Combined microstructural and magneto optical study of current flow in polycrystalline forms of Nd and Sm Fe-oxypnictides

TL;DR

This study combines magneto-optical imaging and microstructural analysis to investigate current flow in polycrystalline Nd and Sm Fe-oxypnictides, revealing anisotropic intragranular currents and extrinsic grain boundary barriers affecting intergranular current.

Contribution

It provides detailed microstructural and magneto-optical insights into the factors limiting current flow in polycrystalline Fe-oxypnictides, highlighting extrinsic grain boundary effects.

Findings

Intragranular current is anisotropic, highest when c axes are perpendicular to the observation plane.

Extrinsic factors like cracks and wetting phases at grain boundaries reduce intergranular Jc.

Sample Jc values are 10-40 times higher than in YBa2Cu3O7-x, but still limited by extrinsic barriers.

Abstract

In order to understand why the inter- and intra-granular current densities of polycrystalline superconducting oxypnictides differ by three orders of magnitude, we have conducted combined magneto-optical and microstructural examinations of representative randomly oriented polycrystalline Nd and Sm single-layer oxypnictides. Magneto optical images show that the highest Jc values are observed within single grains oriented with their c axes perpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lower intergranular Jc is at least partially due to many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are found at many grain boundaries. However, some grain boundaries are structurally clean under high resolution transmission electron microscopy examination. Because the whole-sample global Jc(5K) values of the two samples examined are 1000-4000 A/cm2, some 10-40 times that found in random, polycrystalline YBa2Cu3O7-x, it appears that the dominant obstruction to intergranular current flow of many present samples is extrinsic, though some intrinsic limitation of current flow across grain boundaries cannot yet be ruled out.