Magneto - Optical Observation of Magnetic Flux Penetration and Current Distribution in Y - Ba - Cu - O Single Crystals and Melt - Textured Ceramics

Abstract

The local critical current properties in YBaCuO ceramics,melt-textured on the 211 substrate, were estimated by analyzing the results of magneto-optical imagings. The local critical current density Jc were determined and the pinning mechanisms were argued from measurements of the Jc vs the temperature and 211 additions. The combined experiments show that the magnitude of Jc in single crystals and ceramics is largely determined by the intragrain flux pinning. Experimental values of Jc varied from 2 104 up to (5÷8) 105 A cm−2 for the regions of sample with the different structures. It is shown that areas of sample which initially were 123-compact have the limitations for Jc~2 104 A cm−2, which are common for material manufactured by every kind of melt-processing technique and is determined by the distribution of current-limiting defects.Thus the current has to percolate through the defects network.But the area of the sample,which initially was substrate 211,is highly textured isotropic YBaCuO ceramics capable of carrying current density up to 106 A cm 2.The pinning mechanism is determined by 211 particles as the dominant pinning centers in the 5–77 K temperature region.The Jc can be tailored through variations in size, shape and amount of 211 inclusions: much finer normal particles should be introduced in order to improve the critical current density.