BUTTERFLY MAGNETIZATION IN YBA2CU3-XFEXO7-Y - CORRELATION WITH THE MICROSTRUCTURE AND THE MACROSTRUCTURE

The butterfly anomaly in the critical current density (J) of a large n umber of YBa2Cu3-xFe(x)O(7-y) samples is investigated as a function of the magnetic field (0 less than or equal to H less than or equal to 6 0 kOe), the field sweep rate (2 Oe/s less than or equal to dH/dt less than or equal to 10(3) Oe/s), the temperature (T less than or equal to 55 K), the dimensions of the specimens (10(-2) mm less than or equal to R less than or equal to 0.6 mm), and their microstructures. The mag nitude and the shape of the central peak of J(H) depend strongly on th e size of the specimen, but only weakly on its microstructure. Despite the differences in the defect structures of the various samples, thei r current densities tend towards the same value for R less than or equ al to 50 mu m, T < 60 K, and H less than or equal to H-m2, with H-m2 d enoting the field of the second maximum of the hysteresis cycle. The l atter field generally decreases with the defect concentration (n(ef)) and provides an excellent characterization of the pinning properties o f the material. At low concentration, the pinning force increases with n(ef), while the pinning barriers decreases. The butterfly effect is consistent with the collective creep theory in the field region around H-m2. This held marks a crossover between small and large bundle pinn ing regimes.