CeRu2: A magnetic superconductor with extremely small magnetic moments

Muon-spin-relaxation experiments and ac low-held magnetization measurements have been carried out on the superconductor CeRu2. The relaxation rate of the muon-spin polarization in zero field exhibits a small but significant increase below T-M similar or equal to 40 K, which is suppressed by applying a longitudinal held. 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 CeRu2 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.