The circulation radius and critical current density in type II superconductors

A method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used for determining the critical current density on the basis of magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ and Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_{2}$Cu$_{3}$O$_{x}$ superconductors.