Enhancement of interlayer exchange in an ultrathin two-dimensional magnet

Following the recent isolation of monolayer CH3 (ref.(1)), many more two-dimensional van der Waals magnetic materials have been isolated(2-12). Their incorporation in van der Waals hetero-structures offers a new platform for spintronics(5-9), proximity magnetism(13) and quantum spin liquids(14). 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(1,5,6) 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 (ref.(14)).