Gap formation and magnetic ordering in U ${Ru}_{2}$ ${Si}_{2}$ probed by high-field magnetoresistance

We examine the phase diagram of the antiferromagnetic heavy-fermion superconductor U

{Ru}_{2}

{Si}_{2}

with resistivity experiments in continuous magnetic 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 magnetic moment of

μ = 0.04 μ_{B}

. The resistivity anomaly at

T_{0}

reflects the formation of an energy gap

Δ

. The resulting phase diagram clearly distinguishes the

T_{0} (H)

phase boundary and earlier observed high-field (

μ_{0} H ≃ 40

T) field-induced phase transitions towards a large magnetic moment state. Surprisingly, the magnetic field dependence of

μ

, determined by previous neutron-scattering experiments, strongly differs from that of

T_{0}

and

Δ

, with estimated critical fields of 14.5 T for

μ

and ≃ 40 T for

T_{0}

and

Δ

. This observation suggests that there are two energy scales relevant to the magnetic phase transition of U

{Ru}_{2}

{Si}_{2}

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Gap formation and magnetic ordering in URu2Si2probed by high-field magnetoresistance