A 2021 study of neodymium aluminium silicon (NdAlSi) reported that its neodymium spins form a spiral below about 14 K. The researchers proposed that Weyl fermions help mediate the interactions behind this magnetic pattern, but the observation does not establish NdAlSi as a “Weyl magnet.”
What happens to NdAlSi below about 14 K?
Neutron diffraction experiments at the US National Institute of Standards and Technology reportedly found disordered neodymium spins above about 14 K. Below that temperature, the spins spontaneously arrange into a spiral. The transition temperature is approximate, as reported by Chemistry World on 1 September 2021.
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The spiral’s wavelength was reported to be unrelated to the dimensions of the crystal lattice. That makes the magnetic ordering helimagnetic: its repeating spin pattern has a length scale that does not simply follow the spacing of atoms in the underlying crystal.
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NdAlSi has a non-centrosymmetric crystal structure, which is the basis cited for describing it as a Weyl semimetal. The study’s proposed explanation is that Weyl fermions mediate interactions between neighboring neodymium atoms, helping determine how their spins order.
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In the report, Johns Hopkins University physicist Collin Broholm explained that spin-momentum locking constrains how a Weyl fermion can travel between two sites: its spin must point along the direction from one site to the other, or the opposite direction. That constraint could shape the interaction transmitted between neodymium atoms. Broholm described the broader motivation as understanding what anomalous quasiparticles can do in a collective, correlated system.
Does this prove NdAlSi is a Weyl magnet?
No. The reported spiral magnetic structure is an experimental observation; the role of Weyl fermions is a proposed microscopic interpretation. Princeton University physicist Zahid Hasan told Chemistry World, “There is no clear evidence yet regarding the claim that this material is a Weyl magnet.” The evidence described in the report therefore supports a link worth investigating, not a settled classification of NdAlSi as a Weyl magnet.
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Hasan also said neutron scattering is an interesting way to explore magnetic Weyl semimetals and suggested that further neutron experiments may reveal additional phenomena. Those are his views as reported by Chemistry World, not a demonstrated result of the study.
What the 2021 report establishes
- Material: NdAlSi, described as a Weyl semimetal on the basis of its non-centrosymmetric crystal structure.
- Magnetic pattern: neutron diffraction reportedly found disordered neodymium spins above about 14 K and a spiral below it.
- Interpretation: Weyl-fermion-mediated interactions and spin-momentum locking were proposed as an explanation for the ordering.
- Open question: the report does not establish the stronger claim that NdAlSi is a Weyl magnet.
Chemistry World identified the underlying study as J. Gaudet et al., published in Nature Materials in 2021, DOI 10.1038/s41563-021-01062-8. The experimental details and quotations above are attributed to the Chemistry World report.
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