Origin of the large topological Hall effect in the EuCd$_2$Sb$_2$ antiferromagnet

Abstract

We study the origin of large topological Hall effect in the single-crystalline EuCd$_2$Sb$_2$, which orders antiferromagnetically at the N\'eel temperature $T_{\rm N}=7.4$ K. Measurements of magnetoresistance and Hall resistivity disclose anomalies that evolve with temperature and magnetic field, closely tracking the magnetization process. Analysis of these data identifies three possible mechanisms responsible for the enhanced Berry curvature driving the observed topological Hall effect. Below and above $T_{\rm N}$, Weyl states are the main sources of large momentum-space Berry curvature, though their formation mechanisms differ in these two temperature ranges. Below $T_{\rm N}$, breaking of $C_{3}$ symmetry generates Dirac points that split into Weyl nodes in applied magnetic field, whereas above $T_{\rm N}$, strong spin fluctuations can induce Weyl states. The third contribution, which occurs below $T_{\rm N}$, arises from scalar spin chirality developing within antiferromagnetic domain walls, which generates a real-space Berry curvature.