The irreversibility line in polycrystalline YBa2Cu3O7 superconductors with Y2BaCuO5 inclusions

We have investigated the irreversibility line for bulk YBa2Cu3O7:Y2BaCuO5 composites with varying mass ratios by performing AC susceptibility measurements. The irreversibility line depends on the concentration of the 211 inclusions demonstrating the influence of microstructure and pinning mechanisms. Assuming a general form for the pinning potential Up∝(1-t)α/Hβ where t=Tirr/Tc, the behaviour can be understood in terms of the giant flux creep model in which the irreversibility line follows a power law 1-t=aHq. Different values of a and q are found for each sample. Another important feature reported here is a change in the values of a and q in moving from a low to a high field region of the applied AC magnetic field marking a crossover between two regimes. The frequency dependence of the irreversibility temperature Tirr is also used to test the validity of flux creep and vortex glass models. (orig.)