AC SUSCEPTIBILITY AND LOCAL INTERGRANULAR MAGNETIC-FIELD IN HIGH-TC SUPERCONDUCTORS

AC susceptibility of the granular high-T(c) superconductor La1.8Sr0.2CuO4-delta has been measured at 4.5 K as a function of applied DC field H(a)DC for different AC amplitudes H(a)m. Hysteresis related to thermal and magnetic history of the sample was studied and higher screening capabilities in the field-cooled virgin samples compared to the zero-field-cooled virgin samples were observed for all AC amplitudes. For both conditions the real and imaginary susceptibility qualitatively shows the DC and AC dependence predicted by Kim's critical state model. AC losses and enhanced diamagnetic response upon reducing H(a)DC in a field-cooled condition were studied as a function of H(a)m. We have used Kim's intergranular critical state model to evaluate AC susceptibility as a function of H(a)DC for different H(a)m. As the applied field differs from the local field in intergranular regions, we performed an appropriate scaling to describe measured values of AC susceptibility. Through this scaling process the local field at the junctions is determined for different magnetic conditions.