Spiral Magnetism in a Weyl Semimetal

We present evidence of spiral magnetic ordering in the Weyl semimetal SmAlSi from transport and neutron diffraction experiments. This material is a member of RAlSi family where R is the rare-earth. We show that a large magnetocrystalline anisotropy in PrAlSi, CeAlSi, and NdAlSi prevents the spiral order although chiral interactions may be present. However, SmAlSi shows a nearly isotropic crystal electric field which leads to its spiral order. We map the phase diagram of this material and reveal an A-phase at finite field and temperature. Within this phase, we observe a sizable topological Hall effect. We also find that the magnetic k-vector observed in SmAlSi matches the nesting between its Weyl nodes on the Fermi surface. As such, SmAlSi seems to be the first topological semimetal where spiral order is established from the direct engagement of Weyl electrons in magnetic interactions.