Flux pinning by ${\mathrm{Y}}_2$${\mathrm{BaCuO}}_5$ precipitates and field- and temperature-driven pinning centers in melt-powder-melt-growth processed ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Magnetic hysteresis, flux pinning, and flux creep in melt-powder-melt-growth processed ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (Y 1:2:3) containing nominal 0, 25, and 40 mol % concentration of ${\mathrm{Y}}_2$ ${\mathrm{BaCuO}}_5$ (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 ${\mathit{U}}_{\mathrm{eff}}$ versus current density J reveal a diverging behavior of ${\mathit{U}}_{\mathrm{eff}}$(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 ${\mathit{U}}_{\mathrm{eff}}$ 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 ${\mathit{J}}_{\mathit{c}}$ that is an increasing function of H (or B). Moreover, it has been demonstrated that the additional pinning leads to an increase in ${\mathit{U}}_{\mathrm{eff}}$ in an H-T region in which the butterfly is developed.