Irreversibility line and non-linearity in the AC response caused by flux pinning in high- Tc superconductors

Abstract

We have studied the irreversibility lines (IL) obtained by AC magnetic measurements in the region with notable dependence of the AC susceptibility on the AC field amplitude. The commonly used procedure of taking temperatures of maxima in the imaginary part of AC susceptibility χ″ is shown to be consistent with the definition of the IL as the line of constant critical current density. The data obtained in experiments on melt-grown YBaCuO samples were used to compare this method with two other ones: (a) searching for the appearance of flux pinning in a non-linear flux-flow regime, (b) construction of current-voltage characteristics from AC magnetic measurements. We demonstrated that it is possible to reach good agreement between irreversibility lines acquired under the conditions of a non-linear AC regime by different methods if the current densities and the electrical fields in the sample are comparable.