Abstract
In order to understand why the inter- and intra-granular current densitiesof polycrystalline superconducting oxypnictides differ by three orders ofmagnitude, we have conducted combined magneto-optical and microstructuralexaminations of representative randomly oriented polycrystalline Nd and Smsingle-layer oxypnictides. Magneto-optical images show that the highestJc values are observed within single grains oriented with theirc axesperpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lowerintergranular Jc is at least partially due to many extrinsic factors, because cracks anda ubiquitous wetting As–Fe phase are found at many grain boundaries.However, some grain boundaries are structurally clean under high resolutiontransmission electron microscopy examination. Because the whole-sample globalJc (5 K) values of the two samples examined are1000–4000 A cm−2, some 10–40 times higher than that found in random polycrystallineYBa2Cu3O7−x, it appears that the dominant obstruction to intergranular current flow of many presentsamples is extrinsic, though some intrinsic limitation of current flow across grainboundaries cannot yet be ruled out.
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