Pressure-induced magnetic phase transition of the 5f electron system URu2Si2

A puzzling phase transition at T-o similar to 17.5 K of the heavy-electron 5f system URu2Si2 is characterized by development of an unusually small sta ggered moment mu(o) concomitant with large bulk anomalies and sharp magnon excitations. Through elastic and inelastic neutron scattering, we have stud ied the influence of pressure P up to similar to 2.8 GPa on this phase tran sition. For pressures less than 1.3 GPa, mu(o) at 1.4 K is quasi-linearly e nhanced from similar to 0.017(3) mu(B) to similar to 0.25(2) mu(B), while i ts onset temperature increases only ten present. We have also observed a sh arp phase transition at P-c similar to 1.5 GPa, above which a 3D-Ising type of antiferromagnetic phase (mu(o) similar to 0.4 mu(B)) appears. Sharp mag non excitations at Q = (1,0,0) and (1,0.4,0) collapse in this high-P phase. The lattice parameter a decreases discontinuously at P-c even for T > T-o, indicating that pressure induces a significant change in electronic states in a higher characteristic energy scale. We show that the observed magneti c instability could qualitatively well be explained by competition between quadrupolar and dipolar interactions in a non-trivial Gamma(5) doublet.