From graphene to black phosphorus and molybdenum disulfide, the past decade has seen fast progress in the field of two-dimensional materials. Made up of atomically thin layers with nothing but weak out-of-plane van der Waals forces to hold them together into a bulk solid, the materials are alluring from both basic and applied points of view. The physics of isolated 2D layers has proven rich and often surprising, in part because the reduced dimensionality changes the way quantum fluctuations compete with long-range order. Device engineers have already fashioned 2D transistors and dream of building all-2D circuitry. (See the article by Pulickel Ajayan, Philip Kim, and Kaustav Banerjee, Physics Today, September 2016, page 38.)
Of all the types of condensed-matter behavior that have been observed and studied in 2D materials, ferromagnetism has been notably absent. But now two teams—one led by Xiang Zhang of the University of California, Berkeley,...
