Large Topological Hall Effect and Spiral Magnetic Order in the Weyl Semimetal SmAlSi

Abstract

Weyl electrons are intensely studied due to novel charge transport phenomena such as chiral anomaly, Fermi arcs, and photogalvanic effect. Recent theoretical works suggest that Weyl electrons can also participate in magnetic interactions, and the Weyl-mediated indirect exchange coupling between local moments is proposed as a new mechanism to induce spiral magnetic ordering by involving chiral Weyl electrons. Here, we present evidence of Weyl-mediated spiral magnetism in SmAlSi from neutron diffraction, transport, and thermodynamic data. We show that the spiral order in SmAlSi results from the nesting between topologically nontrivial Fermi pockets and weak magnetocrystalline anisotropy, unlike related materials (Ce,Pr,Nd)AlSi, where a strong anisotropy prevents the spins from freely rotating. We map the magnetic phase diagram of SmAlSi and reveal anAphase where topological magnetic excitations may exist. Within theAphase, we find a large topological Hall effect whose variation with the magnetic field direction suggests a dominant helical instead of cycloidal character, as theoretically predicted for the Weyl-induced spiral order.

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Plain Language Summary

Weyl semimetals are materials in which the energy of electrons is linearly proportional to their velocity—that is, they are relativistic. In addition, the spin and momentum of their electrons are locked together, which classifies them as “chiral.” These special characteristics produce exciting electrical properties, but it is unknown whether these characteristics can also affect magnetic properties. A few recent theoretical works have proposed that the Weyl electrons can couple to localized spins in a material, and directly mediate magnetic interactions. Their relativistic and chiral properties are predicted to create a spiral magnetic order. In this work, we present the first experimental realization of such a Weyl-mediated spiral order.

So far, there is only one candidate material for Weyl-mediated magnetism in the literature: NdAlSi. In this compound, wave vectors that correspond to the magnetic structure connect the Weyl electrons. This property is called nesting. However, the spins in NdAlSi have strong anisotropy and tend to point along a specific crystallographic direction. As such, they are not free to rotate in a spiral magnetic order. We find that the spins in a different material, SmAlSi, do not have such a strong anisotropy and thus, they settle in spiral order.

Our work provides a recipe for finding materials with Weyl-mediated spiral order. The key ingredients are the proper nesting vectors and isotropic spins. The next step might be to realize Weyl-mediated magnetism at higher temperatures and use these magnetic structures in high-density magnetic storage devices.