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-drive). The onset of a significant third harmonic voltage component (V-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-3f/(f I-drive) should be a universal function of I-drive/I-sc if the Bean critical state model is applicable, Here I-sc is a scaling current that is proportional to J(c)d (J(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(c)'s covering two decades and drive frequencies 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.