Strong flux pinning and anomalous anisotropy of Sr_0.6K_0.4Fe₂As₂ superconducting tapes

A comprehensive study on the resistivity, magnetization and microstructure is carried out to understand the superconductivity and flux pinning behavior of hot-pressed Sr_0.6K_0.4Fe₂As₂ tapes. The Sr_0.6K_0.4Fe₂As₂ tapes possess excellent properties of high T _c = 36.2 K, large transport J _c of ∼10⁵A cm⁻² at 10 T and 4.2 K, and high pinning potential U ₀/k _B with small field-dependence, demonstrating a great potential in high-field applications. The magnetic J _c at various temperatures and flux pinning mechanism are exhibited for both B//ab and B//c directions. Notably, a point pinning in combination with surface pinning appears in our samples, resulting in a strong flux pinning force. And the characteristic ratio B _irr ^//c(0 K)/B _irr ^//ab(0 K) is as low as 1.2, indicating that the vortex pinning abilities are nearly isotropic. Further transmission electron microscope characterization reveals that the anomalous pinning behavior is originated from the largely increased small-sized defects and the morphological differences along two directions. These positive results indicate that tailoring the pinning characteristic by creating micro-defects is crucial to improving the J _c value and suppressing the anisotropy in iron pnictides. (paper)