Flux dynamics of YBCO/Ag composite

Abstract

DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andH _max, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJ _cgm on low-field M-H loop. The estimation of intergranular critical current densityJ _cjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJ _c(T) with silver content under low AC fields only, consistent with the transportJ _c data; beyond thatJ _c(T) decreases. This improvement inJ _c(T) and transportJ _c with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (H _max>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionH _c2j and lower critical field of intragrain regionH _c1g also decrease with silver content.