Magnetic anti-skyrmions and triangular antiferromagnetism in Mn₃X and Mn₂XY compounds

It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that, rather, the non-collinear triangular antiferromagnet hexagonal Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals¹. Our theoretical calculations demonstrate that the Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect comparable to that of platinum. In the chiral triangular antiferromagnet cubic Mn3Ir, we observe a large spin Hall effect but no evidence for any anomalous Hall effect². We speculate that this is because the sign of the spin Hall effect is independent of the chirality of the triangular antiferromagnet structure but that the sign of the anomalous Hall effect changes sign with the chirality of the antiferromagnetic structure. Finally, we discuss the discovery of anti-skyrmions in several tetragonal inverse Heusler compounds using Lorentz transmission electron microscopy³ and variable temperature magnetic force microscopy. We discuss the topological Hall effect that we observe in these systems.

1 Nayak, A. K. et al. Large anomalous Hall effect driven by non-vanishing Berry curvature in non-collinear antiferromagnet Mn₃Ge Sci. Adv. 2, e1501870, (2016).
2 Zhang, W. et al. Giant facet-dependent spin-orbit torque and spin Hall conductivity in the triangular antiferromagnet IrMn₃. Sci. Adv. 2, e1600759, (2016).
3 Nayak, A. K. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561-566, (2017).