X-rays reveal handed spin waves in manganese telluride, a candidate altermagnet

Most people know two kinds of magnetic order: the ferromagnetism of a fridge magnet and the hidden, self-cancelling order of antiferromagnets. Altermagnetism, which the authors describe as a magnetic phase that bridges the two, has a further twist. In a paper in Science, researchers report direct experimental evidence that magnetic ripples in manganese telluride (MnTe), a candidate altermagnet, carry a definite handedness. The authors call this excitation a chiral altermagnon. They argue that its behaviour bears the fingerprint of the material's altermagnetic symmetry.

In a ferromagnet, the spins of electrons line up and produce a net magnetisation. Spins are the tiny magnetic moments that electrons carry. In an antiferromagnet, neighbouring spins point in opposite directions and cancel out. Altermagnets combine features of both. A key signature mentioned in the abstract is momentum-dependent spin splitting. An electron moving through the crystal has an energy that depends on whether its spin points up or down, and the size and sign of that difference change with its direction of travel. The symmetry of the crystal and its magnetic arrangement dictates this pattern.

The authors note that the same symmetry also sets the conditions for chiral magnons to appear. A magnon is the quantum unit of a spin wave. In a spin wave, the spins across a crystal tip slightly out of alignment in a coordinated, travelling pattern, much as a sound wave is a travelling ripple of atoms. A chiral magnon has a handedness: the spins circle in one rotational sense rather than the other, and the wave carries a matching spin polarisation.

To look for such modes, the team used circular-dichroism resonant inelastic X-ray scattering, or RIXS. In RIXS, X-rays are tuned to an energy that a particular element in the sample absorbs strongly. Some X-rays are then re-emitted after giving up a little energy and momentum to excitations inside the material. Measuring those losses maps out what the excitations are and how they move. Circular dichroism means comparing the signal for X-rays whose polarisation rotates one way with the signal for X-rays whose polarisation rotates the other way. A difference between the two shows that an excitation has a preferred handedness.

According to the study, MnTe hosts a magnon mode of this kind. Its spin polarisation depends on momentum in a distinctive way. The authors find this consistent with the g-wave symmetry expected of altermagnetic MnTe. The label g-wave is borrowed from the naming of atomic orbitals. It describes a pattern that changes sign several times as one moves around the different directions in momentum space. The authors say their polarisation-resolved data support the existence of a separate class of magnetic excitations. They also say the data show how altermagnetic order shapes the way spins move collectively.

Several questions remain open. The abstract calls MnTe an altermagnetic candidate, and the result concerns a single material. The authors suggest the findings could advance spintronics, which uses electron spin as well as charge to store and process information, and quantum technologies. The abstract describes no device, however. How handed spin waves like these might be put to use is left for future work.