We examine the phase diagram of the antiferromagnetic heavy-fermion superconductor U${\mathrm{Ru}}_2$${\mathrm{Si}}_2$ with resistivity experiments in continuous magnetic fields up to 25 T. The phase transition that occurs in zero field at $T_N=T_0=17.5\mathrm{}$ K is accompanied by the formation of a tiny staggered magnetic moment of $\mu =0.04\mathrm{}{\mu }_B$. The resistivity anomaly at $T_0$ reflects the formation of an energy gap $\Delta$. The resulting phase diagram clearly distinguishes the $T_0\mathrm{}\left(H\right)\mathrm{}$ phase boundary and earlier observed high-field (${\mu }_{0}H\simeq 40$ T) field-induced phase transitions towards a large magnetic moment state. Surprisingly, the magnetic field dependence of $\mu$, determined by previous neutron-scattering experiments, strongly differs from that of $T_0$ and $\Delta$, with estimated critical fields of 14.5 T for $\mu$ and ≃ 40 T for $T_0$ and $\Delta$. This observation suggests that there are two energy scales relevant to the magnetic phase transition of U${\mathrm{Ru}}_2$${\mathrm{Si}}_2$.

• Received 2 November 1995

DOI:https://doi.org/10.1103/PhysRevB.53.R6014