Pressure-induced magnetism and hidden order in URu2Si2

We discuss the discovery of pressure-induced antiferromagnetism in URu2Si2, in the context of neutron, NMR and muSR results. The identification of a critical pressure separating mean-field and Ising phase transitions leads us to propose that the system lies close to a bicritical point associated with magnetic and (non-magnetic) hidden order. We note that the recent observation of an isotropic, field-independent component in the silicon NMR line-width implies that the hidden order parameter breaks time-reversal invariance. Relying on simple estimates for the observed local fields, we conclude that URu2Si2 is a strong candidate for an incommensurate orbital antiferromagnet. (C) 2002 Elsevier Science B.V. All rights reserved.