Phonons have long been considered to possess negligible magnetic moments. However, recent research conducted by scientists at Nanjing University and the Chinese Academy of Sciences has challenged this notion. In their study, featured in Nature Physics, the researchers investigated the phonon magnetic moments of Fe2Mo3O8, a polar antiferromagnet, and uncovered giant phonon magnetic moments enhanced by spin fluctuations in this material. This groundbreaking discovery opens up new possibilities for understanding the interplay between magnetism and phonons, as well as the potential for phononic control of magnetic dynamics and spin information devices.
The researchers were inspired by recent findings of large phonon magnetic moments in non-magnetic topological systems. They wanted to explore the magnetic properties of phonons in a spin-ordered system and determine the role of many-body correlations and fluctuations in the formation of these moments. Fe2Mo3O8 was chosen as the material of interest due to its strong spin-lattice coupling and remarkable thermal Hall coefficient.
To investigate the phononic nature of Fe2Mo3O8, the researchers employed two key techniques: magneto-Raman spectroscopy and inelastic neutron scattering. These techniques allowed them to identify a pair of low-lying excitations at 42 cm-1 in Fe2Mo3O8 single crystals and determine the phonon magnetic moments of these modes through the phonon Zeeman effect. The phonon magnetic moments were measured by analyzing the phonon frequency shift under magnetic fields in polarization-resolved Raman spectroscopy.
The most striking finding of the study was the discovery of a sixfold enhancement of the phonon magnetic moment near the boundaries between the antiferromagnetic and paramagnetic phases in Fe2Mo3O8. This represents a 600% ferrimagnetic fluctuation enhancement of the phonon magnetic moment, surpassing the magnetic moment of an electron or a magnon mode. Furthermore, this enhancement has the potential to diverge with the magnetic susceptibility, opening up exciting possibilities for further exploration.
The study was a collaborative effort between Prof. Yuan Wan’s team at the Chinese Academy of Sciences and Prof. Jinsheng Wen’s lab at Nanjing University. Prof. Wan’s group conducted a symmetry analysis to develop a minimal model that captured the essential physics underlying the experiment. Prof. Wen’s team synthesized the Fe2Mo3O8 samples and performed the neutron measurements, which provided crucial insights into the phonon magnetic moments.
The discovery of giant phonon magnetic moments in Fe2Mo3O8 opens up new avenues for research into the interplay between magnetism and phonons. The researchers are particularly interested in exploring the non-equilibrium regime and investigating phenomena such as chiral phonons-driven magnetic dynamics and transient ferromagnetism. These future studies, along with the theoretical microscopic model summarizing the observations from this work, have the potential to yield novel insights and pave the way for exciting discoveries in the field of phononic control of magnetism.
The investigation of the phonon magnetic moments in Fe2Mo3O8 has defied the conventional understanding of these quasi-particles and revealed their significant magnetic properties. The discoveries made by the researchers at Nanjing University and the Chinese Academy of Sciences have not only provided valuable insights into the interplay between magnetism and phonons but also opened up new possibilities for phononic control of magnetic dynamics and spin information devices. As further research unfolds, the implications of this work could have far-reaching impacts on various scientific disciplines, ranging from materials science to quantum technologies.
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