A simple measurement technique for critical current density by using a permanent magnet

We developed a novel nondestructive and contactless method for measurements of critical current density (J_c) in high-temperature superconductor (HTS) thin films by using a permanent magnet (Sm₂Co₁₇). This measurement technique is based on a repulsive force (F_r) and attractive force (F_a) between the magnet and HTS film caused by the shielding currents and pinning forces. The F_r and F_a were measured by using a high-resolution load sensor changing the distance (L) between the magnet and HTS film. An effective maximum repulsive force (F_m,eff) could be determined from an extrapolated value of the F_r vs. L curve for L=0 mm. We investigated the relationship of the J_c to the F_m,eff in many HTS (YBa₂Cu₃O_7-δ and ErBa₂Cu₃O_7-δ) samples. The standard J_c of HTS films was measured by using an inductive measurement system produced from THEVA GmbH. We found that the J_c is almost proportional to the two-thirds power of the F_m,eff normalized by the film thickness. This result indicates that the simple permanent magnet technique can be easily estimate the J_c of the HTS films. This system has good reproducibility for the J_c measurements and is useful to provide a rapid, routine characterization of the superconductivity in the HTS materials.