Nanoscale Texture and Microstructure in NdFeAs(O,F)/IBAD-MgO Superconducting Thin Film with Superior Critical Current Properties

TL;DR

This study investigates the nanoscale texture and microstructure of NdFeAs(O,F) superconducting thin films grown by molecular beam epitaxy, revealing microstructural features that contribute to their high critical current density and superconducting performance.

Contribution

It provides detailed microstructural analysis of NdFeAs(O,F) films and links grain boundary characteristics to their superior superconducting properties, using advanced electron microscopy techniques.

Findings

High critical current density of 2 x 10^6 A/cm^2 at 4.2 K

99% of grain boundaries have misorientation angles below the critical angle

Triple junctions of low-angle grain boundaries act as effective flux pinning centers

Abstract

This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain boundaries perpendicular to the $ab$-plane, good superconducting properties are measured: a critical temperature, $T _{\rm c}$, of 46 K and a self-field critical current density, $J_{\rm c}$, of $2 \times 10^6 \,{\rm A/cm^2}$ at 4.2 K. Automated crystal orientation mapping by scanning precession electron diffraction in transmission electron microscopy is employed to analyze the misorientation angles between adjacent grains in a large ensemble (247 grains). 99% of the grain boundaries show in-plane misorientation angles ($\Delta \gamma$) less than the critical angle $\theta_{\rm c}$, which satisfies one of the necessary conditions for the high $J_{\rm c}$. Comparing the columnar grain size distribution with the mean distance of the flux line lattice, the triple junctions of low-angle grain boundaries are found to be effective pinning centers, even at high temperatures ($\ge$35 K) and/or low magnetic fields.