Anomalous anisotropy of critical currents in (Sr, K)Fe₂As₂ tapes

We measured critical currents and irreversibility fields of Ag-sheath (Sr, K)Fe₂As₂ tapes at various temperatures, magnetic fields and magnetic field directions. The sample tapes were made by a powder-in-tube with a hot-press process in the final heat treatments. The inverse anisotropy in J _c in comparison with the effective mass anisotropy can be found in the hot-pressed (Sr, K)Fe₂As₂ tapes. We found that the broad c-axis peak in angular dependence of J _c appears in a low field region and it decreases with increasing field and temperature. In addition, the crossover of J _c anisotropy appears around 10 T at 4.2 K and the crossover field decreases with increasing temperature. It is considered that these unusual anisotropic J _c behaviors may be originated by the strong random pinning center which is larger than the coherence length,