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.
- Publication:
-
Physical Review B
- Pub Date:
- June 1989
- DOI:
- 10.1103/PhysRevB.39.11345
- Bibcode:
- 1989PhRvB..3911345H
- Keywords:
-
- 74.70.Hk;
- 75.20.Hr;
- 76.75.+i;
- Local moment in compounds and alloys;
- Kondo effect valence fluctuations heavy fermions;
- Muon spin rotation and relaxation