Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides

Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either widely spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto-optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. If the highest current density loops lie only within grains, their magnitude is similar to 5 x 10(6) A cm(-2) at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at 1000 -10 000 A cm(-2), are some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain a significant second phase, some of which wets the grain boundaries and may cause a superconducting -normal -superconducting proximity effect in the whole sample critical current.