Abstract
In magnetic crystals, despite the explicit breaking of time-reversal symmetry, two equilibrium states related by time reversal are always energetically degenerate. In ferromagnets, this time-reversal degeneracy is reflected in the hysteresis of the magnetic field dependence of the magnetization and, if metallic, in that of the anomalous Hall effect (AHE). Under time-reversal, both these quantities change signs but not their magnitude. Here we show that a time-reversal-like degeneracy appears in the metallic kagome spin ice HoAgGe when magnetic fields are applied parallel to the kagome plane. We find vanishing hysteresis in the field dependence of the magnetization at low temperature, but finite hysteresis in the field-dependent AHE. This suggests the emergence of states with nearly the same energy and net magnetization but different sizes of the AHE and of the longitudinal magnetoresistance. By analysing the experimental data and a minimal tight-binding model, we identify a time-reversal-like operation connecting these near-degenerate states, which is related to the non-trivial distortion of the kagome lattice in HoAgGe. Our work demonstrates the diagnostic power of transport phenomena for identifying hidden symmetries in frustrated spin systems.
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Data availability
All data presented in this paper, if not present in the main text or supplementary materials, are available from the corresponding authors upon reasonable request. Source data are provided with this paper.
Code availability
The code for the theoretical analyses and model calculations of this study is available from the corresponding authors upon reasonable request.
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Acknowledgements
We thank O. Tchernyshyov, M. Udagawa, H. Deng, I. Kezsmarki, J. Shen, H. Ren, S. Su, T. Dong and N. Wang for helpful discussions and experimental support. The work in Augsburg (K.Z., Y.T. and P.G.) was supported by the German Research Foundation (DFG) through SPP1666 (project no. 220179758), TRR80 (project no. 107745057) and TRR360 (project no. 492547816) and via the Sino-German Cooperation Group on Emergent Correlated Matter. K.Z. acknowledges the support by the National Key R&D Program of China (Grant No. 2023YFA1406003), National Natural Science Foundation of China (Grants No. 12274015), the Beijing Nova Program (Grant No. Z211100002121095), and the Fundamental Research Funds for the Central Universities. H.C. acknowledges the support by NSF CAREER grant DMR-1945023. A portion of this work was conducted at the Synergetic Extreme Condition User Facility (SECUF).
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K.Z. and P.G. proposed the experiments. K.Z. synthesized single crystals and measured magnetic properties, specific heat and electronic transport properties. Y.T. conducted magnetic Grüneisen parameter measurements. H.C. proposed the theoretical analysis and provided model calculations. K.Z., H.C. and P.G. wrote the manuscript with input from all authors.
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Supplementary Notes 1–15, Figs. 1–34 and Tables 1 and 2.
Source data
Source Data Fig. 1
The anomalous part \({\rho }_{{ca}}^{\rm{AH}}\) (constant-in-field) of the Hall resistivity \({\rho }_{{ca}}\), together with corrected magnetization M(H) data (Fig. 1b); summary of magnetic susceptibility, magnetization M(H), magnetic specific heat Cmag and magnetic Grüneisen parameter Γmag of HoAgGe under H//b (Fig. 1c).
Source Data Fig. 2
The magnetization M, a- and c-axis MR ρa and ρc, as well as the Hall resistivity ρca and conductivity σac of HoAgGe under H//b at various temperatures.
Source Data Fig. 3
The magnetic specific heat Cmag, magnetic Grüneisen parameter Γmag, magnetic entropy Smag and Hall resistivity ρca of HoAgGe at 8 K (Fig. 3a) and 3 K (Fig. 3b).
Source Data Fig. 4
The magnetic susceptibility of HoAgGe under H along c and b axes (Fig. 4a); the magnetization (M) under H//c at 2 K, with dM/dH curves (Fig. 4b); the a-axis MR ρa and the Hall resistivity ρab at 2 K under H//c (Fig. 4c).
Source Data Fig. 5
Band structure and Berry-curvature-related quantities orbital magnetization and linear magnetoconductivity intrinsic AHC (Fig. 5c); intrinsic AHC, Berry-curvature-contributed linear magnetoconductivity and orbital magnetization (Fig. 5d–f).
Source Data Fig. 6
The Hall resistivity ρca (Fig. 6a) and symmetric part of the raw Rca signal (Fig. 6b) of HoAgGe obtained using different field-sweep protocols under H//b at 2 K.
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Zhao, K., Tokiwa, Y., Chen, H. et al. Discrete degeneracies distinguished by the anomalous Hall effect in a metallic kagome ice compound. Nat. Phys. 20, 442–449 (2024). https://doi.org/10.1038/s41567-023-02307-w
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DOI: https://doi.org/10.1038/s41567-023-02307-w
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