Taking the measure of a magnet The recent discovery of magnetism in two-dimensional (2D) materials has inspired efforts to understand its nature. Whereas the magnetism of monolayers of chromium iodide (CrI 3 ) can be understood in terms of out-of-plane magnetic anisotropy, the related material chromium chloride (CrCl 3 ) has spins that lie in the plane. Bedoya-Pinto et al . used molecular beam epitaxy to grow monolayers of CrCl 3 on graphene and studied its magnetic properties. Using x-ray magnetic circular dichroism measurements, the authors found that monolayer CrCl 3 is a ferromagnet, unlike bulk CrCl 3 , which is antiferromagnetic. The scaling of the signal in the critical region indicated that the material belongs to the 2D-XY universality class, distinct from Ising magnetism, which some other 2D magnets exhibit. —JS