Pinning mechanisms in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals

Systematic studies of flux pinning in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals have been undertaken in an attempt to isolate the mechanisms that may cause the so-called fishtail pinning effect, a rise in critical current density (${\mathit{J}}_{\mathit{c}}$) that occurs as the magnetic field (H) increases. Two different sources of point defect pinning, 200 MeV proton irradiation and oxygen annealing, are used to alter the system and to determine how each one changes the ${\mathit{J}}_{\mathit{c}}$ vs H curves. The fishtail pinning effect is extremely robust and survives both in magnitude and with a similar slope of the ${\mathit{J}}_{\mathit{c}}$ vs H curve for a wide range of irradiation and for wide excursions in oxygen content and annealing condition. The data are consistent with an intrinsic granularity arising from cation disorder that survives as oxygen is inserted and removed and as irradiation induced point defects are added and removed.