Phonons, which are quasi-particles associated with sounds or lattice vibrations, have long been considered to possess negligible magnetic moments. However, a recent experiment conducted by researchers at Nanjing University and the Chinese Academy of Sciences has challenged this belief by uncovering giant phonon magnetic moments in Fe2Mo3O8, a polar antiferromagnet. This discovery, featured in Nature Physics, sheds new light on the interplay between phonons and magnetism, potentially opening up new avenues for phononic control of magnetic dynamics and the development of novel spin information devices.
The primary objective of the study conducted by Qi Zhang and his colleagues was to better understand the magnetic properties of phonons in a spin-ordered system. Previous discoveries of large phonon magnetic moments in non-magnetic topological systems prompted the researchers to investigate the possibility of similar properties in a magnetic material. They aimed to determine the roles played by many-body correlations and fluctuations in the formation of giant phonon magnetic moments.
To achieve their research goals, Zhang and his team employed two key techniques: magneto-Raman spectroscopy and inelastic neutron scattering. These methods allowed them to uncover the phononic nature of low-lying excitations in Fe2Mo3O8 and subsequently measure the phonon magnetic moments of these modes using the phonon Zeeman effect. Remarkably, an unusual enhancement of phonon magnetic moments was observed near the boundaries between the antiferromagnetic and paramagnetic phases.
This experimental study was a joint 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, while Prof. Wen synthesized the sample and collected neutron measurements. The most notable finding of the study was the sixfold enhancement of the phonon magnetic moment in the Fe2Mo3O8 sample. This enhancement surpassed the magnetic moment of an electron or a magnon mode and even exhibited the potential to diverge with the magnetic susceptibility.
The discovery of giant phonon magnetic moments in Fe2Mo3O8 opens up exciting possibilities for future research. The direct link between lattice vibration and magnetic processes provided by the large magnetic moments of phonons presents new opportunities for phononic control of magnetic dynamics and the development of innovative spin information devices. Moving forward, the researchers plan to extend their work into the non-equilibrium regime, exploring phenomena such as chiral phonons-driven magnetic dynamics and transient ferromagnetism. These avenues of investigation hold promise for uncovering further interesting discoveries about the interplay between magnetism and phonons.
The recent experimental study conducted by Zhang and his colleagues revealed the surprising existence of giant phonon magnetic moments in Fe2Mo3O8. This groundbreaking discovery challenges the long-held belief that phonons possess negligible magnetic moments. By uncovering the interplay between magnetism and lattice vibrations, this research opens up new possibilities for manipulating magnetic dynamics through phononic control and the development of spin information devices. As work in this field continues, further investigations into the non-equilibrium regime and the exploration of transient ferromagnetism may yield even more intriguing findings.