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

TL;DR

The paper investigates the origin of a large topological Hall effect in EuCd$_2$Sb$_2$, combining magnetotransport, heat capacity, and magnetic measurements on high-quality single crystals. It identifies three intrinsic mechanisms for Berry-curvature–driven signatures: momentum-space Berry curvature from Weyl/Dirac nodes formed by broken $C_3$ symmetry in the AFM state and by spin fluctuations above $T_{ m N}$, and real-space Berry curvature from scalar spin chirality in AFM domain walls below $T_{ m N}$. Through careful decomposition of Hall data and analysis of conductivities, the work shows how these mechanisms produce distinct features in $ ho_{xy}$, $ ho^{ m T}_{xy}$, and $ ho_{xx}$ across temperatures and magnetic fields. The findings advance understanding of THE in Eu-based 122 compounds and highlight the interplay between lattice symmetry, magnetic order, and topological electronic structure in determining transport signatures.

Abstract

We study the origin of large topological Hall effect in the single-crystalline EuCd$_2$Sb$_2$, which orders antiferromagnetically at the Néel 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.

Figures (11)

• Figure 1: (a) Temperature dependence of the heat capacity of EuCd$_2$Sb$_2$. The red solid line shows the fit of Eq. 1 to $C_p(T)$. The inset shows the heat capacity anomaly due to the phase transition to AFM ordered state. (b) Temperature dependence of the magnetic contribution $C_\text{mag}$ to heat capacity capacity, (c) and the magnetic entropy ($S_{\rm mag}$). The $C_p, C_{\rm mag}$ and $S_{\rm mag}$ are all shown in $\rm J\,mol^{-1} K^{-1}$ units.
• Figure 2: Transverse (a, b) and longitudinal (c,d) magnetoresistance of the EuCd$_2$Sb$_2$ as a function of magnetic field for different temperatures ranging from 2 to 150 K. (a) and (c) show data collected at $T<T_{\rm N}$, (b) and (d) those collected at $T>T_{\rm N}$. Inset to (c) shows a blowup of the low-field region. The measurement geometries are shown in the insets (with z parallel to crystallographic c-axis).
• Figure 3: Magnetic field dependence of Hall resistivity of EuCd$_2$Sb$_2$. (a) $\rho_{xy}(H)$ for various temperatures from the range of 2-150 K (curves are offset for clarity). (b) Anomalous Hall resistivity $\rho_{xy}^{\rm A}(H)$ at several temperatures, an arrow marks the region in which there are additional humps at low temperatures. (c) topological Hall resistivity as a function of the magnetic field at several temperatures.
• Figure 4: (a) The extrema of topological Hall conductivity, $\sigma_{xy}^{\rm T}$ versus the longitudinal conductivity, $\sigma_{xx}$. (b) Temperature dependence of these extrema of $\sigma_{xy}^{\rm T}$ (right axis) and temperature dependence of the magnetic field strength, in which they occur, $\mu_0 H_{\text{min}}$, (left axis) (c) The color map of the topological Hall resistivity, $\rho_{xy}^{\rm T}$ plotted in ($T$,$H$) coordinates superimposed on the phase diagram for $\mathbf{H}\parallel c$, constructed based on the magnetic measurements. Stars mark $T_{\rm N}$ values in different fields, derived from supplemental $M(T)$ data SupM.
• Figure 5: Magnetic field dependence of the magnetization, transverse magnetoresistance and topological Hall resistivity for $T$ = 2 K (a), 4 K (b) and 6 K (c). Red and blue vertical dashed lines mark $H_{\rm s}$ and $H_{\rm s}/\sqrt {2}$, respectively.