Note on the interpretation of magnetic diffraction in NdAlSi: helical or fan?

We revisit the magnetic structure analysis reported in Nat. Mater. 20, 1650 (2021), which concluded that a Weyl semimetal candidate NdAlSi hosts a helical magnetic order. This conclusion was based on magnetic neutron diffraction peaks corresponding to modulation vectors ${\vec{k}}_{in}=(1\mathrm{/}3,1\mathrm{/}3,0)$ and ${\vec{k}}_{out}=(2\mathrm{/}3,2\mathrm{/}3,0)$, attributed to in-plane and out-of-plane components of the magnetic moments, respectively. Upon careful reanalysis, we suggest that a fan-type magnetic structure–rather than the helix–provides a more consistent interpretation of these data. Unlike a helical structure, fan structures do not exhibit handedness. The distinction has significant implications for interpreting the electromagnetic responses in this material. We believe that our suggestions motivate a re-examination of magnetic structures in NdAlSi and other tetragonal siblings, where the interplay between magnetism and topology is under active investigation.