Flux pinning by Y2BaCuO5 precipitates and field- 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 Ueff versus current density J reveal a diverging behavior of Ueff(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 Ueff 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 Jc that is an increasing function of H (or B). Moreover, it has been demonstrated that the additional pinning leads to an increase in Ueff 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, Ueff(J,T,H)=Ui[G(T)/Hn](Ji/J)μ, where Ui is the scale of the activation energy, G(T) =[1-(T/Tx)2]m, and Tx is close in value to Tirr(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 Ueff which have μ values that agree qualitatively with the theoretical predictions (=7/9 and 3/2) for a three-dimensional flux-line lattice.