Rapid and simple measurement of critical current density in HTS thin films using a permanent magnet method

We present the results of non-destructive and contactless measurement of the critical current density (Jc) of high-temperature superconductor (HTS) thin films using a permanent magnet method. A cylindrical permanent magnet (Sm2Co17) with a height of 5.0 mm and diameter d of 5.0 mm was used. To investigate the margin of the sample setting for the vertical direction, load vs. distance curves for a minimum distance Lmin which varied from 0.5 to 2.0 mm were measured. Here, L is the distance from the permanent magnet to a surface of the HTS film. The effective maximum repulsive force Fm,eff was almost constant below Lmin = 1.2 mm. This result indicates that the measurement system has a wide margin with respect to sample setting in the vertical direction. We investigated the dependence of Fm,eff on the sample area from diameter l = 20-4 mm and found that the relationship of the critical current density Jc = 1.7 × 10−7 (Fm,eff/t) can be applied for l/d > 2 and the required sample size of the HTS films is approximately l = 2d for this system. Here, t is the thickness of the HTS thin film. These results indicate that the permanent magnet method can be used to easily estimate the Jc of HTS films at 77 K. Additionally, this system has good reproducibility for Jc measurements. It is thus promising as a replacement for conventional inductive measurements and for measurements for Jc mapping of HTS films, tapes, and wires using a small magnet.