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 ${\mathrm{URu}}_2{\mathrm{Si}}_2$. 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 $0.02{\mu }_B∕\mathrm{U}(\sigma =0)$ to $0.22{\mu }_B∕\mathrm{U}$ (0.25 GPa). The rate of increase $\partial {\mu }_o∕\partial \sigma$ is $\sim 1.0{\mu }_B∕\mathrm{GPa}$, which is four times larger than that for the hydrostatic pressure $(\partial {\mu }_o∕\partial P\sim 0.25{\mu }_B∕\mathrm{GPa})$. Above 0.25 GPa, ${\mu }_o$ shows a tendency to saturate, similar to the hydrostatic pressure behavior. For $\sigma \Vert \left[001\right]$, ${\mu }_o$ shows only a slight increase to $0.028{\mu }_B∕\mathrm{U}(\sigma =0.46\mathrm{GPa})$ with a rate of $\sim 0.02{\mu }_B∕\mathrm{GPa}$, 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 ${\mu }_o\left(\sigma \right)$ curve obtained for $\sigma \Vert \left[110\right]$ 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.

• Received 7 October 2003

DOI:https://doi.org/10.1103/PhysRevB.72.214419