Inductive measurements of critical current density in superconducting thin films

A noncontacting method of probing the current-induced breakdown of superconductivity (i.e., J_c) in thin films is described which makes use of a single pancake coil pressed against the film surface. The technique has a sensitivity that is approximately 100 times greater than direct transport measurements using room-temperature electronics, and it eliminates many of the attendant difficulties of the latter. Preliminary results on Nb and Y-Ba-Cu-O films at 4.2 K reveal an exponential voltage-current dependence, as expected from the activated flux creep model. It is noted that, this being the case, no unique critical-current density can be defined. In the case of the oxide superconductors the flux-pinning parameters are such that even a practical J_c definition is probably not useful