Chiral phonons, tiny whirlpools of atomic motion inside crystals, have just revealed a hidden magnetic side that researchers are only beginning to understand. By firing neutrons into a carefully chosen magnetic material, a team has mapped how these swirling vibrations carry their own magnetic fingerprints and intertwine with the material’s spins. The result is a fresh view of how sound, heat and magnetism interact at the quantum level, with implications that stretch from spintronics to heat management in chips.
The work turns an abstract concept into something experimentally tangible: chiral phonons are no longer just a theoretical curiosity but measurable actors that can tug on magnetic moments. As scientists refine ways to see and control these motions in space and time, the prospect of steering information and energy through solids with unprecedented precision is moving from speculation toward design.
From simple vibrations to swirling chiral phonons
For decades, phonons were introduced in textbooks as simple back and forth vibrations, with atoms oscillating along a line as if they were weights on springs. In that picture, phonons carry energy and momentum but no angular momentum, so they lack any intrinsic twist. Recent theory and experiment have overturned that view, showing that phonons can also execute circular motion and therefore behave as if they carry their own tiny angular momentum.
From a fundamental perspective, chiral or rotational phonons are defined as coherent or incoherent motions of atoms around their equilibrium positions along circular or elliptical paths. That description, laid out in work coordinated by Peter Baum within the ChiPS (Chiral phonons for spintronics) initiative, turns an abstract quantum of vibration into a literal microscopic dance. Since the influential papers by Zhang and Niu highlighted that such phonons can execute circular motion either clockwise or counterclockwise, researchers have treated chirality as a new quantum label for lattice vibrations, on par with energy and momentum.
How circular vibrations acquire magnetism
Once atoms move in circles instead of straight lines, their charge traces out loops and generates magnetic moments. Circularly polarized lattice vibrations, also called chiral phonons, lead to orbital motions of the atoms in a crystal, producing a collective phonon magnetic moment that earlier theory placed in the range of a few nuclear magnetons. Detailed calculations of circularly polarized modes show that this moment can be significantly enhanced by orbit lattice coupling, which ties the ionic motion to the electronic orbits.
Phonons are traditionally regarded as linear back and forth atomic motions, thus possessing no angular momentum, and it was not until recent theoretical work that the concept of angular momentum of phonons was formalized. Within that framework, circular rotations of charge carrying ions generate magnetic moments that, for ordinary modes, are smaller than 3 µB or less, but can grow when phonons couple strongly to spin excitations. Studies of circular rotations and of phonons coupled to magnons show that the resulting magnetic moments can reach the µB scale, large enough to be detected and manipulated with external fields.
Neutrons as a microscope for the magnetic dance
Until recently, this magnetic dance was primarily observed using optical techniques that probe phonons only at the center of the Brillouin zone, the region of momentum space where the momentum is nearly zero. Experiments such as the ultrahigh resolution lifetime measurements by Chae and co measured optical phonons at the center of the first Brillouin zone, which meant that the momentum change of the electrons was close to 0 and the view of chiral behavior remained localized. That optical focus left most of the phonon spectrum, especially at finite momentum, effectively uncharted in terms of chirality and magnetism.
Song Bao of Nanjing University in China and his collaborators have now broadened the view of momentum space by using inelastic neutron spectroscopy to track phonons and magnons across the Brillouin zone. Using neutron spectroscopy, a team led by Song Bao at Nanjing University mapped magnetic signatures linked to chiral phonons in a ferrimagnetic material, revealing strong interactions between lattice vibrations and magnetic excitations. Neutron scattering has provided a new and broader view of the twirling collective atomic vibrations in a magnetic crystal, with neutron scattering experiments giving simultaneous access to nuclear and magnetic scattering over a wide range of energies and momenta.
Magnetic fingerprints in a ferrimagnet
The core of the new result lies in a ferrimagnetic crystal cooled below its magnetic ordering temperature TC, where the spins align in a pattern that still leaves a net magnetization. Below TC, researchers observe enhanced magnetic scattering of phonons at small momenta, arising from strong magnon phonon coupling that effectively dresses the lattice vibrations with spin character. The same experiments report out of plane intensity modulation, phonon mode splitting and field induced Zeeman shifts that are closely associated with the chiral phonon modes and their interaction with the magnetic background.
In contrast to optical probes, neutron scattering offers a powerful alternative by providing simultaneous access to both nuclear and magnetic scattering, which allows the team to resolve magnetic moments of phonons reaching the µB scale. According to the neutron study, low energy phonons generate measurable magnetic scattering signals that can be separated from purely nuclear contributions through polarization analysis and field dependence. The featured analysis of Zeeman shifts shows that the phonon energies move with magnetic field in a way consistent with chiral modes carrying angular momentum, which provides direct evidence that these vibrations possess their own magnetic fingerprints.
From microscopic quirks to future devices
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. The work of Bao and collaborators offers a panoramic view of the behavior of chiral phonons throughout the Brillouin zone, thereby providing essential data for building theoretical models and developing practical applications that might steer or divert the flow of heat. Better understanding how the magnetic particles in the system behave could lead to advanced applications in heat transport and novel devices with improved energy efficiency and performance, a goal highlighted in neutron science roadmaps for better energy control.
On the more speculative side, the ability to launch and manipulate chiral phonons in time and space, as pursued in the ChiPS program coordinated by Visualization of chiral motion projects at the University of Konstanz, dovetails with a broader push toward ultrafast control of quantum states in solids. The ultrafast control demonstrated in recent all optical polariton transistors, which could facilitate the manipulation of quantum states at unprecedented rates and impact ultrafast photonics and emerging quantum computing architectures, hints at how chiral phonon based schemes might eventually integrate with ultrafast photonics. If engineers can route angular momentum and heat through solids with the same finesse that photonics now brings to light, the magnetic fingerprints of chiral phonons revealed by neutrons may become design tools for the next generation of spintronic and thermal devices.
