Magnetoelastic phenomena in antiferromagnetic uranium intermetallics: the $\mathrm{UAu_{2}Si_{2}}$ case

TL;DR

This study investigates the magnetoelastic properties of $ ext{UAu}_2 ext{Si}_2$, revealing strong anisotropic effects, a collapse of unit-cell volume, and complex magnetic phase transitions under magnetic field and pressure.

Contribution

It provides the first comprehensive analysis of magnetoelastic coupling and phase diagram in $ ext{UAu}_2 ext{Si}_2$, highlighting anisotropic effects and pressure-dependent magnetic transitions.

Findings

Large anisotropy in magnetoelastic properties.

Collapse of unit-cell volume in the antiferromagnetic state.

Field-induced metamagnetic transition from second to first order.

Abstract

Thermal expansion, magnetostriction and magnetization measurements under magnetic field and hydrostatic pressure were performed on a $\mathrm{UAu_{2}Si_{2}}$ single crystal. They revealed a large anisotropy of magnetoelastic properties manifested by prominent length changes leading to a collapse of the unit-cell volume accompanied by breaking the fourfold symmetry (similar to that in $\mathrm{URu_{2}Si_{2}}$ in the hidden-order state) in the antiferromagnetic state as consequences of strong magnetoelastic coupling. The magnetostriction curves measured at higher temperatures confirm a bulk character of the 50 K weak ferromagnetic phase. The large positive pressure change of the ordering temperature predicted from Ehrenfest relation contradicts the more than an order of magnitude smaller pressure dependence observed by the magnetization and specific heat measured under hydrostatic pressure. A comprehensive magnetic phase diagram of $\mathrm{UAu_{2}Si_{2}}$ in magnetic field applied along the $c$ axis is presented. The ground-state antiferromagnetic phase is suppressed by a field-induced metamagnetic transition that changes its character from the second to the first order at the tricritical point.