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Applied Superconductivity, IEEE Transactions on

Issue 2  Part 2 • Date June 1997

 This issue contains several parts.Go to:  Part 1  | Part 3 

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Displaying Results 1 - 25 of 275
  • IEEE Transactions on Applied Superconductivity

    Publication Year: 1997 , Page(s): 0_1 - 0_22
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  • Proximity effect current strengths in NbTi multifilamentary samples with and without Nb barriers and processed under various conditions

    Publication Year: 1997 , Page(s): 1117 - 1121
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    Conventional magnetization measurements of proximity effect (PE) coupling in superconducting (S) multifilamentary (MF) strands have been made, and a previously developed technique has been used to quantify the coupling strength in terms of a PE coupling supercurrent, J/sub cn/. This quantity has been determined for a number of MF strand types processed under various conditions. All are NbTi/Cu MF composites in some of which the NbTi filaments were surrounded by Nb "barriers", while in the others they were not. The magnetic field dependence of J/sub cn/ was also explored. The measured differences in J/sub cn/ are attributed to differences in the properties of the filament/matrix (normal metal, N) interface treated as a S1-S2-N trilayer. These results, together with those of an earlier series of experiments confirm that PE coupling between the filament and the matrix, and hence the normal-state coherence length, /spl xi//sub N/, is enhanced by the presence of a Nb (S2) barrier; and furthermore that the field-dependent coupling strength, gauged in terms of J/sub cn/ or /spl xi//sub N/, depends on the thickness of that barrier. It is demonstrated that magnetic PE-coupling studies not only provide a technique for extracting /spl xi//sub N/, a quantity central to artificial pinning center (APC) design, but also may help in the investigation of S-N boundary conditions in the clean limit. View full abstract»

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  • High field flux pinning and the upper critical field of Nb-Ti superconductors

    Publication Year: 1997 , Page(s): 1122 - 1125
    Cited by:  Papers (6)
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    Nb-Ti wires of different composition and metallurgical state have been studied near the upper critical field (H/sub c2/) in an attempt to separate H/sub c2/ the irreversibility field (H*) and the resistively measured transition field (H/sub r/). For optimized multifilamentary Nb-47wt.%Ti wire, we find that H*=10.2 T, H/sub c2/=10.8 T and H/sub r/=11.8 T at 4.2 K. The transitions were studied by extended electric field vs. current density curves, magnetization and small current transport measurements. We found a strong correlation between magnetization, bulk flux pinning (F/sub p/) extrapolation and extended E-J curves for the value of H*, all giving H*=10.2 T. We have also shown that the maximum in H/sub r/ occurs for Nb-44wt.% Ti. View full abstract»

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  • Variable composition NbTi superconductors produced by artificial pinning center process

    Publication Year: 1997 , Page(s): 1126 - 1129
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    An artificial pinning center (APC) process, that utilizes pure niobium and titanium as the starting material, has been used to manufacture NbTi superconductors. The Ti content has been varied from 45.4wt% to 59.3wt% by adjusting the relative thicknesses of the Nb and Ti sheets in the starting monofilaments. Any composition can be produced in this manner, as compared to the conventional alloy which is commercially available in only one composition. The higher Ti content is designed to improve critical current density (J/sub c/s) in the low field range (<5T) and lower raw material costs, thus reducing the overall cost to the end user. The APC process does not suffer from the poorer ductility of the conventional NbTi approach when utilizing higher Ti content alloys. The new conductors would be suitable for applications such as magnetic resonance imaging (MRI), superconducting magnetic energy storage (SMES) and detector magnets. View full abstract»

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  • Flux pinning in multifilamentary superconducting wires with ferromagnetic artificial pinning centers

    Publication Year: 1997 , Page(s): 1130 - 1133
    Cited by:  Papers (1)
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    We demonstrate that ferromagnets are effective artificial pinning centers for the enhancement of critical current (J/sub c/) in multifilamentary superconducting wires. We have analyzed theoretically the proximity effect due to the FM pins near the final size of several nanometers and determined that one should achieve a large pinning force by such centers at these sizes. There is also additional pinning strength resulting from the interaction between ferromagnetic moments of the pins and the magnetic field gradient of the fluxon lattice. The measured results of J/sub c/, T/sub c/ and H/sub c2/ are analyzed, and compared with our model analysis. View full abstract»

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  • Pinning and vortex lattice structure in NbTi alloy multilayers

    Publication Year: 1997 , Page(s): 1134 - 1137
    Cited by:  Papers (4)
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    We made thin film multilayers of Nb/sub 0.37/Ti/sub 0.63//Nb and Nb/sub 0.37/Ti/sub 0.63//Ti (d/sub NiTi/=14-27 nm and d/sub N/=4-11 nm) to examine geometries and materials relevant to flux pinning in commercial NbTi conductors. Samples were characterized by transport measurements between 4.2 K and T/sub c/, in magnetic fields nearly parallel to the layers, up to 6 T. For some multilayers, pinning forces had a large peak at intermediate fields whose onset occurred near /spl sim/0.2 H/sub c2/. We suggest this peak effect is caused by a change in the vortex lattice structure, driven by the strong intrinsic pinning. We have measured the highest pinning force density (113 GN/m/sup 3/ at 4.2K and 5 T) ever achieved in the NbTi system. View full abstract»

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  • Critical current anisotropy in conventional and artificial pinning center round wire Nb-Ti superconductors

    Publication Year: 1997 , Page(s): 1138 - 1141
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    Critical current density anisotropy was detected in conventional and artificial pinning center (APC) Nb-Ti round wires by means of a relevant increase of the magnetization M as the wire aspect ratio A/sub w/ (length L/filament diameter d/sub f/) was increased. However, contrary to what is observed in APC wires, the critical current density J/sub /spl par// calculated from the SQUID measured saturated-magnetization in conventional wires using an anisotropic Bean model (BM), is significantly different from the transport critical current density J/sub ct/. We suggest that the mismatch between SQUID measured J/sub /spl par// and J/sub ct/ is due to a combination of the discontinuity of the ribbons along the wire length in conventional Nb-Ti wires and also to the low electric fields generated in SQUID measurements. The low electric fields allows the magnetization-induced currents to redistribute inside the wire due to the local inhomogeneities associated with the presence of /spl alpha/-Ti ribbons. In this case anisotropic BM cannot be used to extract J/sub c/ from M because it will be function of the intrinsic lengths of these ribbons. View full abstract»

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  • Artificial pinning center Nb-Ti superconductors with alloyed Nb pins

    Publication Year: 1997 , Page(s): 1142 - 1145
    Cited by:  Papers (4)
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    A magnetic interaction has been proposed as the dominant fluxon-defect interaction in conventional and artificial pinning center (APC) Nb-Ti, emphasizing the proximity length (/spl xi//sub N/) of the pinning center as the important parameter for optimizing the high field critical current density. We have characterized APC composites containing 25 vol.% of Nb pins alloyed with 7.5 wt.% Ta and 10 wt.% W to deliberately test the predictions of this model. We found that the bulk flux pinning force of the Nb10W-pin composite (/spl xi//sub N/(Nb10W)/spl sim/32 nm) exhibited a magnetic field (H) and pin size (d/sub p/) dependency that is more consistent with conventionally processed Nb47Ti (/spl xi//sub N/(/spl alpha/-Ti)<32 nn) than with existing pure Nb-pin APC composites (/spl xi//sub N/(Nb)/spl sim/83 nm). The Nb7Ta-pin composite (/spl xi//sub N//spl sim/59 nm) had intermediate F/sub p/(H,d/sub p/) behavior, thus qualitatively supporting the model. View full abstract»

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  • Flux pinning scaling behaviors of ultrafine multifilamentary NbTi superconducting wires with Nb island-type artificial pins

    Publication Year: 1997 , Page(s): 1146 - 1149
    Cited by:  Papers (2)
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    In order to engineer composites for high critical current densities Jc, the establishment of an artificial pinning center (APC) composite technique based on the flux pinning mechanism is desired. For that purpose, we investigated temperature and magnetic field scaling behaviors on the flux pinning properties of several kinds of Nb island-type artificial pin wires. The temperature and magnetic field dependence of bulk pinning force densities Fp was evaluated from magnetization curves at 25-7.5K to exclude influences due to external effects such as filament sausaging. As a result it was found that the scaling law holds true in a wide range of temperatures and magnetic fields for all of the APC specimens, which is in contrast to /spl alpha/-Ti specimens. This result suggests that Nb island-type pinning is much stronger than ribbon-like /spl alpha/-Ti pinning. View full abstract»

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  • Irreversibility lines and pinning force density of aligned (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ single crystals

    Publication Year: 1997 , Page(s): 1150 - 1153
    Cited by:  Papers (1)
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    DC magnetic properties of aligned (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ single crystals have been measured for the first time using a SQUID magnetometer in field up to 5 T and in the temperature range from 5 K to 150 K. The results confirm the existence of a broad temperature and field regime with completely reversible magnetization for these crystals. The temperature dependence of the irreversibility field H*(T) is determined from both M-H and M-T curves with fields oriented perpendicular to the ab plane of the crystal lattice. A strong field dependence of critical current density has been observed in these pristine single crystals with very weak pinning. These observations emphasize the importance of artificially introducing pinning sites in BSCCO material used for applications. Non-linear scaling models have been used to fit our experimental results to estimate the elemental pinning force density and the effective pinning potential. View full abstract»

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  • Preparation and magnetic properties of Ba/sub 1-x/K/sub x/BiO/sub 3+y/ and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// single crystal films

    Publication Year: 1997 , Page(s): 1154 - 1157
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    Large area Ba/sub 1-x/K/sub x/BiO/sub 3+y/ and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films were grown by LPE techniques on (Ba,K)BiO/sub 3/ and NdGaO/sub 3/ substrates, respectively. The good quality of epitaxy and small crystal mosaicity of the films were confirmed by X-ray measurements. Chemical homogeneity and microstructure of the films and interfaces were investigated by SNMS, HREM and SEM, depending on growth parameters. A comparative study of magnetic hysteresis loops in the reversible (irreversible) regions were done for both types of films. The VSM data show that the irreversible line follows the flux creep model. View full abstract»

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  • Hysteretic flux dynamics in a detwinned YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// crystal from SQUID picovoltmeter measurements

    Publication Year: 1997 , Page(s): 1158 - 1160
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    We report measurements of resistivity hysteresis in both temperature and magnetic field performed on a detwinned YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// single crystal in the region of the vortex lattice melting transition. We have found evidence that the observed hysteresis cannot be explained by current-induced nonequilibrium effects. The measured voltage-current characteristics for the sample in the overheated state also display hysteretic behaviour. This strongly supports the idea that the vortex solid can be melted by transport current. View full abstract»

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  • Transport characteristics and flux dynamics in high T/sub c/ superconductors under the influence of pin fluctuation

    Publication Year: 1997 , Page(s): 1161 - 1164
    Cited by:  Papers (6)
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    Flux dynamics in a random pin medium have been studied with the Monte-Carlo method. It has been shown that flux depinning occurs percolatively in the disordered system and the scaling of electric field, E, vs. current density, J, curves reflect the stochastic property of the pin fluctuation. The analytical expressions for temperature and magnetic field dependent E-J curves have also been derived. The analytical expressions agreed quantitatively with the simulation and the in a Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film at various temperature and magnetic field. Moreover, combining the analytical E-J relationship with Maxwell's equations, we estimated the internal current distribution for alternating bias currents. View full abstract»

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  • Hysteretic and ac dynamic losses of high T/sub c/ superconductors by numerical solution of the nonlinear magnetic diffusion equation

    Publication Year: 1997 , Page(s): 1165 - 1168
    Cited by:  Papers (2)
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    The complex ac susceptibility (/spl chi/) of high T/sub c/ superconducting materials has been described in terms of hysteretic and dynamic losses. By including both flux creep and flux flow resistivities in the expression of the flux flow resistivities in the expression of the flux diffusivity, induction profiles have been numerically calculated from the nonlinear flux diffusion equation. The imaginary part of /spl chi/ has been evaluated as function of temperature, frequency and amplitude of the applied magnetic field. The role of pinning and dynamic losses occuring in high T/sub c/ materials is discussed. View full abstract»

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  • Static and dynamic properties of Josephson fluxons and their implications on critical currents, especially in cuprate superconductors

    Publication Year: 1997 , Page(s): 1169 - 1172
    Cited by:  Papers (2)
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    Below 1 Tesla pinned JF yield j/sub c//spl ap/j/sub cJ//spl les/5/spl middot/10/sup 7/A/cm/sup 2/ with j/sub cJ/ the critical Josephson current. The pinning is due to irregularities in the Josephson junctions (weak links) consisting of insulating seams of finite length and varying width, e.g., grain or twin boundaries. At higher fields Abrikosov fluxons (AF) in single crystalline parts may dominate j/sub c/, where j/sub c/ still has to be carried by strong and weak links in series, i.e., by Josephson fluxons (JF) strongly pinned. In addition, the insulating regions at strong and weak links amount to the largest concentration of insulating material in cuprate superconductors able to pin fluxons. Dynamic properties, like, the strong field and frequency dependencies of the surface impedance or the fast transition from normal-to-superconducting state are solely explainable by fast JF with their low loss insulating cores. Because j/sub c/ is given by a critical amount of dissipation, the viscous dynamics of JF- or AF-sliding along weak links play a critical role in superconductors containing strong or weak links. View full abstract»

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  • Current dependence of pinning energy and flux dynamics in high temperature superconductors

    Publication Year: 1997 , Page(s): 1173 - 1176
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    We show that the usually observed logarithmic U(J) dependence at large magnetic fields is related to the increasing number of free flowing vortices distributed in the weak pinning centers, with respect to the vortices in the strong pinning centers in the flux creep regime. Increasing the flux density (i.e. Increasing the external magnetic field) the flux distribution on the pinning centers has a larger dispersion (/spl sigma//sub u//spl Gt/k/sub B/T) and this leads to a logarithmic U(J) behavior. The comparison with the experimental data seems to prove this idea. View full abstract»

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  • Influence of magnetic field on the flux motion in superconducting YBCO/PYBCO superlattice

    Publication Year: 1997 , Page(s): 1177 - 1180
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    Angular dependent critical current densities J/sub c/(H,/spl theta/) for the YBa/sub 2/Cu/sub 3/O/sub 7-y//Pr/sub 0.5/Y/sub 0.5/Ba/sub 2/Cu/sub 3/O/sub 7-y/ (120 /spl Aring//24 /spl Aring/)/spl times/8 superlattice under various magnetic fields were measured to investigate the influence of the applied magnetic fields on the dimensionality of the flux pinning and flux dynamics. J/sub c/(H,/spl theta/) was obtained by applying current through x- or y-direction and analyzed by Kes model. It is found that the dissipation of the flux line system in the mixed state is mainly due to the pancake motion along ab-plane as H<1 T, while the flux lines begin to move along c-axis when the magnetic field reached 1 T. The causes will be discussed. View full abstract»

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  • Continuous YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// film deposition by optically controlled reactive thermal co-evaporation

    Publication Year: 1997 , Page(s): 1181 - 1184
    Cited by:  Papers (9)
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    We built a thermal co-evaporation system to deposit YBCO films on large areas in a continuous process which requires some basic modifications to the established standard co-evaporation setup. An optical rate control system based on atomic absorption spectroscopy has been developed to measure the individual metal fluxes, in addition the metal sources can be reloaded in-situ for long times. The performance of these new components is discussed and results of first YBCO films grown on small samples are shown. View full abstract»

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  • Superconducting NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films prepared by the laser ablation method

    Publication Year: 1997 , Page(s): 1185 - 1188
    Cited by:  Papers (2)
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    NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//(NBCO) thin films were deposited on (100) SrTiO/sub 3/(STO) substrates by the laser ablation method. The transition temperature of thin films was very sensitive to the deposition parameters such as substrate temperature, chamber pressure, and the distance between target and substrate. Thin films of T/sub c//spl sim/93 K were obtained when we deposited at 770/spl deg/C and 800 mTorr of oxygen pressure. The NBCO thin films were grown epitaxially with c-axis preferred orientation. The critical current density of the thin films was /spl sim/1/spl times/10/sup 6/ A/cm/sup 2/ at 77 K. We have also studied the surface morphology with AFM, and the typical smoothness is /spl sim/3 nm. View full abstract»

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  • High-quality YBCO thin films grown by off-axis PLD geometry

    Publication Year: 1997 , Page(s): 1189 - 1192
    Cited by:  Papers (1)
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    In order to improve the surface quality of the pulsed laser deposited YBCO thin films an off-axis deposition geometiy is proposed. Using this new PLD geometry, YBCO thin films have been grown on SrTi03 substrates. These films were found to have a good surface morphology without droplets or other particulates, and good supelrconducting and structural properties. We have obtained Tc(R=0)=89 K, R(300)/R(100)=2.6, ΔT=1.5 K, Jc=2×106 A/cm2 (at 77 K and zero magnetic field), and the FWHM of the rocking curve of (005) YBCO peak is about 0.12°. A study regairding the influence of the deposition parameters on the film quality is also presented. View full abstract»

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  • Fabrication of high quality YBa/sub 2/Cu/sub 3/O/sub y/ thin films using pulsed laser deposition

    Publication Year: 1997 , Page(s): 1193 - 1196
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    High quality YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) thin films for directly coupled DC-SQUIDs were fabricated by pulsed laser deposition. Several critical parameters have been optimized through systematic studies. The best quality YBCO thin films on MgO and on LaAlO/sub 3/ substrates were made under the O/sub 2/ pressure of 400/spl sim/500 mTorr at the substrate temperature of 780/spl sim/800/spl deg/C and target to substrate distance of 50/spl sim/60 mm. Films showing a T/sub c/ above 91 K and J/sub c/ above 2/spl times/10/sup 6/ A/cm/sup 2/ at 77 K were routinely obtained. Extensive AFM and x-ray diffraction studies have been conducted for morphological and structural analyses. The directly coupled DC-SQUIDs were fabricated from the YBCO thin films deposited on SrTiO/sup 3/ bicrystals under the optimized conditions. Measurement of 2I/sub c/ and swing voltage gives values of 420 /spl mu/A and 30 /spl mu/V at 77 K, respectively. View full abstract»

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  • Development of APC NbTi superconductors with internal high purity aluminum stabilizer

    Publication Year: 1997 , Page(s): 1197 - 1200
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    It has long been known that high purity aluminum is superior to copper as a stabilizer for NbTi conductors, due primarily to its low density and excellent low temperature thermal and electrical conductivity. Unfortunately, it is difficult to co-process high purity aluminum with conventional NbTi conductors due to mechanical and diffusional problems caused by precipitation heat treatment, Supercon NbTi artificial pinning center (APC) material makes it possible to eliminate these problems. The concept has been demonstrated in APC composites containing /spl sim/15% high purity aluminum by volume. The recent development effort will be described and the results of mechanical and electrical testing will be given. View full abstract»

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  • Nb/sub 3/Sn artificial pinning microstructures

    Publication Year: 1997 , Page(s): 1201 - 1204
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    Extension of the APC approach to Nb/sub 3/Sn requires that a second phase be incorporated into the Nb/sub 3/Sn layer. The second phase would increase pinning strength by either reducing the grain size or by the second phase pinning the flux itself. The following criteria for elements to be candidates for the APC approach are: (1) they must form intermetallic compounds with Cu or Sn and (2) they must have negligible solubility in Cu and Nb or they must be strong oxide formers. Many of the rare earth elements satisfy these criteria. To circumvent the large strains required to produce wires with a fine distribution of the second phase, film deposition techniques have been used. Critical current densities for Nb films doped with Ti and Y are about 4000 A/mm/sup 2/ at 6T and 4.2K. View full abstract»

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  • Multiple peaks in the angular and field dependence of the critical currents in Nb-47wt%Ti/Cu multilayers

    Publication Year: 1997 , Page(s): 1205 - 1208
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    The effect of Nb-Ti layer thickness d/sub s/ on the angular and field dependencies of J/sub c/ in Nb-47wt.% Ti/Cu multilayers was investigated. J/sub c/ exhibits several peaks in the field and angular dependencies for superconductor and pin layer thickness comparable to the coherence and proximity lengths. The shape of the J/sub c/(B) curve strongly depends on the thickness of Nb-Ti layer. A single peak is present for d/sub s/>10 nm and multiple peaks (two and three) are observed in a sample with d/sub s/=10 nm. The low field peak is temperature independent, while the high-field peaks have a pronounced temperature dependence. We suggest that a crossover between dominant pinning mechanisms and orientational transitions in the vortex lattice is responsible for the observed behavior of J/sub c/(B). View full abstract»

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  • Angular dependent flux pinning studies of superconducting layered structures

    Publication Year: 1997 , Page(s): 1209 - 1212
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    Angular dependences of the critical current density (J/sub c/(/spl theta/)) in superconducting multilayers based on Nb/NbO/sub x/ were investigated in this work. In rotating of the sample at a fixed magnetic field, it,vas found that the direction of the external field at which the maximum of J/sub c/(/spl theta/) was obtained formed an angle with the planar direction. The rotation leads to the existence of the kink structure even in a magnetic field parallel to the layers. This results from the difficulties of the transition of the flux vortex lattice into the ground state with minimum energy. In accordance with the model, bringing of the lattice into the ground state before every measurement at a new angle results in the symmetrical angular dependence of the critical current. View full abstract»

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Aims & Scope

IEEE Transactions on Applied Superconductivity contains articles on the applications of superconductivity and other relevant technology.

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Meet Our Editors

Editor-in-Chief
Britton L. T. Plourde
Syracuse University
bplourde@syr.edu
http://asfaculty.syr.edu/pages/phy/plourde-britton.html