Magnetic properties of URu₂Si₂ under uniaxial stress by neutron scattering

The aim of this study is to compare the magnetic behavior of URu ₂Si₂ under uniaxial stress along the a axis with the behavior under hydrostatic pressure. Both are very similar, but uniaxial stress presents a critical stress σxa [0.33(5) GPa] that is smaller than the hydrostatic critical pressure p_x (0.5 GPa) where the ground state switches from a HO (hidden-order) to AF (antiferromagnetic) ground state. From these critical values and Larmor neutron diffraction, we conclude that the magnetic properties are governed by the shortest U-U distance in the plane (a lattice parameter). Under stress, the orthorhombic unit cell stays centered. A key point shown by this study is the presence of a threshold for the uniaxial stress along the a axis before the appearance of the large AF moment, which indicates no mixture of the order parameter between the HO ground state and the AF one as under hydrostatic pressure. The two most intense longitudinal magnetic excitations at Q₀ =(1,0,0) and Q₁ =(0.6,0,0) were measured in the HO state: the excitation at Q₀ decreases in energy while the excitation at Q₁ increases in energy with the uniaxial stress along the a axis. The decrease of the energy of the excitation at Q ₀ seems to indicate a critical energy-gap value of 1.2(1) meV at σxa. A similar value was derived from studies under hydrostatic pressure at p_x.