CeRu2: A magnetic superconductor with extremely small magnetic moments

Muon-spin-relaxation experiments and ac low-field magnetization measurements have been carried out on the superconductor Ce${\mathrm{Ru}}_2$. The relaxation rate of the muon-spin polarization in zero field exhibits a small but significant increase below $T_M\simeq 40$ K, which is suppressed by applying a longitudinal field. This result taken together with magnetization measurements provides definite evidence for the occurrence of static electronic magnetism involving extremely small magnetic moments. Our work shows that Ce${\mathrm{Ru}}_2$ is a member of a restricted family of superconducting compounds that order magnetically with extremely small magnetic moments at a temperature much higher than that at which they become superconducting.