Magnetic symmetry implications of the zero- and applied-field Hall effect of UNi$_4$B

Abstract

The zero-field and applied-field Hall effects in noncollinear antiferromagnets provide evidence for topological states of matter and are tied to materials' magnetic symmetry. For UNi$_4$B, the antiferromagnetic state with $T_\mathrm{N}=20$ K at zero and low magnetic field is debated due to recent magnetoelectric measurements and theory work calling into question the proposed toroidal arrangement of magnetic dipole moments. For a magnetic field applied within the plane of uranium magnetic moments, the field-dependent Hall resistivity of UNi$_4$B shows a curved response for $ρ_{yz}$ ($H{\parallel}x$, $I{\parallel}z$) and $ρ_{zx}$ ($H{\parallel}y$, $I{\parallel}x$) up to $\sim$8 T at 2 K, while an out-of-plane field results in linear behavior of $ρ_{yx}$ ($H{\parallel}z$, $I{\parallel}x$) up to 16 T. Analysis using conventional empirical relationships for the Hall effect indicate that an intrinsic effect from momentum-space Berry curvature contributes significantly to the curved transverse resistivity. Moreover, a finite zero-field Hall effect emerges at the onset of magnetic order for $ρ_{yz}$ and $ρ_{zx}$, further supporting an intrinsic origin of the Hall response. Symmetry arguments for a finite Berry curvature, an observable magnetoelectric effect, and reported magnetic structures suggest that the previously proposed magnetic space groups for the zero-field magnetic structure cannot account for the observed finite zero-field effect for two Hall orientations. Instead, we propose that $Cm'$ or $Pm'$ magnetic symmetry, depending on the parent nonmagnetic space group, is consistent with Hall resistivity, neutron diffraction, and magnetoelectric effect measurements.