidence of a 2D-XY universality class phase transition.

Potential platform for studying topological spin textures.

Abstract

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.