Evidence for supercurrent connectivity in conglomerate particles in NdFeAsO_1-δ

Here we report the use of 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 contribute to 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 10³Acm^-2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ∼120νm it is clear that, on smaller scales, there is better current connectivity with a critical current density of the order of 5 × 10⁴Acm^-2. We find enhanced flux creep around the second peak anomaly in the magnetization curve and an irreversibility line significantly below H_c2(T) as determined by ac calorimetry.