Competition between hidden order and antiferromagnetism in URu_2Si_2 under uniaxial stress studied by neutron scattering

TL;DR

This study investigates how uniaxial stress influences the competition between hidden order and antiferromagnetism in URu2Si2, revealing directional dependence and metastability of the induced antiferromagnetic phase.

Contribution

It provides new insights into the anisotropic response of URu2Si2 to uniaxial stress and highlights the importance of lattice strain and c/a ratio in phase competition.

Findings

Antiferromagnetic order develops under uniaxial stress along [100] and [110] directions.

The AF moment increases with stress, saturating above 0.25 GPa.

The AF phase is metastable and separated from the hidden order by a first-order transition.

Abstract

We have performed elastic neutron scattering experiments under uniaxial stress sigma applied along the tetragonal [100], [110] and [001] directions for the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with sigma along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment mu_o continuously increases from 0.02 mu_B (sigma=0) to 0.22 mu_B (0.25 GPa). The rate of increase partial mu_o/partial sigma is ~ 1.0 mu_B/GPa, which is four times larger than that for the hydrostatic pressure (partial mu_o/partial P sim 0.25 mu_B/GPa). Above 0.25 GPa, mu_o shows a tendency to saturate, similar to the hydrostatic pressure behavior. For sigma||[001], mu_o shows only a slight increase to 0.028 mu_B (sigma = 0.46 GPa) with a rate of ~ 0.02 mu_B/GPa, indicating that the development of the AF state highly depends on the direction of sigma. We have also found a clear hysteresis loop in the isothermal mu_o(sigma) curve obtained for sigma||[110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.