Evaluation of grain-coupling strength of YBaCuO superconductors by their magnetic-field profile measurements

In order to evaluate the strength of intergrain coupling, which determines the transport critical current of polycrystalline oxide superconductors, the relation between the transport critical current and the spatial distribution of the magnetic flux density for YBaCuO thin films as well as bulk superconductors has been studied. The authors have measured the profile of the magnetic flux trapped inside bulk YBaCuO samples with a small Hall magnetic sensor and revealed the correlation between critical-current densities and the slope of the trapped-flux profile, as described by the critical-state model. The measurements of the magnetic-field penetration depth of YBaCuO thin films in a Meissner state is also carried out by measuring the loop inductance of a DC SQUID placed on the YBaCuO thin film to be measured. Observed magnetic-field penetration depths of polycrystalline films with thicknesses of 1-3 μm deposited on MgO single-crystal substrates amount to ~1 μm for samples with T_c, (end) ~80 K and the critical-current density J_c ~10⁴ A/cm² at 4.2 K