Secondary-phase orientation probed by EPR:mY2BaCuO5sin

Abstract

Textured YBa 2 Cu 3 O 7Ϫx samples have been studied by electronic paramagnetic resonance. EPR spectroscopy allows us to analyze the Y 2 BaCuO 5 second phase present as inclusions in the YBa 2 Cu 3 O 6 textured matrix. All samples were deoxygenated in order to limit the skin effect due to conductivity. This is applied to samples textured according to different techniques. Preferential orientation of these particles according to the 123 matrix can be observed. For instance, if texturing under a magnetic field, the particular Y 2 BaCuO 5 orientation is observed of the b 211 axis ͑bϭ7.132 Å͒ parallel to c 123 and to the magnetic field direction. By appropriate calculations from initial spectra, the fraction of preferred orientation of these Y 2 BaCuO 5 grains can be quantified. This orientation comprises 16Ϯ2 % of the Y 2 BaCuO 5 weight for texturing under a magnetic field, presently one of the highest values obtained. It involves the longest size of Y 2 BaCuO 5 elongated grains lying in YBa 2 Cu 3 O 6 planes, facilitating the texturation process. Texturing on Y 2 O 3 by the modified melt textured growth method leads to a preferential orientation of Y 2 BaCuO 5 grains too, but in another direction, i.e., b 211 perpendicular to c 123 . There is no Y 2 BaCuO 5 preferential orientation if texturing with a thermal gradient or on a monocrystalline MgO substrate. This type of analysis can be applied easily to other systems ͑nanocomposites, dispersoids, etc.͒, providing that the signal of the secondary phase can be clearly separated from the signal of the primary one.