S. Senoussi et al., BUTTERFLY MAGNETIZATION IN YBA2CU3-XFEXO7-Y - CORRELATION WITH THE MICROSTRUCTURE AND THE MACROSTRUCTURE, Physical review. B, Condensed matter, 53(18), 1996, pp. 12321-12329
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.