FLUX-PINNING BY Y2BACUO5 PRECIPITATES AND FIELD-DRIVEN AND TEMPERATURE-DRIVEN PINNING CENTERS IN MELT-POWDER-MELT-GROWTH PROCESSED YBA2CU3O7

Magnetic hysteresis, flux pinning, and flux creep in melt-powder-melt-growth processed YBa2Cu3O7 (Y 1:2:3) containing nominal 0, 25, and 40 mol% concentration Of Y2BaCuO5 (Y 2:1:1) inclusions were investigated. The strong pinning due to 2:1:1-phase precipitates in these samples allows for characterization of the hysteretic response as a function of pinning-site concentration over a large portion of magnetic-field-temperature space. We have found the following: (i) The curves of effective pinning energy U(eff) versus current density J reveal a diverging behavior of U(eff)(J) in the low-J regime. This supports the existence of a vortex-glass state, and is a signature of a vanishing resistance as the current density approaches zero. (ii) Both the U(eff) and the J values obtained from magnetic hysteresis loops were observed to increase with Y 2:1:1 concentration. The appearance of the butterfly-shaped (or ''fishtail'') hysteresis loops indicates a J(c) that is an increasing function of H (or B). Moreover, it has been demonstrated that the additional pinning leads to an increase in U(eff) in an H-T region in which the butterfly is developed. The derived effective pinning energy is fit, from the instantaneous experimental relaxation data, to the relation, U(eff)(J,T,H) = U(i)[G(T)/H(n)](J(i)/J)mu, where U(i) is the scale of the activation energy, G(T) = [1 - (T/T(x))2]m, and T(x), is close in value to T(irr) (H) (the irreversibility line of the material). This description breaks down in the vicinity of the ''butterfly'' peak. We observed two power-law regimes of J dependence of U(eff) which have mu values that agree qualitatively with the theoretical predictions (= 7/9 and 3/2) for a three-dimensional flux-line lattice.