Aligned gadolinium barium copper oxide thick films formed by in situ crystallization in a magnetic field
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Dennis Clougherty Department of Physics, University of Vermont, Burlington, Vermont 05405 (Received 19 January 1993; accepted 28 May 1993)
G d - 1 : 2 : 3 films were synthesized from acetate precursors and spun-on to polycrystalline substrates of yttria-stabilized zirconia. The substrates were fired in air at 500 °C in between each applied coat. After applying 10-30 coats, they were crystallized by heating to 900 °C in Ar or O2 with or without the application of a 1 T magnetic field. The applied magnetic field was found to produce c-axis alignment for films < 4 /xm. Annealing in argon produced alignment, but the effect was less pronounced than annealing in oxygen. Substrates with films >20 //,m were not aligned for either environment.
I. INTRODUCTION Grain-aligned bulk samples and films have been prepared by various techniques and have been found to have a significant improvement in the critical current density Jc in comparison to non-aligned samples. Alignment methods include uniaxial pressing of powder samples,1'2 directional solidification from the melt,3 thin film processing,4 and magnetic field alignment (discussed below). Recently, other work has shown that alignment of thin films can be achieved by decreasing the pOl during the crystallization of —100-200 nm thick amorphous films formed by laser ablation5'6 or by RF sputtering.7 Farrell et al.8 first showed that the application of a 0.94 T magnetic field aligned Y - l : 2 : 3 single crystals under ambient conditions. Although the yttrium is nonmagnetic, the alignment arises from the anisotropy in the susceptibility parallel and perpendicular to the c-axis in the C u - 0 planes. With the inclusion of highly magnetic ions, such as some of the rare earth elements, this effect is even more pronounced.9 Magnetic alignment of grains has been done by suspending powders in epoxy and curing in a magnetic field9'10 and by mixing the powders in a volatile solvent and evaporating the solution in a magnetic field.11 Unfortunately, all of these techniques rely on powder processing of relatively coarse-grained materials that are usually chemically inhomogeneous. In order for magnetic alignment to be a feasible processing route, epoxy-cured materials and porous, solvent-dried materials are not viable. Chemical decomposition techniques have been used in tandem with other processing routes such as dippingpyrolysis,12 spin-on coatings,13 and spray drying14 to produce dense films of YBa 2 Cu 3 07_^. These coatings are amorphous and crystallize at elevated temperatures 2440 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 10, Oct 1993 Downloaded: 20 Mar 2015
as the water and volatile organic solvents are evaporated. No work has been done on crystallizing an amorphous precursor in a magnetic field in order to achieve grain alignment. It is expected that the crystallites and their subsequent growth orientation will be influenced by the magnetic field, resulting in a grain-aligned structure. This study was undertaken to examine G d - 1 : 2 : 3 amorphous films produced
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