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

AC susceptibility of the granular high-T(c) superconductor La1.8Sr0.2C uO4-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 therm al 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 b oth conditions the real and imaginary susceptibility qualitatively sho ws the DC and AC dependence predicted by Kim's critical state model. A C losses and enhanced diamagnetic response upon reducing H(a)DC in a f ield-cooled condition were studied as a function of H(a)m. We have use d Kim's intergranular critical state model to evaluate AC susceptibili ty as a function of H(a)DC for different H(a)m. As the applied field d iffers 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 deter mined for different magnetic conditions.