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  • ItemOpen Access
    Individual grain boundary properties and overall performance of metal-organic deposition coated conductors
    (American Physical Society, 2010) Weigand, M; Speller, SC; Hughes, GM; Rutter, S; Grovenor, CRM; Durrell, JH; Durrell, John [0000-0003-0712-3102]
    have investigated single grain boundaries (GBs) isolated in coated conductors produced by metal-organic deposition. When a magnetic field is swept in the film plane, an angle-dependent crossover from boundary to grain limited critical current density J(c) is found. In the force-free orientation, even at fields as high as 8 T, the GBs still limit Jc. We deduce that this effect is a direct consequence of GB meandering. We have employed these single GB results to explain the dependence of Jc of polycrystalline tracks on their width: in-plane measurements become flatter as the tracks are narrowed down. This result is consistent with the stronger GB limitation at field configurations close to force-free found from the isolated boundaries. Our study shows that for certain geometries even at high fields the effect of GBs cannot be neglected.
  • ItemOpen Access
    Practical vortex diodes from pinning enhanced YBa2Cu3O7-delta
    (American Institute of Physics, 2009) Harrington, SA; MacManus Driscoll, JL; Durrell, JH; Durrell, John [0000-0003-0712-3102]
    We identify a scalable, practical route to fabricating a superconducting diode. The device relies for its function on the barrier to flux vortex entry being reduced at the substrate interface of a superconducting pinning enhanced YBa2Cu3O7-d nano-composite film. We show that these composite systems provide a practical route to fabricating a useful superconducting diode and demonstrate the rectification of an alternating current.
  • ItemOpen Access
    Importance of low-angle grain boundaries in YBa2Cu3O7-delta coated conductors
    (IOP Science, 2009) Durrell, JH; Rutter, NA; Durrell, John [0000-0003-0712-3102]
    Over the past ten years the perception of grain boundaries in YBa2Cu3O7-δ conductors has changed greatly. They are no longer a problem to be eliminated but an inevitable and potentially favourable part of the material. This change has arisen as a consequence of new manufacturing techniques which result in excellent grain alignment, reducing the spread of grain boundary misorientation angles. At the same time there is considerable recent evidence which indicates that the variation of properties of grain boundaries with mismatch angle is more complex than a simple exponential decrease in critical current. This is due to the fact that low-angle grain boundaries represent a qualitatively different system to high angle boundaries. The time is therefore right for a targetted review of research into low-angle YBa2Cu3O7-δ grain boundaries. This article does not purport to be a comprehensive review of the physics of grain boundaries as found in YBa2Cu3O7-δ in general; for a broader overview we would recommend that the reader consult the comprehensive review of Hilgenkamp and Mannhart (Rev. Mod. Phys., 74, 485, 2002). The purpose of this article is to review the origin and properties of the low-angle grain boundaries found in YBa2Cu3O7-δ coated conductors both individually and as a collective system.
  • ItemOpen Access
    Benefits of current percolation in superconducting coated conductors
    (American Institute of Physics, 2005) Rutter, NA; Durrell, JH; Blamire, MG; Macmanus Driscoll, JL; Wang, H; Foltyn, SR; Durrell, John [0000-0003-0712-3102]; Blamire, Mark [0000-0002-3888-4476]
    The critical currents of MOD/RABiTS and PLD/IBAD coated conductors have been measured as a function of magnetic field orientation and compared to films grown on single crystal substrates. By varying the orientation of magnetic field applied in the plane of the film, we are able to determine the extent to which current flow in each type of conductor is percolative. Standard MOD/RABiTS conductors have also been compared to samples whose grain boundaries have been doped by diffusing Ca from an overlayer. We find that undoped MOD/RABiTS tapes have a less anisotropic in-plane field dependence than PLD/IBAD tapes and that the uniformity of critical current as a function of in-plane field angle is greater for MOD/RABiTS samples doped with Ca.
  • ItemOpen Access
    Scanning Raman spectroscopy for characterizing compositionally spread films
    (American Chemical Society (ACS), 2005) Venimadhav, A; Yates, KA; Blamire, MG; Blamire, Mark [0000-0002-3888-4476]
    Composition-spread La1-xSrxMnO3 thin films were prepared by pulsed laser deposition technique from LaMnO3 and SrMnO3 targets. The films were epitaxial with a continuous variation of the out-of-plane lattice parameter along the direction of composition gradient. Scanning Raman spectroscopy has been employed as a nondestructive tool to characterize the composition-spread films. Raman spectra showed the variation of the structural, Jahn Teller distortions and the presence of coexisting phases at particular compositions that are in agreement with the previous observation on the single-crystal samples. Raman spectra on the continuous composition-spread film also reveal the effect of disorder and strain on the compositions.
  • ItemOpen Access
    Characteristics of strong ferromagnetic Josephson junctions with epitaxial barriers
    (American Physical Society (APS), 2005) Bell, C; Loloee, R; Burnell, G; Blamire, MG; Blamire, Mark [0000-0002-3888-4476]
    We present the measurement of superconductor/ferromagnetic Josephson junctions, based on an epitaxial Nb bottom electrode and epitaxial Fe20Ni80 barrier. Uniform junctions have been fabricated with a barrier thicknesses in the range 2-12 nm. The maximum critical current density similar to 2.4 +/- 0.2 x 10(9) Am-2 was found for a device with a 3-nm-thick barrier at 4.2 K, corresponding to an average characteristic voltage ICRN similar to 16 mu V. The ICRN showed a nonmonotonic behavior with Fe20Ni80 thickness. The variation of the resistance of a unit area AR(N), of the junctions with barrier thickness gave a Nb/Py specific interface resistance of 6.0 +/- 0.5 f Omega m(2) and Fe20Ni80 resistivity of 174 +/- 50 n Omega m, consistent with other studies in polycrystalline samples.
  • ItemOpen Access
    Flux line lattice structure and behavior in antiphase boundary free vicinal YBa2Cu3O7-delta thin films
    (American Institute of Physics, 2003) Durrell, JH; Mennema, S; Jooss, C; Gibson, G; Barber, ZH; Zandbergen, HW; Evetts, JE; Durrell, John [0000-0003-0712-3102]
    Field angle dependent critical current, magneto-optical microscopy and high resolution electron microscopy studies have been performed on YBa2Cu3O7-delta thin films grown on miscut substrates. High resolution electron microscopy images show that the films studied exhibited clean epitaxial growth with a low density of antiphase boundaries and stacking faults. Any antiphase boundaries (APBs) formed near the film substrate interface rapidly healed rather than extending through the thickness of the film. Unlike vicinal films grown on annealed substrates, which contain a high density of antiphase boundaries, magneto-optical imaging showed no filamentary flux penetration in the films studied. The flux penetration is, however, asymmetric. This is associated with intrinsic pinning of flux strings by the tilted a-b planes and the dependence of the pinning force on the angle between the local field and the a-b planes. Field angle dependent critical current measurements exhibited the striking vortex channeling effect previously reported in vicinal films. By combining the results of three complementary characterization techniques it is shown that extended APB free films exhibit markedly different critical current behavior compared to APB rich films. This is attributed to the role of APB sites as strong pinning centers for Josephson string vortices between the a-b planes. (C) 2003 American Institute of Physics.
  • ItemOpen Access
    Critical current of YBa2Cu3O7-delta low-angle grain boundaries
    (American Physical Society, 2003) Durrell, JH; Hogg, MJ; Kahlmann, F; Barber, ZH; Blamire, MG; Evetts, JE; Durrell, John [0000-0003-0712-3102]; Blamire, Mark [0000-0002-3888-4476]
    Transport critical current measurements have been performed on 5degrees [001]-tilt thin film YBa2Cu3O7-delta single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.
  • ItemOpen Access
    Effect of varying material anisotropy on critical current anisotropy in vicinal YBa2Cu3O7-delta thin films
    (American Institute of Physics, 2003) Durrell, JH; Evetts, JE; Rossler, R; Delamare, MP; Pedarnig, JD; Bauerle, D; Durrell, John [0000-0003-0712-3102]
    The high $T_{c}$ cuprate superconductors are noted for their anisotropic layered structure, certain of these materials indeed tend toward the limit of a Lawrence-Doniach superconductor. However, YBa$_2$Cu$_3$O$_{7-\delta}$ has a smaller anisotropy than would be expected from its interlayer spacing. This is due to the cuprate chains in the structure. To investigate the influence of the chain oxygen on transport properties critical current versus applied field angle measurements were performed on fully oxygenated and de-oxygenated YBa$_2$Cu$_3$O$_{7-\delta}$ thin films and optimally oxygenated Y$_{0.75}$Ca$_{0.2}$Ba$_2$Cu$_3$O$_{7-\delta}$ thin films. The films were grown on 10$^{\circ}$ mis-cut SrTiO$_3$ substrates to enable the intrinsic vortex channelling effect to be observed. The form of the vortex channelling minimum observed in field angle dependent critical current studies on the films was seen to depend on film oxygenation. The vortex channelling effect is dependent on a angular dependent cross-over to a string-pancake flux line lattice. The results obtained appear to be consistent with the prediction of Blatter et al. [Rev. Mod. Phys., 66 (4): 1125 (1994)] that increased superconducting anisotropy leads to the kinked string-pancake lattice existing over a smaller angular range.
  • ItemOpen Access
    Critical currents in vicinal YBa2Cu3O7-delta films
    (American Physical Society (APS), 2004) Durrell, JH; Burnell, G; Tsaneva, VN; Barber, ZH; Blamire, MG; Evetts, JE; Durrell, John [0000-0003-0712-3102]; Blamire, Mark [0000-0002-3888-4476]
    Most measurements of critical current densities in YBa2Cu3O7-delta thin films to date have been performed on films where the c axis is grown normal to the film surface. With such films, the analysis of the dependence of the critical current, j(c), on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of j(c), such as the measurement geometry and disposition of pinning centers, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed flux line lattice structure models on the basis of angular critical current density measurements on c-axis films. Films grown on miscut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of j(c) in YBa2Cu3O7-delta are reviewed. Measurements on YBa2Cu3O7-delta thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the flux line lattice in YBa2Cu3O7-delta. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation, and temperature. As a consequence, a simple scaling law cannot, by itself, describe the observed critical current anisotropy in YBa2Cu3O7-delta. The experimentally obtained j(c)(theta) behavior of YBa2Cu3O7-delta is successfully described in terms of a kinked vortex structure for fields applied near parallel to the a-b planes.
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