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

Abstract

Microscopic aspects of magnetism and superconductivity have been studied in the heavy-fermion superconducting alloy systems (U,Th)Be13 and (U,Th)Pt3 using the technique of positive-muon (μ+) spin rotation and relaxation (μSR). In U1−xThxBe13, x=0.033, a striking increase of zero-field Kubo-Toyabe relaxation rate σKT is observed below the temperature Tc2≃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 Tc2. The observed increase of ∼1.5 Oe in the μ+ local field corresponds to an effective moment of 10−3–10−2 μB/U atom. The μ+ Knight shift Kμ in the normal states of U1−xThxBe13, x=0 and 0.033, is proportional to the bulk susceptibility and yields a transferred f-spin-μ+ hyperfine field of -1.99±0.12 kOe/μB. In the superconducting states of U1−xThxBe13, x=0, 0.01, and 0.033, a large reduction of the magnitude of Kμ is observed for x=0 but not for x=0.033, and an intermediate reduction is found for x=0.01. This behavior suggests (1) spin-singlet pairing in undoped UBe13, and (2) conventional reduction of the spin susceptibility suppression in (U,Th)Be13 by spin-orbit scattering from Th impurities. In undoped UPt3 small increases of σKT and the transverse-field Gaussian relaxation rate σG below ∼6 K are again evidence for the onset of weak static magnetism similar to that found in (U,Th)Be13. Recent spin-polarized neutron scattering studies of normal-state UPt3 have observed a commensurate antiferromagnetic (AF) structure with a sublattice magnetization of ∼10−2 μB/U atom, in rough agreement with the μSR results. For U1−xThxPt3, x=0.05, zero-field μ+ precession frequencies below 6.5 K indicate an AF structure with a considerably larger moment (∼0.6 μB/U atom). The average μ+ Knight shift in the normal state of undoped UPt3 is much smaller than in UBe13, which suggests multiple μ+ stopping sites with hyperfine couplings of opposite signs.

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