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.

No abstract

No abstract

Magnetic ordering of Er + ions in ErBa2Cu30"with 6.12~x~6.91 has been studied for both orthorhombic and tetragonal phases by means of magnetic-susceptibility and heat-capacity measurements. Superconductivity in the orthorhombic phase samples was confirmed by the onset of a diamagnetic susceptibility.The superconducting transition temperature T, was observed to decrease from 92 to 55 K as x was reduced from 6.91 to 6.31. The tetragonal samples with x=6.23 and 6.12 remained paramagnetic down to 1.2 K and showed no overt trace of diamagnetism. The states of Er'+ ions in ErBa2Cu306 and ErBa2Cu307 were calculated by means of crystalline electric field (CEF) theory using both a point-charge model and scaled inelastic neutron scattering results for HoBa2Cu307. The calculated susceptibility is in excellent agreement with data on the tetragonal specimens for 1.2 K~T~160 K. For the orthorhombic samples, sharp heat-capacity peaks which resemble those for a two-dimensional Ising system and evidently correspond to the magnetic ordering of Er + moments are found. The transition temperature T decreases smoothly from 0.604 to 0.545 K as x is varied from 6.91 to 6.31. In contrast, tetragonal samples show only a broad rounded heat-capacity anomaly having its maximum at 0.5 K and closely resembling that for a one-dimensional XY model. The change of Er'+ ion anisotropy from Ising-like to XYlike is explained by the CEF calculation which shows the ground state to be planar for the tetragonal phase but with the b axis slightly favored in the orthorhombic structure. Randomness in the coupling of Er + moments caused by structural disorder may eA'ectively reduce the dimensionality of magnetic ordering as has been found in the case of random dilution. These results imply that the dominant magnetic interactions of Er'+ ions in these materials are of short range.

Microstructure and magnetic hysteresis have been compared for two samples of the YBa2Cu3O7−δ (δ∼0) high-temperature superconductor, one unirradiated, and one irradiated with fast neutrons (E≳0.1 MeV) to a fluence of 3×1018 n/cm2. Notable changes in the microstructure include strain-induced contrast from regions 2–7 nm in size. An intrinsic critical current density (Jc) of 4.6×106 A/cm2 in zero field at 4 K has been determined from magnetic hysteresis measurements for the irradiated sample while 1.2×106 A/cm2 is noted for the unirradiated sample. We propose that the observed defect structure in the irradiated material is responsible for increased pinning and consequently higher Jc’s.