GAP FORMATION AND MAGNETIC-ORDERING IN URU2SI2 PROBED BY HIGH-FIELD MAGNETORESISTANCE

We examine the phase diagram of the antiferromagnetic heavy-fermion su perconductor URu2Si2 with resistivity experiments in continuous magnet ic fields up to 25 T The phase transition that occurs in zero field at T-N=T-0=17.5 K is accompanied by the formation of a tiny staggered ma gnetic moment of mu=0.04 mu(B). The resistivity anomaly at T-0 reflect s the formation of an energy gap Delta. The resulting phase diagram cl early distinguishes the T-0(H) phase boundary and earlier observed hig h-field (mu(0)H similar or equal to 40 T) field-induced phase transiti ons towards a large magnetic moment state. Surprisingly, the magnetic field dependence of mu, determined by previous neutron-scattering expe riments, strongly differs from that of T-0 and Delta, with estimated c ritical fields of 14.5 T for mu and similar or equal to 40 T for T-0 a nd Delta. This observation suggests that there are two energy scales r elevant to the magnetic phase transition of URu2Si2.