The past decade has seen an explosion in the study of atomically thin materials with diverse properties. Graphene is a semimetal, hexagonal boron nitride is an insulator, molybdenum disulfide is a semiconductor, and so on. (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 in two-dimensional materials, ferromagnetism has been notably absent. But now two teams—one led by Xiang Zhang of the University of California, Berkeley, and the other a collaboration between the groups of the University of Washington’s Xiaodong Xu and MIT’s Pablo Jarillo-Herrero—have observed clear signatures of ferromagnetism in two chromium-based 2D materials. And they’ve both uncovered unexpected new effects.
In work on CrGeTe3, Zhang and colleagues discovered a striking dependence of the magnetic transition temperature Tc on the applied magnetic field. Applying a small field to stabilize the magnetic moments is a typical practice in studies of magnetic materials; in bulk materials, the field doesn’t make much difference to the properties of interest. But in 2D materials, it does: The Tc of a bilayer of CrGeTe3 is 44 K under a 0.3 T field, 28 K under a 0.065 T field, and less than 4 K under zero field. Zhang and colleagues interpret the observation as revealing a key difference between 2D and 3D magnets: Magnetism in 3D is governed by the exchange interaction between spins and their neighbors, whereas 2D magnetism is governed by a material’s magnetic anisotropy, the spins’ preference to align in a particular direction.
Xu, Jarillo-Herrero, and colleagues examined CrI3, the material depicted in the figure. They found robust spontaneous magnetization in monolayer CrI3 at temperatures below 45 K, not too much lower than the material’s bulk Tc of 61 K. Their surprise came when they looked at samples of different thicknesses. Monolayer samples were ferromagnetic, as were trilayers. But bilayers exhibited no net ferromagnetism unless a field greater than 0.65 T was applied. The researchers concluded that bilayers tend toward antiferromagnetic order, with one layer spin up and the other spin down. The reason for the layer-dependent effect is still unknown. (C. Gong et al., Nature 546, 265, 2017; B. Huang et al., Nature 546, 270, 2017. Image courtesy of Michael A. McGuire, Oak Ridge National Laboratory.)
