Topology in van der Waals Antiferromagnets

Two-dimensional (2d) magnetism has been at the center of many decades-long researches as it offers the cleanest test bed for new ideas and physics. The prime example is the Berezinskii–Kosterlitz–Thouless transition of the XY model, which was discovered in the early 1970s. Simply, it heralds the beginning of topological physics, a new chapter in condensed matter physics. Despite the immense interest from the theoretical side, there has been relatively slow progress on an experimental side: most of which has depended on either a quai-2d materials or thin films grown by a pulsed laser deposition technique.<br/><br/>However, the discovery of van der Waals magnets in 2016 has completely transformed the field of 2d magnetism by providing real 2d magnets that can be experimentally studied using many tools. Despite their short lifetime, van der have magnets have been used for excitingly interesting reports and ideas. With so many successes, the eyes now turn to new directions: one of which is the exploration of possible topological physics in 2d van der Waals magnets.<br/><br/>Among several candidates, noncollinear metallic van der Waals antiferromagnets can reveal particularly rich topological physics due to their diverse magnetic ground states. We are particularly interested in triangular lattice antiferromagnets. By using metallic triangular antiferromagnet Co_1/3TaS₂, we show that it exhibits a substantial anomalous Hall effect (AHE) related to its noncollinear magnetic order. We show that AHE in Co_1/3TaS₂ is characterised by the toroidal moment, a vortexlike multipole component that arises from a combination of chiral lattice and geometrical frustration. We will further examine this discovery from the viewpoint of neutron scattering data.