Phonons are the quasiparticles of collective lattice excitations that may carry finite angular momenta, but commonly exhibit negligible magnetic moments. A large phonon magnetic moment enables the direct mutual control of magnetic orders and lattice motions, and could be applied to develop spin–phononic devices. In some non- and paramagnetic systems, a large phonon magnetic moment is found due to coupling with electronic excitations. However, for magnetically ordered systems, a correspondingly large moment has not yet been discovered, and the roles of many-body correlations and fluctuations in phonon magnetism remain unclear. Here we report a phonon magnetic moment that is enhanced by critical fluctuations in a polar antiferromagnet, namely, Fe2Mo3O8. Combining magneto-Raman spectroscopy and inelastic neutron scattering measurements, we show that a pair of low-lying chiral phonons carry large magnetic moments. Once the system is driven to a ferrimagnetic phase, we observe a splitting between the chiral phonons of nearly a quarter of the phonon frequency. We also observe a sixfold enhancement in the phonon magnetic moment in the vicinity of the Néel temperature. A microscopic model based on the coupling between phonons and both magnons and paramagnons accounts for the experimental observations.
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We thank J. Li, J. Liu, S. Yu and O. Tchernsyhyov for valuable discussions. We thank R. Kajimoto and M. Nakamura for their help in carrying out the INS experiment. This work was supported by the National Key Research and Development Program of China (grant nos. 2020YFA0309200, 2021YFA1400400 and 2022YFA1403800), the National Natural Science Foundation of China (grant nos. 11974396, 12225407, 12074174, 12125404, 11974162 and 11834006) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB33020300), as well as the Fundamental Research Funds for the Central Universities. S.B. thanks the support from the China Postdoctoral Science Foundation via grant nos. 2022M711569 and 2022T150315 and Jiangsu Province Excellent Postdoctoral Program via grant no. 20220ZB5. The phonon calculations were carried out using supercomputers at the High Performance Computing Center of Collaborative Innovation Center of Advanced Microstructures, the high-performance supercomputing centre of Nanjing University.
The authors declare no competing interests.
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Wu, F., Bao, S., Zhou, J. et al. Fluctuation-enhanced phonon magnetic moments in a polar antiferromagnet. Nat. Phys. 19, 1868–1875 (2023). https://doi.org/10.1038/s41567-023-02210-4