The coupling between the layers has been studied in a series of six single crystals, together with a and a crystal, in terms of the magnetically measured interlayer critical current. The Ginzburg-Landau thermodynamic anisotropies have been estimated too. As oxygen is removed, the coupling weakens and both and the anisotropy between inter- and intralayer critical current densities, , increases rapidly. The current anisotropy appears to be larger than , and increases as the temperature is reduced, whereas should be temperature independent. These results are discussed in terms of Abrikosov versus Josephson vortices, and their effective dimensionality; possible pinning mechanisms are considered.

The coupling between the CuO2 layers has been studied in a series of six TmBa2Cu3O y (6.40≤y≤6.90) single crystals. As oxygen is removed, the coupling weakens and both the thermodynamic anisotropy, Γ, and the anisotropy betweeninter- andintra-layer critical current densities, Ω, increase rapidly. Ω appears to be larger than Γ, and increases as temperature is reduced. The results are discussed in terms of Abrikosov versus Josephson vortices.

The angular variation of the magnetic moment of a single-domain TmBa2Cu3O7-x single crystal in a magnetic field has been studied. There is a sharp phase transition of the vortex lattice from an essentially unpinned state with vortex ''chains,'' at small values of the angle (theta) between the external magnetic field and the ab plane of the crystal, to a mixed state, which contains both vortex chains and ordinary Abrikosov vortices and which is characterized by a strong vortex pinning, at theta> theta(cr) = (2 degrees-9 degrees). The field dependence theta(cr)(H) is very nonmonotonic. (C) 1995 American Institute of Physics.

The irreversible magnetic moment Delta mab associated with fields applied parallel to the ab-plane of high-temperature superconductor (HTS) crystals provides a crucial measure of the inter-layer coupling. However, because these materials are so anisotropic, Delta mab is small, and easily overwhelmed by the magnetization associated with in-plane screening currents. We show that even in the least anisotropic HTS phase, YBa2Cu3O7, the field alignment with the ab-planes has to be within a fraction of a degree for valid data on Delta mab to be obtained. Furthermore, slight mis-positioning of the sample in the magnetometer sense coils can cause serious artefacts. The addition of a second signal channel to the magnetometer, in which the magnetic moment perpendicular to the applied field is also monitored, allows accurate alignment and enables reliable measurements of Delta mab to be made.

We have studied the form of the irreversible magnetic relaxation in high quality YBCO (123) and BSCCO (2212) crystals with the magnetic field applied parallel to the crystal c axis. In YBCO crystals, three different regines of vortex dynamics can be discerned over a wide region of the H-T palne; with application of a scaling field, the data taken at different temperatures collapse onto a single curve. In BSCCO the absolute value of the relazation is higher than in YBCO, reflecting the weaker pinning in this more two dimensional system and only one of the three regimes of relaxation behaviour see in YBCO, is observed.

Previous attempts to study magnetically the critical current that can flow in the c -direction, i.e. from layer to layer, in YBa 2 Cu 3 O 7 have been hampered by the severe anisotropy of the material. We report and discuss the results of measurements in which the experimental problems have been overcome.

The magnetic relaxation in various moderate quality samples of high-temperature superconductors YBaCuO and BiSrCaCuO has been measured in the temperature range 4.2-80 K using a commencing applied magnetic field strength of 1 kOe. Measurements show an approximately logarithmic time dependence of the relaxation, deviating at short (In (t) 8) times. The authors tentatively suggest that linear ln (t) behaviour could be extended to cover the whole time window of their measurement if the energy distribution function could be more appropriately probed. The relaxation data have been analysed first in terms of a single 'effective' activation energy, and then, more correctly, in terms of a distribution of 'effective' activation energies. The single activation energies extracted are in broad agreement with published results. They find it appropriate to fit the distribution of activation energies to a log-normal function. However, our calculated values of the relaxation time, tau 0, are consistently faster than those previously reported. In the light of the resemblance of the present results to those previously published, the authors suggest the barriers involved in flux pinning are largely inherent to the fundamental microscopic properties of the flux line motion (e.g. vortex entanglement, 'draping' of flux lines around pinning centres) rather than being strongly dependent on the macroscopic sample quality.