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 CeRu 2. The relaxation rate of the muon-spin polarization in zero field exhibits a small but significant increase below TM.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 CeRu2 is a member of a restricted family of superconducting compounds that order magnetically with extremely small magnetic mo- ments at a temperature much higher than that at which they become superconducting. @S0163-1829~96!51138-0# The interplay between magnetism and superconductivity engulfs two of the richest areas of solid state physics. The coexistence of the two phenomena was first extensively stud- ied in the Chevrel phases 1 where in certain pure compounds magnetic order appears below the superconducting transition temperature, T c . For these materials the magnetically or- dered ions are only weakly coupled to the conduction elec- trons. A different situation pertains for some of the heavy fermion superconductors such as U 12xTh xBe 13 , UPt 3, and URu 2Si 2. The small size of the ordered magnetic moments in these compounds relative to their Neel temperatures at- tests to the more complex many-body origins of their magnetism. 2-4 It is remarkable that the latter two compounds have magnetic ordering temperatures roughly an order of magnitude higher than Tc and that the magnetic order per- sists into the superconducting state. In CeCu 2Si 2, the most studied Ce-based heavy fermion superconductor, the magne- tism is relatively strong and in competition with supercon-