Technique for measuring the elementary pinning force in thin films

We have developed a contactless technique which utilizes the ultrahigh sensitivity of a superconducting quantum interference device (SQUID) to measure the temperature dependence of the elementary pinning force of superconducting thin films near their transition temperature. Pinning from the edges of the film does not contribute to our measurement, and we have detected pinning forces as small as ${10}^{\mathrm{-}6}$ dyn/cm. Initial measurements on niobium films indicate that the pinning force for films with grain boundary pinning sites is larger and has a steeper temperature dependence than films for which the pinning centers are crystal dislocations. The $f_p$ values that we measure for niobium are smaller than those reported recently for lead-bismuth films by Hyun et al.