Magnetization studies of the high- T c compound Y 1 Ba 2 Cu 3 O z
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D. K. Christen, S. T. Sekula, J. Brynestad Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6061
Y. C. Kim Department of Physics, University of Tennessee, Knoxville, Tennessee 37996-1200 (Received 1 July 1987; accepted 29 July 1987) The superconductive state magnetization of the high-7"c compound YjBa2Cu3O2 has been investigated. Single-phase, polycrystalline samples of this archetype were prepared by solidstate reaction. The onset of the superconductive transition, measured magnetically, was 90.7 K. In addition to low-field dc magnetization results, measurements of the magnetic critical current density Jc {H, T) are presented as a function of magnetic field and temperature. The low values of Jc, especially at high temperature, are discussed in terms of intergranular supercurrents that flow within individual grains in superconductive layers of reduced dimensionality.
I. INTRODUCTION The recent discovery by Bednorz and Miiller1 of superconductive materials with transition temperatures Tc in the 30 K regime spurred intense activity in the field. These efforts quickly led to the discovery of related materials with superconductive transitions near 90 K 2 and to the subsequent determination of the structure of the high-71,, phase,3 thereby allowing single-phase materials to be produced. The archetype of this latter class of materials is Y!Ba2Cu3Oz withz=7. These oxygen deficient, copper oxide-based materials crystallize in an orthorhombic structure with nearly equal lattice parameters in the basal plane and a "c" axis length that is about a factor of 3 greater. The current availability of singlephase material allows for a systematic study of intrinsic properties of polycrystalline samples. In this article we present the results of a magnetic investigation of single-phase Y!Ba2Cu3Oz materials together with information on the electrical resistivity. The magnetic measurements below Tc have been analyzed to obtain values for the critical current density Jc by use of the critical state model. II. EXPERIMENTAL ASPECTS The materials investigated were synthesized by reaction in the solid state. For starting materials, powdered Y2O3, BaCO3, and CuO of 99.999 at. % purity were utilized. Prior to usage, the Y 2 O 3 and other powders were dried by heating in an inert atmosphere to 1100 and 200 °C, respectively. Appropriate quantities of the starting materials were mixed, cold pressed into cylindrical pellets, and sinter reacted overnight in O2 or J. Mater. Res. 2 (6), Nov/Dec 1987
dry air near 900 °C. The pellets were repeatedly reground, pressed into pellets, and fired at 950 °C in O2 gas. Finally, the materials were annealed in O 2 at 700 °C for 24 h. All mixing, grinding, and pressing operations were performed in an oxygen-monitored dry box backfilled with He gas. From the change in weight during high-temperature processing, an oxygen stoichiometry z very near 7 was found. The resulting materials were single phase according to analysis by x-ray diffraction. Most of the results presented here were obtained on a sample
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