EFFECTIVE MAGNETIC GRAIN-SIZE AND ITS INFLUENCE ON THE INTRAGRAIN CRITICAL-CURRENT IN YBACUO POLYCRYSTALS

Magnetization curves M(H) were studied for a series of YBa2Cu3O7-delta polycrystalline samples with different microstructures (from fine-grained with a grain size R similar to 1 mu m to coarse-grained with R similar to 50 mu m) synthesized from mechanically activated oxide mixture using both an original method and according to well known methods (cryochemical and traditional ceramic). For external magnetic fields, some lower than the lower critical field of grains H-c1, quantitative estimation of the sizes of the shielded regions was carried out. It was shown that this size could be taken as the effective grain size R(eff) of polycrystalline samples. This R(eff) value may be used both for estimation of intragrain critical current density j(c) and for comparative study of samples with different microstructures. The values of j(c) for a series of samples with grain sizes from 1 to 50 mu m were calculated taking into account the values of R(eff) obtained. Values of effective penetration depths were obtained using M(H) curves in the range H-c1 << H << H-c2. A correlation between j(c) and grain size was observed. It was shown that fine-grained samples had j(c) an order of magnitude greater than coarse-grained ones. It was revealed that magnetization curves M(H) for some coarse-grained mechanically activated ceramic samples had a peculiar shape in fields H > 1000 Oe (a so-called 'fishtail' shape).