Neutrons Illuminate the Magnetic Dance of Chiral Phonons
Phonons—quantized conveyors of sound and heat in solids—are usually visualized as collective vibrations in which atoms simply bounce back and forth, almost as if they were weights on springs. However, atoms can sometimes form “chiral phonons” that twirl and swivel clockwise or counterclockwise, in a way that resembles a coordinated dance [1]. When these circular, chiral motions entrain ionic charge, they generate a magnetic moment, which suggests that there might be a way to control sound and heat using magnetic fields. Until recently, this magnetic dance was primarily observed using optical techniques, granting access to only one corner of the “stage”—the point in the phonon’s momentum space where the momentum is nearly zero. Song Bao of Nanjing University in China and his collaborators have now broadened the view of momentum space by using inelastic neutron spectroscopy. By revealing this elaborate choreography in a magnetic crystal, they have opened the door to testing theoretical models and developing practical applications [2].
The concept of phonons was developed in the 1930s, yet their magnetic properties remained largely unexplored for decades. This was not an oversight; it was a matter of scale. An atomic nucleus is 3 orders of magnitude heavier than an electron, so the magnetic moment generated by an atom’s circular motion is vanishingly small compared with the electron’s magnetic moment. But when chiral phonons arise in a material with existing electronic magnetism, the interaction between the moving atoms and the spins of the electrons can significantly enhance the phonon’s effective magnetic moment, bringing it into a detectable range [3].
Optical probes have successfully identified phonon magnetic moments by observing how their vibrational frequencies split or shift in response to an external magnetic field—an effect known as the phonon Zeeman effect [4]. However, these techniques provide information only about the phonons at the very center of the Brillouin zone, where the momentum is zero. (A Brillouin zone is the basic region of momentum space used to describe electrons in a crystal.) That’s because optical transitions require the electrons’ momentum to be comparable to the photons’ momentum, which is much lower. To develop a comprehensive physical picture and understand the full range of this atomic dance, it is necessary to observe how their properties evolve across the Brillouin zone.
Inelastic neutron scattering is well suited to this task because it can resolve the phonon-dispersion relation (the relationship between frequency and momentum) over a large portion of momentum space. More importantly, neutrons are uncharged but possess a magnetic spin, enabling them to interact with both the atomic nuclei and the local magnetic fields within a solid. When a neutron scatters off a phonon, it can do so via two channels: either through nuclear scattering, which is caused by the strong-force interaction with the nuclei, or through magnetic scattering, which arises when the neutron’s spin interacts with the effective magnetic moments of the phonon. The latter serves as a direct way to witness the magnetic nature of the chiral-phonon dance.
Bao and collaborators chose Fe1.75Zn0.25Mo3O8 (FZMO) as a material platform to study these effects. FZMO is a ferrimagnet: It has a bipartite magnetic structure, in which two different lattice sites are occupied by iron atoms with opposite spin orientation and unequal magnetic moments (Fig. 1) [5]. FZMO is also polar: Its lack of inversion symmetry engenders an electric dipole moment. This combination gives the material a magnetic ordering whose symmetry causes chiral phonons to emerge. Furthermore, the strong coupling between the electron spins and the crystal lattice of FZMO can significantly enhance the magnetic signatures of these phonons, making them more accessible to experimental probes.
Using high-resolution neutron spectrometers, the researchers measured the phonon-dispersion relation in a large single crystal of FZMO. After cooling the sample well below its magnetic-transition temperature, they identified a specific phonon branch that showed an unusual energy splitting. This splitting provided the first indication that the phonons are not merely mechanical but are instead sensitive to the material’s underlying magnetic symmetry.
The primary line of evidence for the magnetic—and, with it, the chiral—nature of these phonons came from a detailed analysis of the scattering intensities. In a standard nonmagnetic material, the intensity of neutron scattering from a phonon increases proportionally to the square of the phonon’s momentum. However, the data from FZMO revealed a striking anomaly. At low temperatures in the ferrimagnetic phase, Bao and coauthors observed an unexpected increase in scattering intensity at low momentum—exactly where the signal should be weakest according to a simple model that considers only the nuclear scattering of the neutrons.
The observed boost at small momentum implies that magnetic scattering is at play. That’s because the boost is dominant at small momentum but suppressed at large momentum. To confirm this inference, the researchers heated the sample above the magnetic-ordering temperature. As the material transitioned into a paramagnetic state, the anomalous low-momentum intensity vanished, and the scattering returned to the standard nuclear behavior. Taken together, the two lines of evidence show that the phonons in FZMO carry significant magnetic moments, which contribute to neutron scattering in addition to the conventional nuclear scattering. These results complement an independent study that used Raman spectroscopy to probe the magnetic properties of chiral phonons at the center of the Brillouin zone by leveraging the phonon Zeeman effect [6].
The work of Bao and collaborators offers a panoramic view of the behavior of chiral phonons throughout the Brillouin zone, thereby enabling a more rigorous testing of theoretical models of chiral phonons in magnetic materials. The next challenge will lie in uncovering the microscopic rules that dictate which phonons become magnetic and how they do so. This step will be crucial for engineering these effects into functional technologies that harness sound waves to create spin current, for example, or use magnetic fields to divert the flow of heat.
References
- H. Chen et al., “Chiral phonons in two-dimensional materials,” 2D Mater. 6, 012002 (2018); D. M. Juraschek et al., “Chiral phonons,” Nat. Phys. 21, 1532 (2025).
- S. Bao et al., “Magnetic signature of chiral phonons revealed by neutron spectroscopy in ferrimagnetic Fe1.75Zn0.25Mo3O8,” Phys. Rev. Lett. 136, 096502 (2026).
- P. Thalmeier and P. Fulde, “Optical phonons of rare-earth halides in a magnetic field,” Z Phys. B 26, 323 (1977).
- G. Schaack, “Observation of circularly polarized phonon states in an external magnetic field,” J. Phys. C: Solid State Phys. 9, L297 (1976); “Magnetic field dependent splitting of doubly degenerate phonon states in anhydrous cerium-trichloride,” Z Phys. B 26, 49 (1977).
- T. Kurumaji et al., “Doping-tunable ferrimagnetic phase with large linear magnetoelectric effect in a polar magnet Fe2Mo3O8,” Phys. Rev. X 5, 031034 (2015); T. Ideue et al., “Giant thermal Hall effect in multiferroics,” Nat. Mater. 16, 797 (2017).
- F. Wu et al., “Magnetic switching of phonon angular momentum in a ferrimagnetic insulator,” Phys. Rev. Lett. 134, 236701 (2025).