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

Abstract

Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either wide-spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium rare-earth 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. Assuming that the highest current density loops are restricted to circulation only within grains leads to values of ~5×10 6 A/cm 2 at 5 K and self field, while whole-sample current densities, though two orders of magnitude lower are 1000-10000 A/cm 2 , 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 significant second phase, some of which wets the grain boundaries and produces evidences of SNS proximity effect in the whole sample critical current. 2 Introduction:The recent discovery of superconductivity in the LaFeAsO 1-x F x compound [1] has stimulated a rapid exploration of superconductivity in the rare earth iron oxypnictides [2][3][4][5][6][7][8][9][10][11][12][13][14]. It has now been established that the iron oxypnictides can be superconducting when doped to x ~0.05-0.2 and that they can have transition temperature T c above 40 K when La is replaced by Ce [5] and above 50 K by Pr, Nd, Sm and Gd [7][8][9][10][11]. In a recent paper [12] we addressed the issue of electromagnetic granularity in polycrystalline La iron oxypnictides, finding an asymmetric M(H) loop that indicated an irreversible moment due to hysteretic bulk currents that was almost as small as the reversible magnetization of the superconducting state. In that case we were not able to distinguish definitively between a state where the intragrain pinning was very weak, leading to very low intragrain current densities or to the state where currents were largely confined to the intragrain regions and might have been rather high. Based on the rather high upper critical field B c2 (0) values of 63-65 T observed by Hunte et al.

Evidence for electromagnetic granularity in polycrystalline Sm1111 iron-pnictides with enhanced phase purity

“…Bulk polycrystalline samples were exposed to many cycles of ever increasing magnetic field H a , followed by removal of the field and measurement of the remanent moment, m R . 11 For a pure, homogeneous sample, we expect flux to penetrate when H a /(1-D) first exceeds the lower critical field H c1 , where D is the relevant demagnetizing factor. For weakly coupled polycrystals, flux penetration occurs preferentially at grain boundaries, pores or nonsuperconducting second phases at lower fields than are needed to penetrate into the grains.…”

“…11,13,14,15,16,17 In our previous study, evidence for two distinct scales of current flow was found in polycrystalline Sm-and Nd -1111 samples using magneto-optical imaging (MO) and analysis of the field dependence of the remanent magnetization (RM). 11 A global current density J c global flowing in the whole sample of order 4000 A/cm 2 was determined at 4 K in self-field. This value, which appears to be more than one order of magnitude larger than for early results on randomly-oriented polycrystalline cuprates, 18 is still one of the highest for polycrystalline pnictides reported to date.…”

Combined microstructural and magneto-optical study of current flow in polycrystalline forms of Nd and Sm Fe-oxypnictides

“…The discovery of superconductivity in the LaFeAsO 1-x F x compound [1] has been followed by rapid exploration of many aspects of the superconducting behaviour of the broad class of rare earth iron oxypnictides [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] whose transition temperature T c can reach above 40 K when La is replaced by Ce [5] and above 50 K when the rare earth is Pr, Nd, Sm and Gd [7][8][9][10][11]. Hunte et al reported that even the La Fe-oxypnictide with Tc ~ 26 K exhibits a very high upper critical field H c2 of ~65 T [6] while H c2 over 200 T was deduced for the Sm and Nd Fe-oxypnictides [17], strongly suggesting a large high field domain for the Fe-oxypnictides.…”

“…Foreseeing practical applications, there has been immediate interest in the critical current density too. But all polycrystalline samples of La, Sm and Nd Fe-oxypnictides [12][13][14][15][16] examined to date show signs of less than full grain-to-grain connectivity, raising the same concern of depression of superconducting order parameter at grain boundaries that has so greatly complicated applications of the cuprates [18]. Grain boundary order parameter suppression is fundamental detriment to applications since it means that a randomly aligned grain structure will not pass the full current that can be sustained by intragrain vortex pinning, thus reducing the global or whole-sample current density below that circulating in the grains.…”

Structural and critical current properties in polycrystalline SmFeAsO1−xFx

“…On the other hand, many workers have focused on magnetic and transport properties of the series [10][11][12][13][14][15][16][17]. Detailed phase diagram and transport properties of SmFeAsO 1-x F x were studied, and it was found that superconductivity emerges at x=0.10 and optimal doping takes place at x=0.2 with the highest Tc=54 K [10].…”