Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu_2Si_2

We have performed the elastic neutron scattering experiments under uniaxial stress sigma along the tetragonal [100], [110] and [001] directions for URu2Si2. For sigma // [100] and [110], the antiferromagnetic moment mu_o is strongly enhanced from 0.02 mu_B (sigma=0) to 0.22 mu_B (sigma=2.5 kbar) at 1.5 K. The rate of increase dmu_o/dsigma is roughly estimated to be ~ 0.1 mu_B/kbar, which is much larger than that for the hydrostatic pressure (~ 0.025 mu_B/kbar). Above 2.5 kbar, mu_o shows a tendency to saturate similar to the behavior in the hydrostatic pressure. For sigma // [001], on the other hand, mu_o shows only a slight increase to 0.028 mu_B (sigma = 4.6 kbar) with a rate of ~ 0.002 mu_B/kbar. The observed anisotropy suggests that the competition between the hidden order and the antiferromagnetic state in URu2Si2 is strongly coupled with the tetragonal four-fold symmetry and the c/a ratio, or both.