New Type of Non-magnetically Sheathed MgB2 Wires��First Sight to AC Losses with Numerical Simulations

AC losses of MgB₂ wires can be substantially decreased with nonmagnetic sheath materials and multifilament structure. In this paper, AC losses of two low loss samples are computed in AC?CAC domain, which means that the samples were exposed to alternating transport current and field simultaneously. The losses were computed with modified Brandt??s method, which takes J _c(B)-dependence into account, and for these computations, the superconductors?? cross-section is determined directly from photographs. The losses were computed in wide magnetic field range with various transport currents. The results suggest that the AC losses of these samples are relatively low because large magnetization current loops are not formed. Additional simulations show that below the penetration field, the losses can be approximated using constant critical current density based on the self-field critical current of the sample. However, after the penetration field, this leads to too high loss values. Finally, the results are compared to analytical loss formulas in case of sole transport current and sole magnetizing field.     

Pinning in superconductors with anisotropic defects

By calculating the volume pinning force in superconductors with anisotropic pinning centers we have shown that the critical current densitiesj _c in NbTi by both current and magnetization measurements can be explained. The critical current density in fields parallel toj _c can also be included. We conclude therefore that in superconductors with strong pinning centers the critical state is always determined by the equilibrium condition between the Lorentz force and the volume pinning force and not by the instability of the force-free configuration. By detailed investigation of thej _c dependence on field direction, useful information can be obtained with respect to the volume and surface contributions to the pinning force.     

The effect of LiOH addition to the superconducting properties of YBa2Cu3O7-x

The effect of LiOH addition to YBa₂Cu₃O_7-x is presented. The crystal structure remains orthorhombic, bat the orthorhombic splitting decreases from 0.0171 at 2 at.% Li to 0.0158 at 50 at. / Li. The critical temperature exhibits a decrease from 91.18 to 83.79 K in the same range of lithium concentrations. From the magnetization curves were obtained the lower critical fields and the intragranular critical current density. The latter exhibits a negative power law,j _c∞ B^-α with α≈ 1/2.V-I characteristics are typical for grained superconducting materials:Vα(I-I_c)_n(B,t). The transport critical current dependence on temperature and magnetic field is also presented.     

Pinning and vortex dynamics in superconducting (K,Ba)BiO 3 single crystals

Pinning and vortex dynamics have been investigated in the 3-dimensional copper free (K, Ba)BiO₃superconductor (T_c 31K) by magnetization and transport measurements up to 30 Tesla. The magnetization curves present a pronounced fishtail effect which persists for time scales down to 10^–4s (pulsed field measurements). We show that it is an intrinsic feature of the critical current which can in part be well described by the collective pinning theory. Furthermore, this system presents evidence for a vortex liquid /glass transition for vanishingly small currents. As the current density is increased, dissipation in the glass state is dominated by creep effects. The temperature and current dependence of the activation energy is discussed.     

Transport ac loss of a superconducting cylinder with field dependent critical current density

The transport ac loss per cycle per unit length of a hard superconducting cylinder is calculated from the critical state model assuming a Kim-type and an exponential field dependent critical current density. Without such dependence, the results are consistent with Norris’ equations for an ellipse bar, in which the critical current density is assumed not to depend on the flux density. Based on Norris’ equations, the expressions of the loss are derived for a finite length cylinder. It is shown that the field dependence decreases and increases the loss at low and high ac currents, respectively, and the effects of the parameter p on the loss are related to the magnetization process. Compared to Norris’ prediction, the results for the Kim and exponential model show the same trend with respect to the external transport current.     

Pore Structure and Transport Properties in Bulk YBa2Cu3O7?δ Doped with Sb2O3

In this work we have studied the pore structure and electrical transport properties of superconducting YBa₂Cu₃O_7?y polycrystalline samples doped by the addition of different Sb₂O₃ concentrations, i.e. resulting in (YBa₂Cu₃O_7?y )_1?x (Sb₂O₃) _x . The samples were prepared through the solid-state reaction method. Rietveld analyses of X-ray diffraction data were used to investigate how the lattice parameters are modified by doping. Specific superficial area measurements identified the principal characteristics of the pore structure of the samples and how these properties change with doping. The superconducting properties were studied by using zero field cooling magnetization and transport critical current measurements. The critical temperature of the samples does not depend on the doping level, but their transport critical current density strongly decreases as the Sb₂O₃ concentration is increased. Our experimental results suggest that for the samples studied here there is not a direct correlation between the modification by doping of both, the pore structure and the transport critical current density.     

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB<Subscript>2</Subscript> Wires

This study has shown the effect of excessive mechanical deformation by pressing and consecutive heat treatment, respectively, on the microstructure and transport properties of mono-core sintered in situ Fe/MgB₂ wires. These wires were pressed into short tapes under 1 GPa uniaxial pressure. Transport properties like critical temperature, transition width and engineering critical current density of the same samples were determined before and after pressing and after annealing. Phase formation and grain size of the samples were studied with XRD. The SEM analysis indicated that the mechanical deformation reduced initial voids to a large extent but caused cracks, increased the number of grain boundaries and introduced weak links. The subsequent heat treatment on the pressed samples improved the microstructure and the grain connectivity. Possibility of a final densification of the reacted in situ wires was discussed in terms of transport properties.     

Scaling properties of the anisotropic critical current density in bulk textured YBaCuO. Evidence toward a 3D flux line lattice

We have measured dc transport critical current densities of melt texture grown and magnetically melt textured bulk YBaCuO at 77 K and in magnetic field. A maximum value of over 31,000 A/cm² is obtained with a field of 7 teslas applied parallel to the (a, b) planes. Over the rest of the angular range the critical current is shown to be determined mainly by thec-axis component of the applied field. Although this dependency is expected in the presence of two-dimensional vortices, in fact the data are shown to correspond better to the behavior expected of an anisotropic three-dimensional superconductor. These results are compared to magnetization measurements on the same samples. We show that when the field is directed close to the c-axis, superconducting transport currents flow at fields well above the field at which the irreversible magnetization disappears.     

Field, temperature, and angle dependence of the critical current density in Bi2Sr2CaCu2O10/Ag ribbons

Current-voltage characteristics of high-critical-current Bi₂Sr₂CaCu₂O₁₀/Ag ribbons were measured using both transport and magnetization techniques. The slope of these curves changes with magnetic field and temperature in a way very similar to the observedj _c (H, T) behavior. This correspondence between the critical current and the slope of theI–V characteristics can be explained within the thermally activated flux creep framework. The dependence ofj _c on the angle between field and ribbon is compared to the existing intrinsic anisotropy models.     

Magnetization of a Current-Carrying Superconducting Disk with B-Dependent Critical Current Density

In the frame work of the critical state model (CSM), the magnetic response of a thin type-II superconducting disk that carries a radial transport current and is subjected to an applied magnetic field have been studied. To this end, we have studied the process of the magnetic flux-penetration. For a disk initially containing no magnetic flux but carrying a radial current, when a perpendicular magnetic field is applied, magnetic flux-penetration occurs in three stages: (1) the magnetic flux gradually penetrates from the edges of the disk until an instability occurs, (2) there is a rapid inflow of magnetic flux into the disk’s central region, which becomes resistive, and (3) magnetic flux continues to enter the disk, while persistent azimuthal currents flow in an outer annular region where the net current density is equal to J _c . Also the behavior of a current-carrying disk subjected to an AC magnetic field is calculated. The magnetic flux, the current profiles and the magnetization hysteresis loops are calculated for several commonly used J _c (B) dependences. Finally, the results of the applications of the local field-dependent of the critical current density J _c (B) are compared with those obtained from the Bean model.     

The Influence of BaZrO3 on the Magnetic Response of (Bi,Pb) : 2223 High-Temperature Superconductors

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO₃ are presented. The results are discussed in the frame of the critical state model, and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system, BaZrO₃ impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.     

Tensile strain dependence of critical current of RHQ-Nb3Al wires

Nb₃Al superconducting wires produced by rapid heating and quenching (RHQ) have been developed for high-field accelerator magnets. It is known that critical currents of A15 superconductors (e.g., Nb₃Al and Nb₃Sn) have a dependence on stress/strain. It is thus important to clarify the stress/strain behavior of the RHQ-Nb₃Al wires for the development of high-field accelerator magnets. We recently started experimentally investigating the strain dependence of the critical current of the RHQ-Nb₃Al wires with a Ta interfilament by measuring their critical currents under longitudinal tensile strains. To evaluate the effect of residual strain in Nb₃Al filaments induced by thermal contraction of the materials in the wire, neutron diffraction measurements were performed at room temperature.     

Improvement of superconductive and mechanical properties of bulk and thin-wall MT-YBCO ceramics in oxygenation

The possibility to oxygenate the YBa₂Cu₃O_7?δ(Y123) structure to 7?δ ≈ 6.9 atoms (which ensures the highest temperature of transition into the superconductive state of this compound) at high temperature (800°C) and relatively low pressure (16 MPa) of oxygen has been first shown. This fact differs from the generally accepted notions of the equilibrium in the given system. It has been found that the MT-YBCO oxygenation at enhanced pressures and high temperatures decreases the oxygenation time and the amount of cracks and increases the twin density in the material, which positively affects the critical current density and mechanical characteristics of the ceramics. The experiments have shown that twins are largely responsible for high density of the critical currents j _c and irreversibility fields in the MT-YBCO ceramics. In the case of high dislocation density (10¹² cm^?2) and low twin density (0–1 μm^?1) in the Y123 structure, the critical current turned out to be an order of magnitude lower than in the case of high twin density (22 μm^?1) and absence of dislocations and stacking faults. The density of twins and microcracks (parallel to the ab plane) in the structure of the YBa₂Cu₃O_7?δ phase has been found to depend on the size of the Y₂BaCuO₅ inclusions and the pattern of their distribution, which in turn is defined by the initial materials. A process has been developed of the oxygenation of the thin-wall (cellular) MT-YBCO ceramics under the conditions of the controllable oxygen pressure from 0.5 kPa to 16 MPa and temperatures from 900 to 800°C. The process allows one to attain record high j _c values and double the trapped magnetic field as compared to both the bulk MT-YBCO ceramics oxygenated under the same conditions and thin-wall MT-YBCO ceramics oxygenated at atmospheric pressure and optimal temperature.     

Effects of Electrolytic Current Density on Structural, Magnetic Properties and GMI Behavior in Electrodeposited Bilayer FeNi/Cu Composite Wires

In this article, a systematic study has been performed on structural, magnetic properties and the GMI effect in electrodeposited FeNi/Cu composite wires in the range of current density from j=15.9?mA/cm² to 79.5?mA/cm². Our obtained results reveal a correlation between the structure, magnetic softness, and GMI behavior in prepared wires. Among the samples investigated, the softest magnetic property (lowest coercivity) was found for the wire-sample plated at j=47.7?mA/cm², which arises from the smallest nanograin size. This results in the largest values of GMI ratio and its sensitivity of this sample, which is ascribed to the optimized domain structure with a well-defined circular anisotropy. It is worthy to note that the maximum GMI ratio and the field sensitivity reached the highest values of 90?% and 7?%/Oe for at a measuring frequency of 5 MHz. This result is very promising for developing high-performance GMI-based sensors applications. Our finding demonstrates that the changes in the surface morphology and the microstructure of prepared wires has led to modification of the surface magnetic properties, and hence GMI effect.     

Special features in the synthesis and properties of thin Y-Ba-Cu-O high-temperature superconductor films free of secondary phases

Data on the structure and transport properties of thin Y-Ba-Cu-O (YBCO) high-temperature superconductor films obtained by magnetron sputtering of a stoichiometric target in a system with a 90° off-axis geometry are reported. It is shown that the films prepared under these conditions are free of copper-rich secondary phases and are characterized by the surface roughness height below 10 nm. The films possess a perfect structure and exhibit high superconducting properties: c-axis misorientation in microblocks FWHM(005)YBCO=0.4–0.5°; zero-resistance temperature T _co=89 K; critical (pinning) current density j _p=1.5–2 MA/cm² (77 K).     

Micromagnetic Structure of Co-Rich Amorphous Microwires

1. IntroductionSince the discovery of amorphous materials theirmagnetic prOPerties have been eXtensively studieddue to a great deal of laterest for fundamental investigations and their enormous potelltial in technological applications (in sensors, transducers, magnetic screens). Co-rich amorphous wires are promising candidates for a variety of sensor applications.It is knownlll thst some importal magnetic characteristics for technical applications (for example,low field magnetoresistance, mag…     

Alternating field losses in superconducting wires carrying dc transport currents: Part 1 single core conductors

The influence of dc transport currents on alternating-field hysteresis loss was investigated for a single core superconducting wire. The configuration of the sample coil simulates a tight solenoidal winding of superconducting magnets, and the field is a small transverse alternating field applied on top of a large dc bias. The losses were determined as a function of transport current and external field from simultaneous measurements of the magnetization and the dynamic resistance. The results were compared with calculations based on a slab model. When the parameter of the slab-equivalent of an array of superconducting wires is chosen properly, the calculations explain the observed results quite well.     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Magnetization and critical current of high-temperature superconductors with artificial pinning centers

We present new data on magnetization at T = 4.2 and 77 K for polycrystalline high-temperature superconductors of the Bi₂Sr₂Ca₂Cu₃O_10+δ system containing various amounts of nanodimensional inclusions of tantalum carbide, niobium carbide, or niobium nitride. The introduction of these nanoparticles (≤30 nm in size) leads to an increase in the magnetization and the critical current density. It is established for the first time that the dependence of the normalized critical current on the bulk concentration of indicated dopants is described by the same universal curve. The interval of the optimum dopant concentrations is found, in which the additives lead to the maximum increase in the critical current density.     

Multifilamentary V3Ga wires and tapes with composite cores

Results on the critical current behaviour of 19 core composite superconducting wires and tapes in magnetic fields up to 9 T are reported. The composite cores used in the present studies are vanadium tubes filled with Cu-Ga matrix. Two V₃Ga layers are formed on either side of the tubular vanadium. For a given matrix volume to vanadium surface area ratio, the inner V₃Ga layer has much faster growth rate and consequently finer grains than the corresponding outer layer. An increase of about 50% in overall critical current density compared to the 19 core wires using the same starting materials but prepared with conventional geometry has been obtained. The critical current density of V₃Ga (jc) as well as the overall critical current density (J_c) of the tape specimens with composite cores is significantly higher than the corresponding wires. Enhanced surface flux pinning seems to be responsible for the higher current density in the tapes. Addition of 6 at% gallium to vanadium and 0.5 at% Mg to CuGa (19 at%) matrix leads to enhanced critical current densities in these specimens in conformity with our earlier studies.