Muon-spin relaxation studies of weak magnetic correlations in ${\mathrm{U}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Th}}_{\mathrm{x}}$${\mathrm{Be}}_{13}$

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.