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 Weyl semimetal, NdAlSi, that offers an example. Using neutron diffraction, we found a long-wavelength helical magnetic order in NdAlSi, the periodicity of which is linked to the nesting vector between two topologically non-trivial Fermi pockets, which we characterize using density functional theory and quantum oscillation measurements. We further show the chiral transverse component of the spin structure is promoted by bond-oriented Dzyaloshinskii–Moriya interactions associated with Weyl exchange processes. Our work provides a rare example of Weyl fermions driving collective magnetism.

外尔半金属中的新兴相对论准粒子是奇异电子特性的来源，例如表面费米弧、异常霍尔效应和负磁阻，所有这些都在真实材料中观察到。尽管这些现象突出了外尔费米子对电子传输特性的影响，但人们对它们可能支持的集体现象知之甚少。在这里，我们报告了一个 Weyl 半金属 NdAlSi，它提供了一个例子。使用中子衍射，我们在 NdAlSi 中发现了长波长螺旋磁序，其周期性与两个拓扑非平凡费米口袋之间的嵌套矢量相关，我们使用密度泛函理论和量子振荡测量对其进行表征。我们进一步表明自旋结构的手性横向分量是由与外尔交换过程相关的键取向的 Dzyaloshinskii-Moriya 相互作用促进的。我们的工作提供了一个罕见的外尔费米子驱动集体磁性的例子。