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Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3

Abstract

Layered α-RuCl3 is a promising material to potentially realize the long-sought Kitaev quantum spin liquid with fractionalized excitations. While evidence of this state has been reported under a modest in-plane magnetic field, such behaviour is largely inconsistent with theoretical expectations of spin liquid phases emerging only in out-of-plane fields. These predicted field-induced states have been largely out of reach due to the strong easy-plane anisotropy of bulk crystals, however. We use a combination of tunnelling spectroscopy, magnetotransport, electron diffraction and ab initio calculations to study the layer-dependent magnons, magnetic anisotropy, structure and exchange coupling in atomically thin samples. Due to picoscale distortions, the sign of the average off-diagonal exchange changes in monolayer α-RuCl3, leading to a reversal of spin anisotropy to easy-axis anisotropy, while the Kitaev interaction is concomitantly enhanced. Our work opens the door to the possible exploration of Kitaev physics in the true two-dimensional limit.

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Fig. 1: Three-dimensional electron diffraction and demonstration of IETS measurements on 1L α-RuCl3.
Fig. 2: Temperature-dependent IETS of few-layer α-RuCl3.
Fig. 3: Magnetic-field-dependent IETS on few-layer α-RuCl3.
Fig. 4: Lateral magnetotransport measurement on 1L α-RuCl3 with dual gates.
Fig. 5: Three primary distortions of 1L α-RuCl3 and magnetic phase diagram.

Data availability

All relevant data within the article and supporting information are available from the corresponding authors upon reasonable request.

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Acknowledgements

A.W.T. acknowledges support from the US Army Research Office (W911NF-21-2-0136), Ontario Early Researcher Award (ER17-13-199) and the National Science and Engineering Research Council of Canada (RGPIN-2017-03815). This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund. R. Hovden acknowledges support from the US Army Research Office (W911NF-22-1-0056), S.H.S. and Y.M.G. acknowledge support from the Keck Foundation. S.M.W. was supported by a pilot grant from the Center for Functional Materials, and performed computations on the Wake Forest University DEAC Cluster, a centrally managed resource with support provided in part by Wake Forest University. D.A.S.K., S.B. and R.V. acknowledge support by the Deutsche Forschungsgemeinschaft through grants VA 1171/15-1 and TRR 288-4 22213477 (project A05). G.Y. and R. He are supported by National Science Foundation CAREER grant no. DMR-1760668 and National Science Foundation grant no. DMR-2104036. H.L. acknowledges support by the National Key R&D Program of China (grant no. 2018YFE0202600), the Beijing Natural Science Foundation (grant no. Z200005) and the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (18XNLG14 and 19XNLG17, respectively).

Author information

Authors and Affiliations

Authors

Contributions

B.Y. and A.W.T. conceived and initiated the study. B.Y. fabricated the α-RuCl3 devices for transport measurements, electron diffraction and Raman spectroscopy. B.Y. performed the IETS and lateral transport measurements. S.H.S., Y.M.G. and R. Hovden conducted the three-dimensional electron diffraction measurements. G.Y. and R. He performed the Raman spectroscopy. S.B., D.A.S.K., R.D., R.V. and S.M.W. performed the ab initio calculations. C.L., S.Y. and H.L. grew the α-RuCl3 crystals. F.C. grew the 1 T′-MoTe2 crystals. S.J. conducted the magnetic circular dichroism measurements. B.Y. and A.W.T. wrote the manuscript with the input of all authors.

Corresponding authors

Correspondence to Stephen M. Winter, Robert Hovden or Adam W. Tsen.

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Nature Materials thanks Hu Miao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 IETS device with Gr contacts.

(a): Colorized optical image of the sample. The device is fabricated in the same geometry as shown in Fig. 1a, except the Td-MoTe2 is replaced by few-layer graphene and the thickness of α-RuCl3 is 5-6 layers. (b): False-color two-dimensional plot of background-subtracted d2I/dV2 spectrum taken from 0.3 K to 10 K for positive bias. The trace at 0.3 K is overlaid in blue. The low-energy peak at 1 meV is reproduced in this sample, which gradually disappears above ~8 K, and the continuum between 4-10 meV is reproduced as well.

Extended Data Fig. 2 Temperature-dependent IETS data on few-layer α-RuCl3.

Upper panels: 1L, 2L, and 3L α-RuCl3 spectra with changing temperature from 2 K to 10 K in 1 K increments without background subtraction. Offset is applied for clarity. Grey lines represent the backgrounds for the IETS data. Lower panels: 1L, 2L, and 3L α-RuCl3 spectra with background subtraction. The background-subtracted data is used for plotting Fig. 2.

Extended Data Fig. 3 Temperature dependence of magnetization for a bulk α-RuCl3 single crystal used in the measurements.

The sharp kink near 8 K indicates the Néel temperature, which is consistent with IETS data.

Extended Data Fig. 4 Original field-dependent IETS data without background subtraction.

1L, 2L, and 3L α-RuCl3 spectra without background subtraction with changing B|| (a) and B (b) from 0 T to 14 T in 1 T increments and offset for clarity.

Extended Data Fig. 5 False-color plot of normalized IETS spectra without background subtraction for 3L α-RuCl3 from 0 T to 14 T.

Evolution of the low-energy magnon peak and the maximum position of the magnon continuum is overlaid in grey and red, respectively.

Extended Data Fig. 6 Magnetic circular dichroism (MCD) measurements comparing 1L α-RuCl3 and 1L CrBr3.

(a): \({{{\mathrm{{\Delta}}}MCD}} = {{{\mathrm{MCD}}}} - {{{\mathrm{MCD}}}}(0{{{\mathrm{T}}}})\) for 1L α-RuCl3 at 3.5 K and 1L CrBr3 at 5 K between ±70 mT. The data for 1L CrBr3 is reproduced from previous work in ref. 27. (b): MCD data for 1L α-RuCl3 at 3.5 K between ± 8 T.

Supplementary information

Supplementary Information

Supplementary Figs. 1–9, Tables 1 and 2 and Sections 1–6.

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Yang, B., Goh, Y.M., Sung, S.H. et al. Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3. Nat. Mater. (2022). https://doi.org/10.1038/s41563-022-01401-3

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