Direct experimental analysis of the relation between the grain structure and the distribution of critical current density in YBa₂Cu₃O_x coated conductors

We present a relatively simple experimental method to correlate the grain structure of YBa₂Cu₃O_x coated conductors with spatial variations of the critical current density on a macroscopic scale. Variations of the current density on a micrometre scale are visualized with magneto-optical imaging, using the flux trapped by colonies of grains to quantify the degree of connectivity. Integrating these trapped flux profiles over larger distances yields direct information of the critical current distribution on a millimetre scale, provided that contributions from negative return flux in colony boundaries are properly eliminated. Flux polarity is determined using the wavelength dependence of the Verdet constant. The validity of this analysis is demonstrated by comparing the results with direct transport data on the lateral current distribution, measured with the magnetic knife technique. The combination of both experiments shows that the critical current density is suppressed over several millimetres of conductor length at areas where a large number of high-angle grain boundaries or defects are present.