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Early studies have found quasi-reversible magnetization curves inpolycrystalline bulk rare-earth iron oxypnictides that suggest eitherwide-spread obstacles to intergranular current or very weak vortex pinning. Inthe present study of polycrystalline samarium and neodymium rare-earth ironoxypnictide samples made by high pressure synthesis, the hystereticmagnetization is significantly enhanced. Magneto optical imaging and study ofthe field dependence of the remanent magnetization as a function of particlesize both show that global currents over the whole sample do exist but that theintergranular and intragranular current densities have distinctively differenttemperature dependences and differ in magnitude by about 1000. Assuming thatthe highest current density loops are restricted to circulation only withingrains leads to values of ~5 MA/cm2 at 5 K and self field, while whole-samplecurrent densities, though two orders of magnitude lower are 1000-10000 A/cm2,some two orders of magnitude higher than in random polycrystalline cuprates. Wecannot yet be certain whether this large difference in global and intragraincurrent density is intrinsic to the oxypnictides or due to extrinsic barriersto current flow, because the samples contain significant second phase, some ofwhich wets the grain boundaries and produces evidences of SNS proximity effectin the whole sample critical current.

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