NA-DOPING EFFECT ON THE MAGNETIC-PROPERTIES OF THE YBCO CERAMICS

The ac magnetization and the microwave surface resistance (at 12 GHz) were measured on a series of Na-doped YBCO ceramic samples at 77 K. Th eir magnetic behavior was explained on the basis of the modified criti cal state model by taking into account the existence of a field-depend ent component J(c1) (due to weakly linked grains) and a field-independ ent component J(c0) (due to perfectly linked grains) in the bulk criti cal current density of the samples. The Na and Cu remaining after the heat treatment of the samples changed the intergranular medium paramet ers and impeded the correlation between the grains. As a result, an in crease of the Na concentration led to a decrease of the specimens crit ical current density and an increase of their surface resistance. At s mall Na concentrations, a certain increase of J(c1) was observed, whic h can be explained by taking into account the possibility of partial p inning of Josephson vortices in the ''weakly seeded'' places in the in tergranular media.