Muon spin rotation in the magnetic and superconducting ground states of (U,Th)Be13 and (U,Th)Pt3.

Microscopic aspects of magnetism and superconductivity have been studied in the heavy-fermion superconducting alloy systems (U,Th)Be/sub 13/ and (U,Th)Pt/sub 3/ using the technique of positive-muon (..mu../sup +/) spin rotation and relaxation (..mu..SR). In U/sub 1/minus//ital x//Th/sub /ital x//Be/sub 13/, /ital x/=0.033, a striking increase of zero-field Kubo-Toyabe relaxation rate sigma/sub KT/ is observed below the temperature /ital T//sub /ital c/2//congruent/0.4 K at which a second phase transition occurs in the superconducting state. This jump is firm evidence for the onset of weak static magnetism below /ital T//sub /ital c/2/. The observed increase of /similar to/1.5 Oe in the ..mu../sup +/ local field corresponds to an effective moment of 10/sup /minus/3/--10/sup /minus/2/ ..mu../sub /ital B///U atom.