Showing papers on "Flux pinning published in 1985"
TL;DR: In this paper, a detailed study of the critical parameters of the ternary addition A.15 material (Nb7.Sw/oTa) 3 Sn manufactured by Vacuumschmelze GMBH, Hanau F.G.
Abstract: Further to our programme of investigations of new, technical superconductive materials which are of interest to designers of practical devices, we present the results of a detailed study of the critical parameters of the ternary addition A.15 material (Nb7.Sw/oTa) 3 Sn manufactured by Vacuumschmelze GMBH, Hanau F.R.G. The basis of the results we report is a comprehensive Jc(B, T) characterization within the range, O
58 citations
TL;DR: In this paper, it was shown that the defect density is very high, the precipitate morphology has a very high aspect ratio and the extreme thinness of the precipitates must permit some superconductivity to be induced in them by the proximity effect.
Abstract: Recent work in our group on the fabricationmicrostructure-superconducting properties of composites of Nb-Ti has produced much new information about the precipitate morphology and origins of high critical current density (J /SUB c/ ) in these materials./sup 1 -4/ Precipitation of Ti-rich phase is seen to commence as a grain boundary film 2 - 4 nm thick, the film then developing into approximately equiaxed particles of ..cap alpha..-Ti at the boundary triple points. The typical size of such precipitates is 50 - 100 nm. Controlled drawing of such a structure produces an array of locally ordered ribbon precipitates. These precipitates are typically 3 - 5 nm thick by 100 - 300 nm long (when observed in transverse section). Their length in longitudinal section appears to be several hundred nm, indicating great ductility in these small ..cap alpha..-Ti precipitates. The typical separation of the precipitates is 20 - 30 nm. Thus the dimensions of the precipitate array are quite comparable to that of the flux lattice since the fluxoid separation is 22 nm at 5 T and the fluxoid diameter of Nb 46.5 wt% Ti is approximately 10 nm. The flux pinning behavior of these precipitate structures is expected to be complex:more » /SUP 2.4/ the defect density is very high, the precipitate morphology has a very high aspect ratio and the extreme thinness of the precipitates must permit some superconductivity to be induced in them by the proximity effect./sup 5/« less
47 citations
TL;DR: A general critical-state model including the effects of both flux-line cutting and flux pinning is used for calculating B-, J-, and E-field distributions during the approach to the quasisteady state of a type-II superconductor subjected to a parallel rotating magnetic field.
Abstract: A general critical-state model including the effects of both flux-line cutting and flux pinning is used for calculating B-, J-, and E-field distributions during the approach to the quasisteady state of a type-II superconducting slab subjected to a parallel rotating magnetic field. Three initial magnetic configurations (diamagnetic, paramagnetic, and nonmagnetic) are considered. It is shown that, depending upon sample parameters and magnetic history, multiple-zone structures develop inside the superconductor. It is also shown that, regardless of the specimen's magnetic history, flux-line-cutting B consumption leads to a final state in which B has a diamagnetic profile near the surface of the specimen. The relation of these calculations to the pioneering experiments of LeBlanc and co-workers is discussed.
42 citations
TL;DR: Homogenized V/sub 3/Si crystals were irradiated with fast neutrons at conditions which result in a proportionality between the induced defect density and the neutron fluence and the resulting rise of the volume pinning force is investigated as a function of the density of the weak interacting defects.
Abstract: Homogenized ${\mathrm{V}}_{3}$Si crystals were irradiated with fast neutrons at conditions which result in a proportionality between the induced defect density and the neutron fluence. The critical current density is corrected for differences in the fluence dependence of the upper critical field and of the Ginzburg-Landau parameter. The resulting rise of the volume pinning force is investigated as a function of the density of the weak interacting defects. A second power law which points to a collective interaction is observed at low defect densities and at low fields. With increasing defect density and increasing field, this quadratic dependence vanishes abruptly and an almost linear relation dominates. The exponent of this first power law decreases continuously with further increase of field or defect density. These observations are compared with the collective pinning theory.
41 citations
TL;DR: In this paper, the dispersion relation is obtained for acoustic plane waves that scatter coherently from an ensemble of parallel magnetic flux tubes when the wave vector is perpendicular to the flux-tube axis.
Abstract: The dispersion relation is obtained for acoustic plane waves that scatter coherently from an ensemble of parallel magnetic flux tubes when the wave vector is perpendicular to the flux-tube axis. When the magnetic flux tubes are distributed uniformly and possess radii that are small compared with the wavelength, the frequency can be calculated exactly. The waves are damped slightly due to a loss of coherence and are shifted downward or upward in frequency relative to a medium devoid of magnetic fibrils, depending primarily on whether the flux tubes are more or less dense than their surroundings. It is suggested that the influence of the fibril magnetic fields observed at the solar surface cannot be ignored in the interpretation of high-1 surface p-mode data.
40 citations
TL;DR: The Larkin and Ovchinnikov (LO) theory for the summation of local pinning forces on flux vortices in type II superconductors describes the collective interaction between a perfectly elastic vortex lattice and a subthreshold pinning potential as discussed by the authors.
Abstract: The Larkin and Ovchinnikov (LO) theory for the summation of local pinning forces on flux vortices in type II superconductors describes the collective interaction between a perfectly elastic vortex lattice and a subthreshold pinning potential. Using a similar minimization procedure, it is shown that the elastic solution derived by LO does not always represent the lowest energy state of the vortex lattice. It is argued that as the magnetic field approaches Bc2, the vortex lattice inevitably becomes defective and that the defect structure determines the total pinning force, the static and dynamic response of the vortex lattice, and history effects when they occur. Calculations are presented for two and three dimensions, in two dimensions, the equilibrium defect structure predicts a peak effect in good agreement with observed critical currents. It is suggested that the situation in three dimensions is always nonequilibrium.
36 citations
TL;DR: In this article, the authors developed a numerical method for finding the magnetic response of type-II superconductors subjected to parallel magnetic fields that change in both magnitude and orientation, and compared their results with related experiments by Cave and LeBlanc, and reported model calculations for the ac behavior when the sample is subjected to a dc bias field and a large-amplitude ac field at various angles (0°, 45°, and 90°) relative to this field.
Abstract: Starting from our general critical‐state theory, which includes the effects of both flux‐line cutting and flux pinning, we develop a numerical method for finding the magnetic response of type‐II superconductors subjected to parallel magnetic fields that change in both magnitude and orientation. We describe model calculations of the time‐evolving magnetic profiles when the direction of an applied magnetic field of fixed magnitude oscillates with large amplitude, and we compare our results with related experiments by Cave and LeBlanc. We also report model calculations for the ac behavior when the sample is subjected to a dc bias field and a large‐amplitude ac field at various angles (0°, 45°, and 90°) relative to this field.
32 citations
TL;DR: In this paper, an approximate closed-form result for the interaction of an isolated magnetic flux line in a type-II superconductor with a grain boundary, due to the scattering of electrons by the boundary, is presented.
Abstract: An approximate closed-form result for the interaction of an isolated magnetic flux line in a type-II superconductor with a grain boundary, due to the scattering of electrons by the boundary, is presented here. The grain boundary need not be infinitely thin, but can include a distribution of scattering centers nearby. The resulting expressions are more suitable for computer simulations of flux pinning and their interpretation is more physically transparent than previous results; they are used here to discuss pinning by nonideal boundaries.
28 citations
TL;DR: In this paper, a discussion of the state of microstructural knowledge about filamentary Nb-Ti and Nb 3 Sn conductors is provided, and some semi-quantitative measures of precipitate flux pinning are described.
Abstract: A discussion is provided of the state of our microstructural knowledge about filamentary Nb-Ti and Nb 3 Sn conductors. The situation is believed to be much clearer for Nb-Ti than for Nb 3 Sn, due to recent successes in elucidating the microstructures of Nb-Ti composites by TEM. Considerable changes in some earlier ideas of flux pinning are proposed and some semi-quantitative measures of precipitate flux pinning in Nb-Ti alloys are described. The situation is less advanced for Nb 3 Sn. The J c is controlled by a greater number of variables and it is in general not clear that individual microstructural measurements can be taken to be typical of the whole composite.
27 citations
TL;DR: In this article, critical current behavior and intrinsic superconducting properties of liquid quenched Nb3Al and its pseudobinaries were studied in fields up to 29 T. The best results were obtained in Nb75Al19Ge6 samples with Tc of 20.0 K, estimated Hc2(0) of 43 T and Jc (4.2 K) in excess of 104 A/cm2 at 30 T.
Abstract: Critical current behavior and intrinsic superconducting properties of liquid quenched Nb3Al and its pseudobinaries Nb3 (Al,Si) and Nb3(Al,Ge) have been studied in fields up to 29 T. Our results show that flux pinning in these materials is dominated by strong precipitate pinning at low temperatures and by grain boundary pinning at high (>10 K) temperatures. The best results were obtained in Nb75Al19Ge6 samples with Tc of 20.0 K, estimated Hc2(0) of 43 T and Jc (4.2 K) in excess of 104 A/cm2 at 30 T.
24 citations
TL;DR: It is shown that the magnetic-field-induced superconducting state recently observed in the system Eu/sub x/Sn/sub 1-x/Mo/sub 6/S/sub 8/ has new and unexpected magnetic properties.
Abstract: It is shown that the magnetic-field-induced superconducting state recently observed in the system Eu/sub x/Sn/sub 1-x/Mo/sub 6/S/sub 8/ has new and unexpected magnetic properties. In particular, for a certain applied field, the external field will penetrate completely and uniformly into the superconductor. Below this field the superconductor contains antivortices and the magnetic field is enhanced in the superconducting regions. A first observation of the paramagnetic nature of this state is presented.
Journal Article•
TL;DR: In this paper, the penetration of a magnetic field into a London superconductor without an inversion center was analyzed and the magnetization produced in the Meissner layer corresponds to a magnetic-induction jump at the super-conductor surface.
Abstract: The penetration of a magnetic field into a London superconductor without an inversion center is analyzed. The magnetization produced in the Meissner layer corresponds to a magnetic-induction jump at the superconductor surface.
TL;DR: In this article, a helical flux flow model was proposed to explain the appearance of the dynamic structure observed by Ezaki et al. with a longitudinal negative electric field under a constant longitudinal magnetization.
Abstract: A fluxoid motion in a resistive state of a current-carrying superconductor under a longitudinal magnetic field is investigated. A helical flux flow is considered to be induced by the driving torque due to the axial current above a critical value. This motion can be expected also from the Poynting vector measured on a specimen surface. The present helical flux flow model explains an appearance of the dynamic structure observed by Ezaki et al. with a longitudinal negative electric field under a constant longitudinal magnetization. This model is supported also by the experiment of Makiej et al. on a transverse electric field which indicates a motion of the longitudinal magnetic flux. And it is shown that the mean longitudinal electric field does not originate from penetration of the azimuthal magnetic field.
TL;DR: In this article, a temperature scaling law of the flux pinning force for superconducting alloys with high field critical current densities Jc and pinning behavior is discussed.
Abstract: High‐field critical current densities Jc and pinning behavior are discussed for Nb‐63a/oTi, Nb‐61.7a/oTi‐3a/oHf, and Nb‐64.7a/oTi‐7.2a/oTa superconducting alloys. Jc properties for 8–12 T in these alloy superconductors under superfluid helium environments can be even superior to those in Nb3Sn at 4.2 K, when they are heavily cold‐worked after final heat treatments. A temperature scaling law of the flux pinning force Fp is found to hold for these alloys in the form of FP=K [Bc2(T)]nbp(1−b)q, where Bc2 is the upper critical field and b≡B/Bc2. The peak reduced field, bm, in the Fp−b curve depends upon how the alloys have been processed. The origins of n, p, and q are discussed in terms of the processing condition.
TL;DR: In this article, the critical currents in melt spun amorphous Zr 70 Cu 30 and their evolution with heat treatment were measured and interpreted in terms of the Larkin-Ovchinnikov two dimensional collective pinning theory.
TL;DR: A reed performing flexural vibrations in a longitudinal magnetic field may be used to measure pinning and viscose motion of the flux lines in type-II superconductors in this paper.
TL;DR: In this article, the critical current behaviour of composite superconducting wires and tapes in magnetic fields up to 9 T was investigated and 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 was obtained.
TL;DR: In this article, the magnetic moment observed in a fine filament NbTi superconductor upon cooling through the transition temperature in a magnetic field large compared with H c1 was observed and this effect was interpreted in terms of surface current, body current and the magnetic moments of the flux lines.
Abstract: Measurements were made of the magnetic moment observed in a fine filament NbTi superconductor upon cooling through the transition temperature in a magnetic field large compared with H c1 . A partial Meissner effect was observed and this effect is interpreted in terms of surface current, body current and the magnetic moment of the flux lines. The interpretation is used to study the surface current behavior.
TL;DR: In this article, a review on recent topics on theories of flux pinning in superconductors is given, covering the problem of estimate of elementary pinning forces of individual pinning centers interacting with flux lines and the summation problem to calculate the global pinning force density.
Abstract: A review is given on recent topics on theories of flux pinning in superconductors. It covers the problem of estimate of elementary pinning forces of individual pinning centers interacting with flux lines and the summation problem to calculate the global pinning force density.As pointed out by Thuneberg, the elementary pinning force of pinning centers much smaller than the BCS coherence length should be reestimated by taking account of the electron-scattering effect. And calculations of the elementary pinning force of grain boundaries from the electron-scattering effect are briefly reviewed. Their dependence on the impurity parameter is opposite to the theoretical result of Thuneberg.The summation theories are reviewed and a significance of instabilities of flux lines appeared in the Labusch theory is emphasized. Then, recent two theories, the Larkin-Ovchinnikov theory and the mean field approximation theory are compared and problems contained in two theories are discussed.
TL;DR: In this paper, a new effect regarding the collective flux pinning in sputtered amorphous Nb3Ge films is reported, which is not the one usually observed in 2DCP, caused by a structural transition in the vortex lattice.
Abstract: We report a new effect regarding the collective flux pinning in sputtered amorphous Nb3Ge films. Thick films (d = 5−8 μm) show the two dimensional collective pinning (2DCP) at low fields and all temperatures. Below 0.7Tc, however, an extremely sharp and large peak develops in the macroscopic pinning force at fields around 0.75Bc2. It is argued that this peak is not the one usually observed in 2DCP, caused by a structural transition in the vortex lattice. Strong evidence is found that the onset of this peak is triggered by a dimensional cross-over in the collective pinning (2D to 3D) expected when the longitudinal correlation length Lc becomes smaller than d/2. A discrepancy with the 3D expression of the Larkin-Ovchinnikov theory for Lc nevertheless remains.
TL;DR: In this article, a phenomenological framework is established for calculating the response of a moving vortex under the action of local forces, and the vortex lattice restoring forces can be treated using a self-consistent mean-field approximation.
Abstract: The current-voltage characteristic of an irreversible type II superconductor depends on the interaction of moving flux vortices with flux pinning defects. Experimental characteristics display curvature and fine structure whose origin is still uncertain. In this paper a phenomenological framework is established for calculating the response of a moving vortex under the action of local forces. It is shown that the vortex lattice restoring forces can be treated using a self-consistent mean-field approximation. Using this approximation the distribution of vortex displacements, the velocity distribution and the mean voltage are calculated for a simple one-dimensional model pinning system.
TL;DR: In this paper, an alternative interpretation of the power law and a practical difficulty of flux pinning for measurements in small fields are pointed out, and experimental examples are presented for very homogeneous granular aluminum films.
Abstract: Recently theories of vortex fluctuations in two-dimensional superconducting films have been compared to the experimentally determined power law dependence of voltage on current and to the temperature-dependent resistance in small applied fields. Here an alternative interpretation of the power law and a practical difficulty of flux pinning for measurements in small fields are pointed out. To illustrate these points, experimental examples are presented for very homogeneous granular aluminum films.
01 Oct 1985
TL;DR: In this article, the authors show that adding Mn to Ti/Nb alloy has little effect on major superconducting phase, but confers strong paramagnetic susceptibility on alpha-phase particles.
Abstract: Addition of Mn to Ti/Nb superconducting alloy increases critical current. Adding Mn to Ti/Nb alloy has little effect on major superconducting phase, but confers strong paramagnetic susceptibility on alpha-phase particles. beta-phase particles become stronger flux pinners, resulting in increase in critical current.
01 Jan 1985
TL;DR: In this article, the form of the scaling law for flux pinning by grain boundaries in A15-structure compounds, primarily in the case of Nb3Sn and its alloys, is discussed, and the effect of alloying and radiation damage on the pinning strength is compared with predictions of the electron-scattering theory of the elementary pinning force.
Abstract: The form of the scaling law for flux pinning by grain boundaries in A15-structure compounds, primarily in the case of Nb3Sn and its alloys, is discussed, and the effect of alloying and radiation damage on the pinning strength is compared with predictions of the electron-scattering theory of the elementary pinning force. In addition, some preliminary results of 2-dimensional computer simulations are discussed in relation to the observed effects of alloying on the form of the scaling law for Nb3Sn.
TL;DR: In this article, a simple series resistance model is used to explain the magnetic field dependence of the pinning force, which is consistent with the phase separation model that explains the evolution of the superconductive penetration depth.
Abstract: Superconducting critical currents were measured as a function of a perpendicular applied magnetic field in disordered La70Cu30. It is shown that the pinning force is very sensitive to inhomogeneities already present in the as-quenched samples, although these inhomogeneities have no obvious influence on other superconducting and transport properties. A simple series resistance model is used to explain the magnetic field dependence of the pinning force. A large decrease of the pinning force was obtained for the annealed samples. This behaviour is consistent with the phase separation model that explains the evolution of the superconductive penetration depth. In order to obtain a better characterization of the structural evolution with annealing of the La70Cu30 system, the behaviour of the electrical resistivity, superconducting critical temperature and differential thermal analysis were measured.
01 Nov 1985
TL;DR: In this paper, the authors investigated the effect of grain boundary faceting and flux-flow rectification on Niobium bicrystals by making four-terminal measurements of critical current and neutron diffraction measurements of the flux-line bending.
Abstract: Flux-line pinning by the grain boundary in niobium bicrystals was investigated by making four-terminal measurements of the critical current and neutron diffraction measurements of the flux-line bending. The elementary pinning interaction is estimated from the critical current data by using an approximate theory of the current distribution on the grain boundary. The data reported here are mainly for a sample in which the applied magnetic field is parallel to the (111) and the (001) crystal directions in the two grains when it is in the plane of the boundary and perpendicular to the current direction. Evidence is seen of grain boundary faceting and of a flux-flow rectification effect that peaks as a function of temperature below 3/sup 0/K. The scale of the grain-boundary pinning is consistent with the quasiparticle-scattering theory.
TL;DR: In this article, the authors compare experimental results with theories on the elementary pinning force and the summation problem and compare with experiments within a regime of strong pinning where conventional calculation of the elementary force holds safely.
Abstract: A review is given on recent topics on experimental studies of flux pinning in superconductors, while theoretical problems were discussed in the preceding review article. Here we compare experimental results with theories on the elementary pinning force and the summation problem.Quantitative comparison of the elementary pinning force is not possible at this moment because of ambiguity in the summation theory. The dependence of the specific pinning force of grain boundaries on the impurity parameter shows a similar tendency to the calculations by Zerweck, Yetter et al., and Welch based on the electron scattering mechanism. This observed tendency is different from the prediction from the microscopic theory of Thuneberg. The summation theories are compared with experiments within a regime of strong pinning where conventional calculation of the elementary pinning force holds safely. Observed pinning forces seem to obey the linear summation derived from the mean field approximation theory.The other topics such as the saturation phenomenon and the strain scaling are also discussed.
TL;DR: The structure and superconducting properties of rapidly quenched Zr-Ni and ZrV alloys have been studied by XRD, EXAFS spectroscopy, TEM, electron diffraction, ED, electrical resistivity and ac susceptibility measurements as discussed by the authors.
Abstract: The structure and superconducting properties of rapidly quenched Zr-Ni and Zr-V alloys have been studied by XRD, EXAFS spectroscopy, TEM, electron diffraction, ED, electrical resistivity and ac susceptibility measurements. Amorphous Zr66Ni34 ribbons quenched with different cooling rates were also examined by flux pinning measurements. Characteristic differences are observed in the flux pinning mechanisms of microcrystalline and amorphous samples. The temperature dependence of upper critical fields have been measured down to 0.4 K and the results are analyzed in terms of WHH theory.