Inhomogeneous magnetism in URu2Si2 studied by muon spin relaxation under high pressure

We have studied the low-temperature phase below 17.5 K in the heavy-fermion superconductor URu2Si2 using the zero-field muSR technique under hydrostatic pressures P up to 0.85 GPa. The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic 'hidden order'. The AF volume fraction, however, develops abruptly at around a pressure of 0.6 GPa, in contrast to the P-linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17.5 K, whereas the nominal Neel temperature seems to be about 20 K. These experimental results suggest that the AF phase exists as a metastable state. (C) 2002 Elsevier Science B.V. All rights reserved.