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

Abstract

Early studies have found quasi-reversible magnetization curves in polycrystallinebulk rare-earth iron oxypnictides that suggest either widely spread obstacles tointergranular current or very weak vortex pinning. In the present study of polycrystallinesamarium and neodymium iron oxypnictide samples made by high pressure synthesis,the hysteretic magnetization is significantly enhanced. Magneto-optical imagingand study of the field dependence of the remanent magnetization as a functionof particle size both show that global currents over the whole sample do existbut that the intergranular and intragranular current densities have distinctivelydifferent temperature dependences and differ in magnitude by about 1000. Ifthe highest current density loops lie only within grains, their magnitude is∼5 × 106 A cm−2 at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at1000–10 000 A cm−2, are some two orders of magnitude higher than in random polycrystalline cuprates. Wecannot yet be certain whether this large difference in global and intragrain current densityis intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because thesamples contain a significant second phase, some of which wets the grain boundaries andmay cause a superconducting–normal–superconducting proximity effect in the wholesample critical current.