High transport critical current densities in textured Fe-sheathed Sr1-xKxFe2As2+Sn superconducting tapes

We report the realization of grain alignment in Sn-added Sr1-xKxFe2As2 superconducting tapes prepared by ex-situ powder-in-tube method. At 4.2 K, high transport critical current densities Jc of 2.5x10^4 A/cm^2 (Ic = 180 A) in self-field and 3.5x10^3 A/cm^2 (Ic = 25.5 A) in 10 T have been measured. These values are the highest ever reported so far for Fe-based superconducting wires and tapes. We believe the superior Jc in our tape samples are due to well textured grains and strengthened intergrain coupling achieved by Sn addition. Our results demonstrated an encouraging prospect for application of iron based superconductors.Comment: 14 pages, 4 figure

Transport properties and anisotropy in rare earth doped CaFe2As2 single crystals with Tc above 40 K

In this paper we report the superconductivity above 40 K in the electron doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction patterns indicate high crystalline quality and c-axis orientation. the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by partial replacement of Ca by rare earth and a superconducting transition reaches as high as 43 K, which is higher than the value in electron doping FeAs-122 compounds by substituting Fe ions with transition metal, even surpasses the highest values observed in hole doping systems with a transition temperature up to 38 K. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding the anisotropy of 2~3. Hall-effect measurements indicate that the conduction in this material is dominated by electron like charge carriers. Our results explicitly demonstrate the feasibility of inducing superconductivity in Ca122 compounds via electron doping using aliovalent rare earth substitution into the alkaline earth site, which should add more ingredients to the underlying physics of the iron-based superconductors.Comment: 21 pages, 7 figure

Effect of starting materials on the superconducting properties of SmFeAsO1-xFx tapes

SmFeAsO1-xFx tapes were prepared using three kinds of starting materials. It shows that the starting materials have an obvious effect on the impurity phases in final superconducting tapes. Compared with the other samples, the samples fabricated by SmAs, FeO, Fe2As, and SmF3 have the smallest arsenide impurity phase and voids. As a result, these samples possess much denser structure and better grain connectivity. Moreover, among the three kinds of samples fabricated in this work, this kind of sample has the highest zero-resistivity temperature ~40 K and largest critical current density ~4600 A/cm^2 in self-field at 4.2 K. This is the highest Jc values reported so far for SmFeAsO1-xFx wires and tapes.Comment: 15 pages, 4 figure

Improved transport critical currents in Ag and Pb co-doped BaxK1-xFe2As2 superconducting tapes

Fe-clad BaxK1-xFe2As2 superconducting tapes were fabricated by the ex situ powder-in-tube method combined with a short high-temperature annealing technique. The effect of annealing time and different dopants on the transport properties of the BaxK1-xFe2As2 tapes were systematically studied. By co-doping with Ag and Pb, the transport critical current density Jc of BaxK1-xFe2As2 tapes was significantly improved in whole field region and the highest transport Jc was up to 1.4x10^4 A/cm^2 (Ic=100 A) at 4.2K in self field. It is proposed that the superior Jc in the co-doped samples are due to the combine effect of Pb doping at low fields and Ag doping at high fields.Comment: 22 pages, 7 figures, to be published in Supercond. Sci. Techno

Morphology and Crystallography of Ausferrite in Austempered Ductile Iron

The microstructure of austempered ductile iron was investigated by electron backscatter diffraction technique. The results show that the orientation relationship between acicular bainitic ferrite and austenite is Greninger–Troiano relationship. A single austenite grain is divided into four packets and each packet contains six variants that share a {011}α (i.e., {111}γ) plane. When two γ grains are twinned, the twins share a {111}γ plane and have seven packets. The adjacent acicular bainitic ferrite plates (or laths) sharing a 〈 001 〉 γ axis have small misorientation of about 5.7°. The adjacent acicular bainitic ferrite plates (or laths) not sharing a 〈 001 〉 γ axis have two high misorientation angles of ~54.3° and ~60.0°. Further, the low angle boundary to high angle boundary ratio is far less than the ratio of the variant pairs with small misorientation to the ones with large misorientation. This work is available for structures obtained as a consequence of the heat treatment of austempering

Superconductivity and phase diagram in Ru-doped SrFe

Single crystals of SrFe2−xRuxAs2 have been successfully synthesized using the FeAs self-flux method. The X-ray diffraction patterns indicate that the substitution of Fe by Ru leads to a decrease of the c parameter. Upon Ru substitution, the resistivity anomaly in the parent compound is progressively suppressed and superconductivity is stabilized at low temperatures for x≥0.45. The superconducting region is dome-like, with an optimal Tc ∼15.5 K found around x=0.667. A phase diagram of temperature vs. doping, based on electrical transport, has been constructed. Results of upper and lower critical field are presented. Our results confirm that the substitution of Fe ions with isoelectronic Ru could suppress the magnetic/structural phase transition in the parent compound, and suggest that there is strong competition between magnetism, structure, and superconductivity in iron-based superconductors

Superconductivity and magnetic properties in SmFe

In this paper the effects of replacing Fe with Co in SmFe1-xCoxAsO (x=0 to 1) are reported. X-ray diffraction patterns indicate that all the samples have formed the ZrCuSiAs-type structure with a space group P4/nmm and a full solid solution forms between SmFeAsO and SmCoAsO. It is found that the antiferromagnetic spin-density-wave (SDW) order in the parent compounds is rapidly suppressed by Co-doping, and superconductivity emerges as 0.05 ≤ x ≤ 0.175. A phase diagram is derived based on the resistivity measurements and a dome-like Tc-vs.-x curve is established. Furthermore, it was shown that the metallic character increases with the addition of more Co. Meanwhile we found that the two magnetic phase transitions in SmCoAsO are suppressed as Co is replaced by Fe. Our work demonstrates that the solid solution has been established in the SmFeAsO-SmCoAsO system. The systemic study in the SmFe1-xCoxAsO system would add more ingredients to the underlying physics of this material

Superconductivity at 22 K in Ir-doped CaFe

In this paper we report superconductivity in single crystals of the new iron-pnictide system CaFe2−xIrxAs2 (x=0, 0.05, 0.10, 0.13) grown by a self-flux solution method and characterized via X-ray, transport and magnetic measurements. The magnetic ordering associated with a structural transition at 160 K present in CaFe2As2 is completely suppressed by substitution of Fe with Ir and superconductivity is induced at a critical temperature Tc=22 K in the sample with x=0.13. We also determined the upper critical fields along the ab-plane and c-axis. The anisotropy of the CaFe2−xIrxAs2 superconductors determined by the ratio of Habc2 and Hcc2 is estimated to be 1.5–2.2. Our results suggest that superconductivity can also be easily induced in the CaFe2As2 family by substituting the Fe with Ir, at almost the similar maximum transition temperatures as for Co, Ni and Rh doping