AC Measurements on Hard Superconductors

Abstract

Abstract Cylindrical samples of PbIn, NbZr, and NbTa were exposed to a longitudinal, static magnetic field with a collinear alternating field, and the voltages produced in pick up coils wound around the samples were observed on an oscilloscope. The shape of the voltage‐time curves can be adequately explained by considering a surface sheath which is able to carry a certain critical surface current and a bulk region where the pinning forces dominate the magnetic behavior. It is shown that an analysis of the voltage‐time curves obtained yields values for the critical surface current and the bulk critical current density which are in good agreement with static measurements. While the bulk critical current density goes to zero when the external field reaches the upper critical field H c2 , the surface currents can be seen up to a field H c3 > H c2 . Values of the bulk critical current density obtained were then used to determine the pinning force per unit volume and the pinning force on a single vortex line, as a function of the vortex line density.