Abstract
Topological spin textures are a spectacular manifestation of the chirality of the magnetic nanostructures protected by topology. Most known skyrmion systems are restricted to a topological charge of one, require an external magnetic field for stabilization, and are only reported in a few materials. Here, we investigate the possibility that the Kitaev anisotropic-exchange interaction stabilizes a higher-order skyrmion crystal in the insulating van der Waals magnet NiI2. We unveil and explain the incommensurate static and dynamic magnetic correlations across three temperature-driven magnetic phases of this compound using neutron scattering measurements, simulations, and modeling. Our parameter optimisation yields a minimal Kitaev-Heisenberg Hamiltonian for NiI2 which reproduces the experimentally observed magnetic excitations. Monte Carlo simulations for this model predict the emergence of the higher-order skyrmion crystal but neutron diffraction and optical experiments in the candidate intermediate temperature regime are inconclusive. We discuss possible deviations from the Kitaev-Heisenberg model that explains our results and conclude that NiI2, in addition to multiferroic properties in the bulk and few-layer limits, is a Kitaev bulk material proximate to the finite temperature higher-order skyrmion crystal phase.
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The data that supports the findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
The authors thank Cristian Batista and Jong-Seok Lee for their fruitful discussion and constructive comments. The authors also thank Maxim Avdeev for his help with powder neutron diffraction measurement at ECHIDNA, ANSTO. The work at SNU was supported by the Leading Researcher Program of the National Research Foundation of Korea (Grant No. 2020R1A3B2079357) and the National Research Foundation of Korea (Grant No. RS-2020-NR049405). The work of O.V. and M.M. was supported by the U.S. National Science Foundation through Grant Nos. NSF-DMR-1750186 and NSF-DMR-2309083. S.M. acknowledges support from the Center for Nonlinear Studies at Los Alamos National Laboratory. The work of Y.K. and S.-J.K. was supported by the Pioneer Research Center Program through the National Research Foundation of Korea, funded by the Ministry of Science, ICT & Future Planning (NRF-2022M3C1A3091988). Part of this research was conducted at the High-Flux Isotope Reactor and Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The beam time was allocated to SEQUOIA on proposal numbers IPTS-27591 and 29888. The beam time was allocated to DEMAND on proposal numbers IPTS-33254. Another neutron experiment was performed at the Materials and Life Science Experimental Facility of the J-PARC Center under a user program 2022BU1201. One of the authors (J.-G.P.) acknowledges the hospitality of the Indian Institute of Science, where the manuscript was finalised, and the financial support of the Infosys Foundation.
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J.G.P. initiated and supervised the project. Y.L., Y.K., and S.J.K. grew the single crystal. C.K. aligned the sample for measurements. C.K., O.V., P.P., Y.A., W.C., M.B.S., A.I.K., S.I., S.A., T.M., M.M., and J.G.P. performed the INS measurement. C.K., Y.H. and M.M. performed the single-crystal neutron diffraction measurement. C.K. analyzed the data and performed Landau-Lifshitz dynamics and linear spin-wave calculations. S.M. contributed to the data fitting using the Bayesian optimization. C.K., M.M., and J.G.P. wrote the manuscript with input from all co-authors.
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Kim, C., Vilella, O., Lee, Y. et al. Kitaev interaction and proximate higher-order skyrmion crystal in the triangular lattice van der Waals antiferromagnet NiI2. npj Quantum Mater. (2026). https://doi.org/10.1038/s41535-026-00851-1
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DOI: https://doi.org/10.1038/s41535-026-00851-1
