Showing papers by "David K. Christen published in 2011"

Thickness dependence of magnetic relaxation and E - J characteristics in superconducting (Gd-Y)-Ba-Cu-O films with strong vortex pinning

Abstract: The dependence of the critical current density ${J}_{c}$ on temperature, magnetic field, and film thickness has been investigated in (Gd-Y)-Ba-Cu-O materials of 0.7, 1.4, and 2.8 \ensuremath{\mu}m thickness. Generally the ${J}_{c}$ decreases with film thickness at investigated temperatures and magnetic fields. The nature and strength of the pinning centers for vortices have been identified through angular and temperature measurements, respectively. These films do not exhibit $c$-axis correlated vortex pinning, but do have correlated defects oriented near the $\mathit{ab}$ planes. For all film thicknesses studied, strong pinning dominates at most temperatures. The vortex dynamics were investigated through magnetic relaxation studies in the temperature range of 5--77 K in 1 and 3 T applied magnetic fields, $H$ \ensuremath{\parallel} surface normal. The creep rate $S$ is thickness dependent at high temperatures, implying that the pinning energy is also thickness dependent. Maley analyses of the relaxation data show an inverse power law variation for the effective pinning energy ${U}_{\mathrm{eff}}$ \ensuremath{\sim} (${J}_{0}$/$J$)${}^{\ensuremath{\mu}}$. Finally, the electric field-current density (E-J) characteristics were determined over a wide range of dissipation by combining experimental results from transport, swept field magnetometry (VSM), and superconducting quantum interference device (SQUID) magnetometry. We develop a self-consistent model of the combined experimental results, leading to an estimation of the critical current density $J$${}_{c}$${}_{0}$($T$) in the absence of flux creep.

Anisotropy of the irreversibility field for Zr-doped (Y,Gd)Ba 2 Cu 3 O 7-x thin films up to 45 T

Abstract: The anisotropic irreversibility fieldBIrr of twoYBa2Cu3O7 x thin films dopedwith additional rare earth (RE)= (Gd, Y) and Zr and containing strong correlated pins (splayed BaZrO3 nanorods and RE2O3 anoprecipitates) has been measured over a very broad range up to 45 T at temperatures 56 K < T < Tc We found that the experimental angular dependence of BIrr ( ) does not follow the mass anisotropy scaling BIrr ( ) = BIrr (0)(cos2 + 2 sin2 ) 1/2, where = (mc/mab)1/2 = 5 6 for the RE-doped Ba2Cu3O7 x (REBCO) crystals, mab and mc are the effective masses along the ab plane and the c-axis, respectively, and is the angle between B and the c-axis For B parallel to the ab planes and to the c-axis correlated pinning strongly enhances BIrr , while at intermediate angles, BIrr ( ) follows the scaling behavior BIrr ( ) (cos2 + 2 RP sin2 ) 1/2 with the effective anisotropy factor RP 3 significantly smaller than the ass anisotropy would suggest In spite of the strong effects of c-axis BaZrO3 nanorods, we found even greater enhancements of BIrr for fields along the ab planes than for fields parallel to the c-axis, as wellmore » as different temperature dependences of the correlated pinning contributions to BIrr for B//ab and B//c Our results show that the dense and strong pins, which can now be incorporated into REBCO thin films in a controlled way, exert major and diverse effects on the measured vortex pinning anisotropy and the irreversibility field over wide ranges of B and T In particular, we show that the relative contribution of correlated pinning to BIrr for B//c increases as the temperature increases due to the suppression of thermal fluctuations of vortices by splayed distribution of BaZrO3 nanorods« less

Intra- and inter-grain currents in coated conductors with arbitrary grain boundary properties from magnetic measurements

Abstract: A generalized methodology is described for determining both the intragranular and through-grain-boundary critical current densities in practical coated conductors from contact-free magnetic hysteresis measurements. The model incorporates the vector nature of current density J within the superconducting grains and current conservation with respect to the grain boundaries. Using physically observed values for the grain aspect ratio as input, the analysis yields the low-field intragranular critical current density JcG from a single field-dependent measurement of the global Jc and provides a consistent description that spans the entire range from the weak-link () to the single-grain () limit. Results are given for ex?situ processed RBCO coatings on RABiTS.

Flux pinning enhancements in YBa2Cu3O7-8 superconductors through phase separated, self-assembled LaMnO3-MgO nanocomposite films.

Abstract: Technological applications of high temperature superconductors (HTS) require high critical current density, Jc, under operation at high magnetic field strengths This requires effective flux pinning by introducing artificial defects through creative processing In this work, we generated correlated disorder for strong vortex pinning in the YBa2Cu3O7- (YBCO) films by replacing the standard LaMnO3 (LMO) cap buffer layers in ion beam assisted deposited MgO templates with LMO:MgO composite films Such films revealed formation of two phase-separated, but at the same time vertically aligned, self-assembled composite nanostructures that extend throughout the entire thickness of the film Measurements of magnetic-field orientation-dependent Jc of YBCO coatings deposited on these nanostructured cap layers showed correlated c-axis pinning and improved in-field Jc performance compared to those of YBCO films deposited on standard LMO buffers The present results demonstrate feasibility of novel and potentially practical approaches in the pursuit of more efficient, economical, and high performance superconducting devices

Pinning Enhancements in YBCO Films via Nanoengineered ${\rm LaMnO}_{3}$:${\rm MgO}$ Composite Cap Layer

Abstract: In the present work, composite LaMnO3:MgO (LMO:MgO) cap buffer layers with varying MgO contents 5 vol% up to 75 vol% have been grown on homo-epi MgO/IBAD(MgO) substrates to enhance the performance of YBa2Cu3O7-x (YBCO) films. Results showed formation of phase separated MgO nanocolumns within the LMO matrix. The impact of these nanocolumns on the superconducting properties of YBCO films deposited using pulsed laser deposition (PLD) on the nanostructured layer was investigated by electrical transport measurements. Such YBCO films showed better in-field performance compared to that of YBCO films on standard LMO cap films. In particular, measurements of the field-angle dependence revealed c-axis correlated pinning for YBCO films on these composite cap layers. The present results demonstrate a practical approach to obtain high performance superconducting wires.