  Emergent relativistic quasiparticles in Weyl semimetals are the source of
exotic electronic properties such as surface Fermi arcs, the anomalous Hall
effect, and negative magnetoresistance, all observed in real materials. Whereas
these phenomena highlight the effect of Weyl fermions on the electronic
transport properties, less is known about what collective phenomena they may
support. Here, we report a new Weyl semimetal, NdAlSi that offers an example.
Using neutron diffraction, we report a long-wavelength magnetic order in NdAlSi
whose periodicity is linked to the nesting vector between two topologically
non-trivial Fermi pockets, which we characterize using density functional
theory and quantum oscillation measurements. Our work provides a rare example
of Weyl fermions driving collective magnetism.
