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-gra ined with a grain size R similar to 1 mu m to coarse-grained with R si milar to 50 mu m) synthesized from mechanically activated oxide mixtur e using both an original method and according to well known methods (c ryochemical and traditional ceramic). For external magnetic fields, so me lower than the lower critical field of grains H-c1, quantitative es timation 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 es timation of intragrain critical current density j(c) and for comparati ve 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 calcu lated taking into account the values of R(eff) obtained. Values of eff ective 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 obse rved. It was shown that fine-grained samples had j(c) an order of magn itude greater than coarse-grained ones. It was revealed that magnetiza tion curves M(H) for some coarse-grained mechanically activated cerami c samples had a peculiar shape in fields H > 1000 Oe (a so-called 'fis htail' shape).