We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe 1-x Co x ) 2 As 2 . Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe 1-x Co x ) 2 As 2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field H c2 both parallel and perpendicular to the c-axis, the irreversibility field H irr c (T) and a rather unusual symmetric volume pinning force curve F p (H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ = H c2 ab /H c2 c deduced from the slopes of dH c2 ab /dT = 4.9T/K and dH c2 c /dT = 2.5T/K decreases from ~2 near T c , to ~1.5 at lower temperatures, much smaller than g for 1111 pnictides and high-T c cuprates.

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122,11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest T c , huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the T c of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families. 2 , to the ab-plane. 12 The temperature dependence is very different in the two directions, strongly departing from the WHH behaviour 16 mainly in the direction parallel to c. The anisotropy evaluated as γ = ab c ab c H H H ⊥ = 2 // 2 / γ, is also strongly temperature dependent, reminiscent of the two-gap behaviour seen in MgB 2 . 17,18 However, a different situation is observed in the 122 family. (Ba,K)Fe 2 As 2 single crystals exhibit nearly isotropic μ 0 H c2 with

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (Tc) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-Tc superconductors next to the high-Tc cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15-25 K, thin films with high critical current density (Jc) > 1 MA/cm2 at ~10 T and 4 K, and an average Jc of 1.3*104 A/cm2 at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high Jc. These properties also make IBSCs promising for applications using high magnetic fields.Comment: Published online in Materials Today. Open Acces

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.

Abstract:We show that despite the low anisotropy, strong vortex pinning and high irreversibility field H irr close to the upper critical field H c2 of Ba(Fe 1-x Co x ) 2 As 2 , the critical current density J gb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe 1-x Co x ) 2 As 2 bicrystals is strongly depressed, similar to high-T c cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry. PACS: 74.70.xa, 74.25.F-______________________________________ *