AC susceptibility study of fluorine-doped Bi-2212 ceramics

Two superconducting ceramics of Bi2Sr2CaCu2OyFx (x=0, 0.4) were elaborated by solid state reaction. Their temperature dependence of AC susceptibility under different magnetic fields were measured on rectangular bar-shaped samples. The study shows that fluorine doping improves the intergrain connectivity and increases the effective superconducting volume fraction fg of the grains. fg increases linearly with the applied magnetic field in the doped sample while it saturates at high fields in the undoped one. For each sample, the maximum of the extracted matrix susceptibility differs slightly from the predicted theoretical values of Bean's critical state model. The deduced intergranular critical current density Jc(T) was observed to increase with fluorine doping. At 5 K, the estimated value of the critical field Hcw of the weak links is higher in the doped sample.