We have investigated magnetic relaxation on a grain-aligned sample of YBa2Cu307-, powder embedded in epoxy with the magnetic field along the c axis. Through a thermal-cycling procedure and through variations in the measuring temperature over the range 10-30 K, we manage to commence magnetic relaxation from a range of values of the magnetization M or current density J. Analysis of relaxation measurements at a magnetic field of 1.0 T over the temperature range 10-30 K reveals a nonlinear dependence of the effective activation energy U, on M or J that is consistent with the logarithmic dependence obtained by Zeldov et al. from transport measurements.This demonstration of a nonlinear dependence of U, (M) is capable of resolving anomalies in the magnitudes and temperature dependence of flux-creep activation energies obtained from magnetic-relaxation studies.

Microscopic aspects of magnetism and superconductivity have been studied in the heavy-fermion superconducting alloy systems (U,Th)Be» and (U,Th)Pt3 using the technique of positive-muon (p ) spin rotation and relaxation (pSR). In U& Th Be», x =0.033, a striking increase of zero-field Kubo-Toyabe relaxation rate o. KT is observed below the temperature T,2-0.4 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,&. The observed increase of -1.5 Oe in the p+ local field corresponds to an effective moment of 10 -10 pz/U atom. The p+ Knight shift K" in the normal states of U& Th, Be», x =0 and 0.033, is proportional to the bulk susceptibility and yields a transferred f-spin-p, " hyperfine field of -1.99+0.12 kOe/ps. In the superconducting states of U&, Th Be», x =0, 0.01, and 0.033, a large reduction of the magnitude of K" 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 UBe», and (2) conventional reduction of the spin susceptibility suppression in (U,Th)Be» by spin-orbit scattering from Th impurities. In undoped UPt3 small increases of a&T and the transverse-field Gaussian relaxation rate o. G below -6 K are again evidence for the onset of weak static magnetism similar to that found in (U,Th)Be». Recent spin-polarized neutron scattering studies of normal-state UPt3 have observed a commensurate antiferromagnetic (AF) structure with a sublattice magnetization of -10 pz/U atom, in rough agreement with the pSR results. For U& Th Pt3, x =0. 05, zero-field p+ precession frequencies below 6.5 K indicate an AF structure with a considerably larger moment (-0.6 pz/U atom). The average p+ Knight shift in the normal state of undoped UPt3 is much smaller than in UBe», which suggests multiple p+ stopping sites with hyperfine couplings of opposite signs.

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