Crystallographic and magnetic structure of UNi$_4$$^{11}$B

TL;DR

This study uses neutron scattering to elucidate the crystallographic and magnetic structures of UNi4B, revealing an orthorhombic lattice, a partially ordered antiferromagnetic state below 19.5 K, and insights into its toroidal magnetic order.

Contribution

It provides the first comprehensive refinement of UNi4B's structure and magnetic order, clarifying the relationship between its structural and magnetic properties.

Findings

Orthorhombic lattice structure at low temperature

Partially ordered antiferromagnetic state below 19.5 K

No magnetic structure change at 330 mK

Abstract

We present an extensive powder and single-crystal neutron scattering investigation of the crystallographic structure and magnetic order of the frustrated metallic $f$-electron magnet UNi$_4$B. We carry out a full refinement of the crystallographic structure and conclude that the low-temperature lattice symmetry is orthorhombic (space group Pmm2; cell parameters: $a = 6.963(4)~\r{A}$ , $b = 14.793(9)~\r{A}$ , $c = 17.126(8)~\r{A}$). We determine the magnetically ordered structure, concluding that below $T_\mathrm{N} = 19.5~\mathrm{K}$ the material undergoes a transition into a partially ordered antiferromagnetic state. The magnetic structure is consistent with the existence of toroidal order in this material. We further test the proposal of a second magnetic transition occurring at $330~\mathrm{mK}$, concluding that the thermodynamic anomalies observed at these temperatures do not reflect modifications of the magnetic structure. Our study provides a consistent picture of the interrelationship of structural and magnetic properties in the frustrated magnet UNi$_4$B previously unresolved.