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

Abstract

We prepared polycrystalline SmFeAsO 1-x F x (Sm1111) bulk samples by sintering and hot isostatic pressing (HIP) in order to study the effects of phase purity and relative density on the intergranular current density. Sintered and HIPped Sm1111 samples are denser with fewer impurity phases, such as SmOF and the grain boundary wetting phase, FeAs. We found quite complex magnetization behavior due to variations of both the inter and intragranular current densities. Removing porosity and reducing second phase content enhanced the intergranular current density, but HIPping reduced T c and the intragranular current density, due to loss of fluorine and reduction of T c . We believe that the HIPped samples are amongst the purest polycrystalline 1111 samples yet made. However, their intergranular current densities are still small, providing further evidence that polycrystalline pnictides, like polycrystalline cuprates, are intrinsically granular. 2 IntroductionThe discovery of superconductivity in LaFeAs(O,F) with critical temperature T c of 26 K was announced by Hosono et al. 1 in February 2008. This event prompted an army of physicists, chemists and material scientists to study these materials and shortly thereafter many other Fe-based superconductors were discovered that are now grouped into 4 families, which are generally referred to as "1111" for REFeAsO, "122" for AEFe 2 As 2 , 2 "111" for LiFeAs 3 and "11" for Fe(Te,Se) 4 where RE denotes rare earth and AE denotes alkali earth. The critical temperatures, in the optimally doped compounds, range from 19 K to 55 K and have very high upper critical fields approaching ~300 T for 1111 5 and ~100 T for 122 doped with potassium 6 or cobalt 7 . These favorable characteristics make it urgent to explore their potential for applications with the hope that practical conductors can be made with these Fe-based superconductors using simpler processes than are needed with high-temperature cuprate superconductors. 8 Actually these new Fe-based superconductors share several characteristics with cuprates including: layered structures, coexistence of different orderings, occurrence of superconductivity upon doping, short coherence length and unconventional pairing. In cuprates all these features have been shown to be unfavorable for applications. On the other hand, the available results show that both impurities 9,10 and grain boundaries 11,12 are less detrimental to the superconducting properties of these Fe-based superconductors than to the cuprates. Use of Fe-based superconductors in largescale applications would be greatly enhanced if polycrystalline samples were not intrinsically electromagnetically granular, as is the case for the cuprates. However, investigations of the critical current density (J c ) of 1111 polycrystalline samples have all shown significant evidence for granularity and low intergranular J c values. 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-optica...

Progress in wire fabrication of iron-based superconductors

“…Global supercurrent flow [36,84], and locally well-connected area [85,86] were confirmed to exist in this system.…”

“…However, Yamamoto et al reported that even for the purest polycrystalline 1111 bulk samples made by the sintering and HIP process, their intergranular current densities are still small [84], providing further evidence that polycrystalline 1111 oxypnictides, like cuprates, are largely affected by weak-linked problems.…”

Density effect on critical current density and flux pinning properties of polycrystalline SmFeAsO1 −xFxsuperconductor

“…Recently, the peak effect of J cm (B) in oxypnictide polycrystal [36] and single crystal [43] was verified to be caused by a first order phase transition from a ordered "elastically pinned" low-field vortex phase to a high-field disordered phase. weak links or local links due to Fe-As wetting phase on the grain boundary [19,25,27], and the number of strong links is very few. The well-connected structure of sample A increases the amount of weak links and local links, leading to the higher V sc and J cm (B) than sample B-F in lower fields.…”

“…However, the REFeAs(O, F) materials have short coherence length [15], low carrier density [16,17], significant evidence for granularity and low intergranular J c [18,19], which are similar to the case of cuprates. Many efforts were devoted to enhance the sample quality [19][20][21]. The SmFeAsO 1-x F x superconducting wires with high transport J c were successfully prepared by powder-in-tube (PIT) technique of both in-situ [22] and ex-situ [23] processes.…”

Exploring the feasibility of Fe(Se,Te) conductors by ex-situ powder-in-tube method

“…Indeed, the best transport critical current density, Jc, up to 10 5 -10 6 A/cm 2 in self field, has been obtained so far with the 122 family. 5,6 Recently, Jc values above the practical level (10 5 A/cm 2 at 10 T) have been Indeed, they suffer the loss of fluorine as a consequence of both the thermal treatment 9 and the strong interdiffusion with the metal sheaths. 10 Recently, a transport critical current density above 10 4 A/cm 2 at 4.2 K and in self-field has been achieved in 1111 tapes, but it is strongly suppressed by the field.…”