Measurement of the critical current and flux creep parameters in thin superconducting films using the single coil technique

A non-contacting method of determining the critical current of a superconducting film has proven useful in several laboratories as a routine indicator of HTS film quality. It consists of a small coil pressed against the film surface and driven with an audio frequency sine wave current (I/sub drive/). The onset of a significant third harmonic voltage component (V/sub 3f/) across the coil indicates that the screening currents in the film have exceeded their critical value. We show in this paper that the quantity V/sub 3f//(f I/sub drive/) should be a universal function of I/sub drive//I/sub sc/ if the Bean critical state model is applicable. Here I/sub sc/ is a scaling that is proportional to J/sub c/d (J/sub c/=critical current density in the film, d is its thickness) and f is the frequency. By varying the temperature of a thin YBCO film between 4.2 K and its transition, this scaling was observed to apply over a range of J/sub c/´s covering more than three decades. The frequency dependent measurements revealed a logarithmic dependence of the critical current on frequency that can be interpreted as a manifestation of flux creep. This data was used to infer the pinning energy within the collective pinning model.