# Ferromagnetic Kitaev interaction and the origin of large magnetic anisotropy in α-RuCl3

## Abstract

α-RuCl3 is drawing much attention as a promising candidate for the Kitaev quantum spin liquid1,2,3,4,5,6,7,8. However, despite intensive research efforts, controversy remains about the form of the basic interactions governing the physics of this material. Even the sign of the Kitaev interaction (the bond-dependent anisotropic interaction responsible for Kitaev physics) is still under debate, with conflicting results from theoretical and experimental studies5,6,9,10,11,12,13,14,15. The significance of the symmetric off-diagonal exchange interaction (referred to as the Γ term) is another contentious question16,17,18. Here, we present resonant elastic X-ray scattering data that provide unambiguous experimental constraints to the two leading terms in the magnetic interaction Hamiltonian. We show that the Kitaev interaction (K) is ferromagnetic, and that the Γ term is antiferromagnetic and comparable in size to the Kitaev interaction. Our findings also provide a natural explanation for the large anisotropy of the magnetic susceptibility in α-RuCl3 as arising from the large Γ term. We therefore provide a crucial foundation for understanding the interactions underpinning the exotic magnetic behaviours observed in α-RuCl3.

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## Data availability

All data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

## Code availability

The computer code used to generate results that are reported in the paper is available from the authors on reasonable request.

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## Acknowledgements

We would like to thank J. Bertinshaw and H. Suzuki for their help with the experiment. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III. Work at the University of Toronto was supported by the Natural Science and Engineering Research Council (NSERC) of Canada, Canadian Foundation for Innovation, Ontario Innovation Trust, and the Center for Quantum Materials at the University of Toronto. Y.B.K. is also supported by the Killam Research Fellowship from the Canada Council for the Arts. This work was performed in part at Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611.

## Author information

Authors

### Contributions

J.A.S. and Y.-J.K. conceived the experiments. J.A.S., P.J.B. and S.F. performed the experiments and J.A.S. analysed the data. J.A.S. synthesized and characterized the sample. S.K. provided the magnetic susceptibility data. L.E.C. and Y.B.K. performed theoretical calculations. J.A.S. and Y.-J.K. wrote the paper with contributions from all co-authors.

### Corresponding author

Correspondence to Young-June Kim.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1 and 2, and discussion.

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Sears, J.A., Chern, L.E., Kim, S. et al. Ferromagnetic Kitaev interaction and the origin of large magnetic anisotropy in α-RuCl3. Nat. Phys. 16, 837–840 (2020). https://doi.org/10.1038/s41567-020-0874-0

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