Phys. Rev. Lett. 111, 125302 (2013)

One of the more widely quoted theorems in condensed-matter physics, ascribed to David Mermin and Herbert Wagner, is broadly taken to mean that long-range magnetic order cannot be sustained in a two-dimensional system as long as the interactions between the spins are short-ranged and isotropic. Alas, reality isn't quite so straightforward. As the enormous experimental literature on systems with reduced dimensionality suggests, long-range order — magnetic or otherwise — can be stabilized in two dimensions, provided the smallest perturbation to the highly idealized scenario considered by Mermin and Wagner is present. The helium isotope 3He physisorbed on graphite is a case in point. The origin of the helium's ferromagnetism is due to nuclear, rather than electronic, spin interactions, and as Andrew Casey and colleagues now confirm experimentally, the system represents an ideal realization of a frustrated two-dimensional magnet. Using a combination of nuclear magnetic resonance and sensitive magnetometry measurements, they show that its ferromagnetism is stabilized by the additional presence of subtle 'multi-site' ring exchange interactions.