Zero-field positive muon-spin relaxation rates in ${\mathrm{U}}_{0.965}$${\mathrm{Th}}_{0.035}$${\mathrm{Be}}_{13}$ provide definitive evidence for the onset of magnetic correlations at the lower phase transition at temperature $T_{c2}$ below the normal-superconducting transition at $T_{c1}$. The transition at $T_{c2}$ is found to be second order and mean field like, with a moment (${10}^{\mathrm{-}3}$–${10}^{\mathrm{-}2}$)${\mu }_B$/f atom. Candidates for the second phase, such as magnetic order and magnetic superconducting states, are discussed.

• Received 31 March 1989

DOI:https://doi.org/10.1103/PhysRevB.40.806