Magnetic relaxation near a fishtail peak of isothermal magnetization in B1 NbCy encapsulated in multiwall carbon nanocages

Isothermal magnetization near a fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 4000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with temperature T and is independent of applied magnetic field H. The results of J(t) and U_eff (T, H) are consistent with the Anderson–Kim flux–creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. U_eff at a fishtail peak field H_fp evolves quickly above a fishtail peak temperature T_fp, but slowly below that temperature. The result suggests that a decrease of flux viscosity coefficient above T_fp at H_fp is the origin of the fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.     

Pinning,thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high Jc YBa2Cu3O7−x films

YBa₂Cu₃O_7?x (Y123) films with quantitatively controlled artificial nanoprecipitate pinning centers were grown by pulsed laser deposition (PLD) and characterized by transport over wide temperature (T) and magnetic field (H) ranges and by transmission electron microscopy (TEM). The critical current density J_c was found to be determined by the interplay of strong vortex pinning and thermally activated depinning (TAD), which together produced a non-monotonic dependence of J_c on c-axis pin spacing d_c. At low T and H, J_c increased with decreasing d_c, reaching the very high J_c ～ 48 MA/cm² ～20% of the depairing current density J_d at 10 K, self-field and d_c ～ 10 nm, but at higher T and H when TAD effects become significant, J_c was optimized at larger d_c because longer vortex segments confined between nanoprecipitates are less prone to thermal fluctuations. We conclude that precipitates should extend at least several coherence lengths along vortices in order to produce irreversibility fields H_irr(77 K) greater than 7 T and maximum bulk pinning forces F_p,max(77 K) greater than 7–8 GN/m³ (values appropriate for H parallel to the c-axis). Our results show that there is no universal pin array that optimizes J_c at all T and H.     

A new method for estimating the critical current density of a superconductor from its hysteresis loop

The critical current density J_c of some of the superconducting samples, calculated on the basis of the Bean’s model, shows negative curvature for low magnetic field with a downward bending near H = 0. To avoid this problem Kim’s expression of the critical current density, J_c = k/(H₀ + H), where J_c has positive curvature for all H, has been employed by connecting the positive constants k and H₀ with the features of the hysteresis loop of a superconductor. A relation between the full penetration field H_p and the magnetic field H_min, at which the magnetization is minimum, is obtained from the Kim’s theory. Taking the value of J_c at H = H_p according to the actual loop width, as in the Bean’s theory, and at H = 0 according to an enhanced loop width due to the local internal field, values of k and H₀ are obtained in terms of the magnetization values M⁺(?H_min), M^?(H_min), M⁺(H_p) and M^?(H_p). The resulting method of estimating J_c from the hysteresis loop turns out to be as simple as the Bean’s method.     

Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

We report the achievement of transport critical currents in Sr_0.6K_0.4Fe₂As₂ wires and tapes with a T_c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J_c was enhanced upon silver addition, and at 4.2 K, a largest J_c of ～1200 A/cm² (I_c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J_c is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J_c at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.     

Critical properties of superconducting Ba1−xKxFe2As2

Magnetisation and magneto-resistance measurements have been carried out on superconducting Ba_1?xK_xFe₂As₂ samples with x = 0.40 and 0.50. From high field magnetization hysteresis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The J_C determined from the high field data is >10⁴ A/cm² at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 T. From the variation of the T_C onset with applied field, dH_C2/dT at T_C was obtained to be ?7.708 T/K and ?5.57 T/K in the samples with x = 0.40 and 0.50.     

Increase of critical current density with doping carbon nano-tubes in YBa2Cu3O7−δ

The effects of carbon nano-tubes (CNTs) on the crystal structure and superconducting properties of YBa₂Cu₃O_7?δ (Y-123) compound were studied. Samples were synthesized using standard solid-state reaction technique by adding CNT up to 1 wt% and X-ray diffraction data confirm the single phase orthorhombic structure for all the samples. Current–voltage measurements in magnetic fields up to 9 T were used to study the pinning energy U_J and critical current density J_c as a function of magnetic field at fixed temperature. We find that while T_c does not change much with the CNT doping (91–92 K), both U_J and J_c increase systematically up to 0.7 wt% CNT doping in a broad magnetic field ranges between 0.1 and 9 T and J_c in the 0.7 wt% CNT doped sample is at least 10 times larger than that of the pure Y-123. The scanning electron microscope image shows that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the Y-123-compounds improve the electrical connection between superconducting grains to result in the J_c increase.     

Influence of Zr and Ce doping on electromagnetic properties of (Gd,Y)–Ba–Cu–O superconducting tapes fabricated by metal organic chemical vapor deposition

(Gd,Y)Ba₂Cu₃O_x tapes have been fabricated by metal organic chemical vapor deposition (MOCVD) with Zr-doping levels of 0–15 mol.% and Ce doping levels of 0–10 mol.% in 0.4 μm thick films. The critical current density (J_c) of Zr-doped samples at 77 K, 1 T applied in the orientation of H 6 c is found to increase with Zr content and shows a maximum at 7.5% Zr doping. The 7.5% Zr-doped sample exhibits a critical current density (J_c) of 0.95 MA/cm² at H 6 c which is more than 70% higher than the J_c of the undoped sample. The peak in J_c at H 6 c is 83% of that at H 6 a–b in the 7.5% Zr-doped sample which is more than twice as that in the undoped sample. Superconducting transition temperature (T_c) values as high as about 89 K have been achieved in samples even with 15% Zr and 10% Ce. Ce-doped samples with and without Ba compensation are found to exhibit substantially different J_c values as well as angular dependence characteristics.     

Reduced crystallization time of YBCO in a fluorine-free MOD process using uv-lamp irradiation

We have studied the crystallization time dependence of the epitaxial YBCO films (t = 0.8 μm) grown on CeO₂-buffered SrTiO₃ substrates by fluorine-free metal–organic deposition using uv-lamp irradiation (uv-MOD). As increasing the time (T₀) for heat treatment at the reaction temperature (760 °C) from 0 to 90 min, J_c and the YBCO 0 0 l XRD intensity are steeply increased and reach their maximum values at T₀ = 10 min. This suggests that the heat treatment required for YBCO crystallization is significantly shortened in uv-MOD compared to conventional all-pyrolytic F-free MOD processes, which consume T₀ = 90–150 min for crystallizing 0.4–0.5-μm-thick films. Scanning electron microscope measurement revealed a drastic change in surface morphology between T₀ = 8 and 10 min, showing a good correspondence to the J_c and XRD data which suggest that the epitaxial growth reaches the film surface at the very early stage in the heat treatment.     

Vortex pinning in bulk-processed Y–Ba–Cu–O with ZrO2 nano-particles: Optimum pinning center size

Crystalline defects on the nano-scale were successfully introduced into YBCO high-temperature superconductors (HTS) by ZrO₂ nanometer particles addition in order to strongly pin the quantized vortices. Three batches of ZrO₂ nano-particles with different particle size distributions were used. The corresponding mean nano-particle diameters are respectively, 287, 536 and 764 nm. Serving as artificial pinning centers (APC), non-superconducting nano-particles cause a remarkable enhancement of critical current density (J_c) at T = 77 K. This improvement has been shown to depend on the size of APC. The pinning strength of nano-particles inclusions has been found to be greater with wide size dispersed nano-particles. Our results indicate that pinning properties and vortex dynamics depend on the size of APCs. The introduction of APCs with controlled size is indispensable to achieve a high J_c.     

Magnetism of UCoGe and U3Co4Ge7

A series of UCoGe and U₃Co₄Ge₇ polycrystalline samples has been prepared by arc melting and studied with respect to the phase composition and crystal structure, magnetization, a.c. susceptibility, electrical-resistivity and specific-heat behavior (down to 350 mK). U₃Co₄Ge₇ has been found to exhibit a spontaneous magnetization below T_C=21 K. Clear anomalies at T_C typical for a ferromagnetic transition have been observed in a.c. susceptibility, electrical-resistivity and specific-heat data. No additional anomaly, which would indicate the second magnetic phase transition below T_C reported in the literature, has been indicated. In all our UCoGe samples a transition to superconductivity has been revealed. On the other hand, no clear evidence of any transition to ferromagnetism with zero-field cooling down to 1.8 K has been obtained. The zero-field state is most probably governed by strong ferromagnetic spin fluctuations and seems to transform to a ferromagnetic state only when applying a magnetic field ≥10 mT. Simultaneously, an increase of the superconducting transition temperature is increasing with a magnetic field up to 10 mT and starts to decrease when increasing the field above this value. Measurements on bulk samples, which are by rule textured, indicate strong magnetocrystalline anisotropy in both investigated compounds.     

Improved critical current density in Zn doped YBCO single crystals

Magnetic measurements were made using pure YBCO and Zn doped YBa₂(Cu_1?xZn_x)₃O_7?σ. Single crystals with Zn concentration of 0.5%, 1.5%, 3.0% and 4.3%. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 ? T/T_c ? 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density J_c increased for low Zn content samples up to 3% Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of J_c in the temperature range of 0.7–0.9T_c, close to critical temperature compared to pure YBCO sample. The irreversibility field H_irr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field H_p above H_c1 and irreversibility field H_irr, both show power law dependence of the form H = m₁(1 ? T/T_c)^m2 in the temperature range of 0.75–0.96T_c. The values of parameter m₂ increased from 1.44 to 1.95 for the samples up to 3% Zn content and decreased to 1.37 for higher Zn contents. The ratio H_irr/H_p was found to be 3–4 for the lower Zn content samples and was 7–8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field H_p and irreversibility field H_irr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution.     

Inelastic neutron scattering on iron-based superconductor BaFe2(As,P)2

Inelastic neutron scattering has been performed on powder sample of an iron-based superconductor BaFe₂(As_0.65P_0.35)₂ with superconducting transition temperature (T_c) = 30 K, whose superconducting (SC) order parameter is expected to have line node. In the normal state, constant-E scan of dynamical structure factor, S(Q, E), exhibits a peak structure centered at momentum transfer Q ～ 1.20 Å^?1, corresponding to antiferromagnetic wave vector. Below T_c, the redistribution of the magnetic spectral weight takes place, resulting in the formation of a peak at E ～ 12 meV and a gap below 6 meV. The enhanced magnetic peak structure is ascribed to the spin resonance mode, evidencing sign change in the SC order parameter similar to other iron-based high-T_c superconductors. It suggests that fully-gapped s_± symmetry dominates in this superconductor, which gives rise to high-T_c (=30 K) despite the nodal symmetry.     

Superconductivity and thermal properties of sulphur doped FeTe with effect of oxygen post annealing

Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe_1?xS_x; (x = 0–30 %) system and study the impact of low temperature oxygen (O₂) annealing as well. Rietveld analysis of room temperature X-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity C_p(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe_0.75S_0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe_0.75S_0.25 sample. The upper critical fields H_c2(10%), H_c2(50%) and H_c2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 °C) O₂ annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [C_p(T)] measurements for FeTe_1?xS:O₂ system are presented and discussed.     

Effect of processing temperature on the superconducting properties of acetone doped MgB2 tapes

Correlation of phase formation, critical transition temperature T_c, microstructure, and critical current density J_c with sintering temperature has been studied for acetone doped MgB₂/Fe tapes. Sintering was performed at 600–850 °C for 1 h in a flowing Ar atmosphere. High boron substitution by carbon was obtained with increasing the sintering temperature; however, the acetone doped samples synthesized at 800 °C contain large size MgB₂ grains and more MgO impurities. Incomplete reaction for the acetone doped samples heated at 600 °C result in bad intergrain connectivity. At 4.2 K, the best J_c value was achieved in the acetone doped sample sintered at 700 °C, which reached 24,000 A/cm² at 10 T and 10,000 A/cm² at 12 T, respectively. Our results indicate that the small grain size and less impurity were also important for the improvement of J_c–B properties besides the substitutions of B by C.     

Analysis of temperature-dependent electrical resistivity of ZnO nano-structures

The temperature–dependent electrical resistivity ρ(T) in metallic and semiconducting phase of ZnO nanostructures is theoretically analysed. ρ(T) shows semiconducting phase in low temperature regime (140 K<T<180 K), shows an absolute minimum near 180 K and increases linearly with T at high temperatures (200 K<T<300 K). The resistivity in metallic phase is estimated within the framework of electron–phonon and electron–electron scattering mechanism. The contributions to the resistivity by inherent acoustic phonons (ρ_ac) as well as high frequency optical phonons (ρ_op) were estimated using Bloch–Gruneisen (BG) model of resistivity. The electron–electron contributions ρ_e?e=BT² in addition with electron–phonon scattering is also estimated for complete understanding of resistivity in metallic phase. Estimated contribution to resistivity by considering both phonons, i.e., ω_ac and ω_op and the zero limited resistivity are added with electron–electron interaction ρ_e–e to obtain the total resistivity. Resistivity in Semiconducting phase is discussed with small polaron conduction (SPC) model. The SPC model consistently retraces the low temperature resistivity behaviour (140 K<T<180 K). Finally the theoretically calculated resistivity is compared with experimental data which appears favourable with the present analysis in wide temperature range.     

Current carrying capability of HTS Roebel cable

We have developed a numerical technique to estimate the current carrying capability of HTS Roebel cable composed from coated conductor strands. The influence of self-field on the critical current density is studied computationally for a Roebel cable using a realistic field and angle dependence behaviour of critical current. The computations are carried out for N/2 (number of strands/strand width in mm), and N/5 Roebel cable for N = 2–15. The local current distribution in each strand satisfies the self-consistent criteria J = J_c(B(J)) except for a small region where the current density is set to zero to maintain the condition of equal currents in all strands. The variation of critical current with vertical and horizontal separation between the strands is also investigated. Results are compared with the measured values of critical current for some of our cables. The comparison shows an error of up to 10% which we attribute mostly to the model not accounting for the spread in I_c values of the constituent strands.