LOW-FIELD AC MAGNETIZATION OF GRANULAR HIGH-T-C SUPERCONDUCTORS

We report the low-field a.c. magnetization of granular high-T-c superc onductors with an anomalous structure, not previously observed. The M- H curve develops a lobe in addition to the main loops, showing intergr anular losses. We attribute this to the possible presence of a second kind of weak-link network, connecting the grains along the applied fie ld direction. This is a consequence of the finite geometry of the grai ns, bringing their demagnetization factor into play. This also explain s the observed double-peak structure in the second-harmonic voltage wi th d.c. fields.