Critical currents of ex situ YBa₂Cu₃O_7– thin films on rolling assisted biaxially textured substrates: Thickness, field, and temperature dependencies

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A. O. Ijaduola,¹ J. R. Thompson,^1,2 R. Feenstra,² D. K. Christen,² A. A. Gapud,² and X. Song³
¹Department of Physics, University of Tennessee, Knoxville, Tennessee 37996-1200, USA
²Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6061, USA
³Applied Superconductivity Center, University of Wisconsin, Madison, Wisconsin 53706-1390, USA

Received 5 December 2005; revised 3 March 2006; published 4 April 2006

Thecritical current density J_c flowing in thin YBa₂Cu₃O_7– (YBCO) filmsof various thicknesses d has been studied magnetometrically, both asa function of applied field H and temperature T, witha central objective to determine the dominant source of vortexpinning in these materials. The films, grown by a BaF₂ex situ process and deposited on buffered rolling assisted biaxiallytextured substrates ("RABiTS") substrates of Ni-5% W, have thicknesses d rangingfrom 28 nm to 1.5 µm. Isothermal magnetization loops M(H;T) and remanentmagnetization M_rem(T) in H=0 were measured with Hc-axis (i.e., normalto film plane). The resulting J_c(d) values (obtained from amodified critical state model) increase with thickness d, peak neard~120 nm, and thereafter decrease as the films get thicker. Fora wide range of temperatures and intermediate fields, we findJ_c [proportional] H^– with alpha ~(0.56–0.69) for all materials. This feature can beattributed to pinning by large random defects, which theoretically haspower-law exponent alpha =5/8. Calculated values for the size and densityof defects are comparable with those observed by TEM inthe films. As a function of temperature, we find J_c(T,sf)~[1–(T/T_c)²]ⁿwith n~1.2–1.4. This points to " delta T_c pinning" (pinning that suppressesT_c locally) in these YBCO materials.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.73.134502
DOI:	10.1103/PhysRevB.73.134502
PACS:	74.25.Qt; 74.25.Sv; 74.72.Bk; 74.78.-w 74.25.Qt Vortex lattices, flux pinning, flux creep in superconductors 74.25.Sv Critical currents in superconductors 74.72.Bk Y-based cuprates (HTSC) 74.78.-w Superconducting films and low-dimensional structures YEAR: 2006
KEYWORDS:	critical current density (superconductivity), yttrium compounds, barium compounds, superconducting thin films, high-temperature superconductors, rolling, flux pinning, remanence, transmission electron microscopy

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