Correlations between microstructure and a.c. magnetic response of bulk superconducting YBa2Cu3O7−x

The magnetic response of granular superconducting YBCO samples with different grain sizes is reported. The analysis is carried out by measuring the low-frequency a.c. magnetic susceptibility as a function of the temperature with an applied magnetic field of amplitude ranging between 0.4 and 800 A/m. The observed magnetic behaviour is interpreted in terms of grain size and weak-links properties. The experimental results evidence the influence of the sample microstructure on the magnetic susceptibility. The intergranular and intragranular onset temperatures are compared with the critical temperature of the samples obtained by means of electrical-resistivity measurements. The temperature dependence of the intergranular critical current density is derived from the susceptibility data.