Muon spin rotation in the magnetic and superconducting ground states of (U,Th)Be13and (U,Th)Pt3

Abstract

Microscopic aspects of magnetism and superconductivity have been studied in the heavy-fermion superconducting alloy systems (U,Th)${\mathrm{Be}}_{13}$ and (U,Th)${\mathrm{Pt}}_3$ using the technique of positive-muon (${\mu }^{+}$) spin rotation and relaxation (μSR). In ${\mathrm{U}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Th}}_{\mathrm{x}}$ ${\mathrm{Be}}_{13}$, x=0.033, a striking increase of zero-field Kubo-Toyabe relaxation rate ${\sigma }_{\mathrm{KT}}$ is observed below the temperature $T_{c2}$≃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 $T_{c2}$. The observed increase of ∼1.5 Oe in the ${\mu }^{+}$ local field corresponds to an effective moment of ${10}^{\mathrm{-}3}$–${10}^{\mathrm{-}2}$ ${\mu }_B$/U atom.