Estimation of Critical Current Densities in Polycrystalline ${\rm Sr}_{0.6}{\rm K}_{0.4}{\rm Fe}_{2}{\rm As}_{2}$ Superconductors

The superconducting Sr_0.6K_0.4Fe₂As₂ (122 type) polycrystalline iron-based pnictides with Pb or Ag addition were prepared by a normal PIT method. The critical current densities were investigated by ac inductive (Campbell´s) method. It was found that a fairly large critical current density over 10¹⁰ A/m² even at the relatively high temperatures flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low global critical current density of 10⁴10⁶ A/m² flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller critical current density in the increasing-field process than that in the decreasing-field process. Basing on the experimental results of the penetrating ac magnetic flux profile and the characteristic of ac magnetic field vs. penetration depth, the inter- and intra-granular super-current, the weak-link and the history effect in these materials were discussed.