High critical current density doped ferropnictide superconductors. (a) Magneto-optical image of a rectangular slab of (Ba0.6K0.4)Fe2As2 bulk material after zero field cooling the sample to 10 K and applying 167 mT. (b) Current stream lines calculated for zero field cooled sample showing uniform current circulating though the bulk. (c) Magneto-optical image showing remnant trapped magnetic flux after zero field cooling to 5.7 K, applying a small field of 166 mT, and then removing the applied field. Credit: Weiss; FSU.

When high-temperature superconducting oxides were discovered in the mid-1980s, it was thought that they would revolutionize electric power delivery. They might still, however, the physics of these intriguing materials has made them tricky to engineer for applications. One problem that persists is the tendency of high-temperature superconducting cuprates, such as YBCO, to have weakly linked grain boundaries, which diminishes the global critical current density.

The weak linking can be overcome by controlling the grain misorientation angle with textured substrates. Another way would be to find a material with high enough local intragrain critical current densities. The ferropnictide family of compounds show some promise in this regard. (The pcnitides are the Group Va compounds in the period table of the elements: N, P, As, Sb and Bi.) The ferropcnitide family of superconducting compounds, in particular, are interesting because of their high critical temperatures and some interesting physics related to their multiband superconductivity and antiferromagnetism.

For example, the pcnitide compound, BaFe2As2 (Ba-122), is of interest because its magnetic and superconducting properties are in a range that makes them useful for applications. There has been a fair amount of research on cobalt-doped Ba-122 (electron-doped). A disadvantage is that this compound has the problem mentioned above, namely intrinsically weak linking of its grain boundaries. However, the critical current density is less sensitive to grain misorientation than is seen in the cuprate compounds. Thus, there is strong interest in studying polycrystalline ferropcnitides.

ACerS member, Eric Hellstrom, and his team at Florida State University recently published a paper in Nature Materials reporting on superconductivity in potassium-doped Ba-122 (hole-doped) wire and bulk material.

The surprising result is that the global critical current density is much higher in K-doped Ba-122 than the Co-doped version. The abstract gets right to the point: “Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba0.6K0.4)Fe2As2 bulks and round wires with high grain boundary density have transport critical current densities well over 0.1MA cm-2 (self field, 4.2 K).”

How “much more positive?” They report critical current densities that are “more than 10 times higher than that of any other round untextured ferropnitide wire and 4-5 times higher than the best textured flat wire,” which they say are “high enough to be interesting for applications.”

The improvement is attributed to enhanced grain connectivity, which in turn, arises from several factors relating to the material’s microstructure, and therefore, processing. Three factors are singled out.

First, the polycrystals were synthesized by chemical reaction, which could be done at temperatures that are low enough to prevent the formation of unfavorable secondary phases like FeAs. Secondary phases have been shown to wet the grain boundaries and block current.

Second, the synthesis process is done under high-pressure conditions, which yields a nearly 100 percent dense material and very good intragranular connectivity.

Third, the material is very fine-grained with grain sizes of approximately 200 nanometers. This means that planar grain boundaries are rare and the anisotropy values are low, which makes the vortex stiffness high. Why does this matter? Even though most of the vortices span grain boundaries, with this material, very little of any vortice actually resides in the grain boundary.

In the paper, the authors suggest that there may be some compound-related factors involved, too, citing a higher critical current density as a function of magnetic field for the K-doped Ba-122 than for the Co-doped material, which could be related to hole- vs. electron-doping.

For full details see “High intergrain critical current density in fine-grain (Ba0.6K0.4)Fe2As2 wires and bulks,” Weiss, et al., Nature Materials (doi: 10.1038/NMAT3333).