  The pressure-temperature phase diagram of the heavy-electron superconductor
URu2Si2 has been reinvestigated by ac-susceptibility and elastic
neutron-scattering (NS) measurements performed on a small single-crystalline
rod (2 mm in diameter, 6 mm in length) in a Cu-Be clamp-type high-pressure cell
(P < 1.1 GPa). At ambient pressure, this sample shows the weakest
antiferromagnetic (AF) Bragg reflections reported so far, corresponding to the
volume-averaged staggered moment of mord ~ 0.011 mB/U. Under applied pressure,
the AF scattering intensity exhibits a sharp increase at P ~ 0.7 GPa at low
temperatures. The saturation value of the AF scattering intensity above 0.7 GPa
corresponds to mord ~ 0.41 mB/U, which is in good agreement with that (~ 0.39
mB/U) observed above 1.5 GPa in our previous NS measurements. The
superconductivity is dramatically suppressed by the evolution of AF phase,
indicating that the superconducting state coexists only with the hidden order
phase. The presence of parasitic ferro- and/or antiferromagnetic phases with
transition temperatures T1star =120(5) K, T2star = 36(3) K and T3star = 16.5(5)
K and their relationship to the low-T ordered phases are also discussed.
