Pressure dependence of magnetism in URu2Si2

Neutron-scattering and specific-heat measurements of the heavy-fermion superconductor URu2Si2 under hydrostatic pressure and with Rh-doping [U(Ru{0.98}Rh{0.02})2Si2] show the existence of two magnetic phase transitions. At the second-order phase transition Tm ≈ 17.5 K, a tiny ordered moment is established, while at TM < Tm, a first-order phase transition (under pressure or doping) gives rise to a large moment. The results can be understood in terms of a hidden OP Psi coupled to the ordered moment m, where m and Psi have the same symmetry.