AC inductance measurements of high-temperature superconductor thin films in DC magnetic fields

An AC inductance technique combined with a variable DC magnetic field has been used to measure at 77 K the flux exclusion properties of thin films of the high-temperature superconductors YBa2Cu3O7-x and Tl2Ba2CaCu2O8, prepared by coevaporation, in situ off-axis sputtering, or laser ablation. These results are compared to those for single crystal YBa2Cu3O7-x. A decrease in flux exclusion with applied DC field is seen for all samples, most strongly for Tl2Ba2CaCu2O8 films. The change in flux exclusion with magnetic field varies widely for YBa2Cu3O7-x samples, with some laser-ablated films showing less change in flux exclusion with magnetic field than single crystal YBa2Cu3O7-x. These results are interpreted in terms of magnetic field penetration through crystalline structures and grain boundaries in these materials. (orig.)