Evidence for Supercurrent Connectivity in Conglomerate Particles in NdFeAsO1-d

TL;DR

This study demonstrates supercurrent connectivity in NdFeAsO1-d superconductors, revealing scale-dependent critical current densities and flux dynamics through magnetometry and microscopy techniques.

Contribution

It provides direct evidence of supercurrent pathways in conglomerate particles and quantifies their critical current densities at different scales.

Findings

Macroscopic critical current density ~10^3 A/cm^2

Microscopic critical current density ~5 x 10^4 A/cm^2

Enhanced flux creep observed near the second magnetization peak

Abstract

Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry.