Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu2Si2

We have performed the elastic neutron scattering experiments under uniaxial stress σ along the tetragonal [100], [110] and [001] directions for URu 2 Si 2 . For σ∥[100] and [110], the antiferromagnetic moment µ o is strongly enhanced from 0.02 µ B /U (σ=0) to 0.22 µ B /U (σ=0.25 GPa) at 1.5 K. The rate of increase ∂µ o /∂σ is roughly estimated to be ∼1.0 µ B /GPa, which is much larger than that for the hydrostatic pressure (∼0.25 µ B /GPa). Above 0.25 GPa, µ o shows a tendency to saturate similar to the behavior in the hydrostatic pressure. For σ∥[001], on the other hand, µ o shows only a slight increase to 0.028 µ B /U (σ=0.46 GPa) with a rate of ∼0.02 µ B /GPa. The observed anisotropy suggests that the competition between the hidden order and the antiferromagnetic state in URu 2 Si 2 is strongly coupled with the tetragonal four-fold symmetry and the c / a ratio, or both.