Evaluation of Critical Current Density of FeAs-based Superconductors

```  Abstract-- In this review paper, we report the characteristics of the critical current density in FeAs based superconductors which is newly discovered by Hosono group of Tokyo Institute of Technology on 2008. Since the many specimens in present stage are not single crystals, there are two kinds of critical current density observed in the specimens which are so-called local and global critical current densities. Therefore, it is necessary to evaluate both kinds of critical current densities. The history effect in which the global critical current density shows different values in increasing and decreasing magnetic field is also observed when the specimens have the local and the global critical current densities. The wire which critical current is 180 A is successfully developed with using the knowledge of abovementioned characteristics of two kinds of critical current densities and the history effect. kinds of critical current density in the specimens. They are so-called local and global critical currents. That is, the connection between the grains in specimen is too week to carry large current, and the global critical current density is far lower than the local critical current density which flows only inside the grain. They are also called intra- and inter-grain shielding current density. It is essential to measure both local and global critical current densities, because the local critical current density will be achieved when the specimen will be prepared as single crystalline in near future which is already done in RE-123 (RE: Rare Earth) superconducting coated wire. Moreover, the global critical current is corresponding to transport critical current density which is usually measured by four probe method. In the present study, the local and the global critical current