High intergrain critical current density in fine-grain (Ba0.6K0.4)Fe2As2 wires and bulks

The K- and Co-doped BaFe₂As₂ (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (H_c2) of well over 50 T, H_c2 anisotropy γ < 2and thin-film critical current densities J_c exceeding 1 MA cm^-2 (refs , , , ) at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behaviour for [001] tilt misorientations of more than about 5°, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba_0.6K_0.4)Fe₂As₂ bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MA cm^-2 (self-field, 4.2 K), more than 10 times higher than that of any other round untextured ferropnictide wire and 4-5 times higher than the best textured flat wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (γ~1-2) when compared with YBa₂Cu₃O_7-x (γ~5).