FLUX DYNAMICS OF YBCO AG COMPOSITE

DC magnetization measurements have been carried out on bulk YBCO/Ag co mposites with silver content up to 20 wt percent. DC Fields in the ran ge 0.5 mT to 200 mT have been used to investigate the inter- and intra granular properties at 77K. The AC susceptibility as a function of tem perature at different AC fields (0-026-0.30 mT) has also been studied. Under small DC fields (almost-equal-to 4 mT), depending on the Ag con tent and H(max), the M-H loop shows a complicated behaviour. This beha viour can be explained on the basis of effect of strong field dependen ce of transport critical current, grain size and intragrain critical c urrent density J(cgm) on low-field M-H loop. The estimation of intergr anular critical current density J(cjm) from these loops does not remai n a simple function of AM/d. The AC susceptibility measurements show a small increase in J(c)(T) with silver content under low AC fields onl y, consistent with the transport J(c) data; beyond that J(c)(T) decrea ses. This improvement in J(c)(T) and transport J(c) with silver can be ascribed to the improved coupling between grains but not to the pinni ng. Also at higher field (H(max) > 20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical Field of intergranular region H(c2j) and lower critical field of intragrain reg ion H(c1g) also decrease with silver content.