# Enhancement of interlayer exchange in an ultrathin two-dimensional magnet

## Abstract

Following the recent isolation of monolayer CrI3 (ref. 1), many more two-dimensional van der Waals magnetic materials have been isolated2,3,4,5,6,7,8,9,10,11,12. Their incorporation in van der Waals heterostructures offers a new platform for spintronics5,6,7,8,9, proximity magnetism13 and quantum spin liquids14. 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 films1,5,6 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).

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## Data availability

The data that support the findings of this study are available at https://dataverse.harvard.edu/dataverse/crcl3.

## Change history

• ### 24 September 2019

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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## Acknowledgements

This work was supported by the Center for Integrated Quantum Materials under NSF grant DMR-1231319 (D.R.K., E.K. and S.F.), the DOE Office of Science, Basic Energy Sciences under award DE-SC0018935 (D.M.), as well as the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4541 to P.J.-H.; D.R.K. acknowledges partial support by the NSF Graduate Research Fellowship Program under grant no. 1122374. R.C. acknowledges support from the Alfred P. Sloan Foundation. Q.S. is supported by the Xu Xin International Student Exchange Scholarship from Nanjing University. E.K. and S.F. are also supported by the ARO MURI award no. W911NF-14-0247. Work done at AmesLaboratory (M.X., R.A.R. and P.C.C.) was performed under contract no. DE-AC02-07CH11358. R.A.R. was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4411. The computations in this paper were run on the Odyssey cluster supported by the FAS Division of Science, Research Computing Group at Harvard University.

## Author information

D.R.K., D.M. and P.J.-H. conceived the project. D.R.K. and D.M. grew the bulk CrCl3 crystals, fabricated and measured the transport devices and analysed the data. Q.S. carried out Raman measurements under supervision of R.C.; D.T.L. and S.F. carried out symmetry analysis and DFT calculations under supervision of E.K.; M.X., R.A.R. and P.C.C. supplied the boron nitride crystals. All authors contributed to writing the manuscript.

Correspondence to Pablo Jarillo-Herrero.

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### Competing interests

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## Supplementary information

### Supplementary Information

Additional theoretical details and Supplementary Figs. 1–13.

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Reprints and Permissions

Klein, D.R., MacNeill, D., Song, Q. et al. Enhancement of interlayer exchange in an ultrathin two-dimensional magnet. Nat. Phys. (2019) doi:10.1038/s41567-019-0651-0

• #### DOI

https://doi.org/10.1038/s41567-019-0651-0

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