Intergrain current flow in a randomly oriented polycrystalline $SmFeAsO0.85$ oxypnictide

We report a direct current transport study of the local intergrain connections in a polycrystalline $SmFeAsO0.85$ (Sm1111) bulk, for which we earlier estimated significant intergranular critical current density $Jc$⁠. Our combined low temperature laser scanning microscopy and scanning electron microscopy observations revealed only few grain-to-grain transport current paths, most of which switched off when a magnetic field was applied. These regions typically occur where current crosses Fe–As, which is a normal-metal wetting-phase that surrounds Sm1111 grains, producing a dense array of superconducting-normal-superconducting contacts. Our study points out the need to reduce the amount of grain boundary-wetting Fe–As phase, as well as the crack density within pnictide grains, as these defects produce a multiply connected current-blocking network.