Song, QianFang, ShiangXu, MingyuRibeiro, R. A.Canfield, P. C.Kaxiras, EfthimiosComin, RiccardoJarillo-Herrero, PabloKlein, DahliaMacNeill, DavidLarson, DanielCanfield, P2019-12-162019-09-16Klein, Dahlia, David MacNeill, Qian Song, Daniel Larson, Mingyu Xu, R. Ribeiro, P. Canfield, Efthimios Kaxiras, Riccardo Comin, and Pablo Jarillo-Herrero. 2019. Enhancement of Interlayer Exchange in an Ultrathin Two-dimensional Magnet. Nature Physics 15, no. 12: 1255-260.1745-24731745-2481http://nrs.harvard.edu/urn-3:HUL.InstRepos:42060276Following the recent isolation of monolayer CrI3, many more two-dimensional van der Waals magnetic materials have been isolated. Their incorporation in van der Waals heterostructures offers a new platform for spintronics proximity magnetism and quantum spin liquids. A primary question in this field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies of CrI3 have shown a different magnetic ground state for ultrathin exfoliated films compared with the bulk, but the origin is not yet understood. Here, we use electron tunnelling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order and find a tenfold enhancement of the interlayer exchange compared with bulk crystals. Moreover, temperature- and polarization-dependent Raman spectroscopy reveals that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low-temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides insight into the connection between stacking order and interlayer interactions in two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets such as RuCl3.en-USGeneral Physics and AstronomyJournal Article2019-12-1610.1038/s41567-019-0651-0