Critical current density in iron-doped Y-Ba-Cu-O single crystals

Critical current densities calculated from magnetization measurements are investigated in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{O}}_7$ single crystals with x=0 to 0.24. The measurements are done over a wide temperature range, 5 K to ${\mathit{T}}_{\mathit{c}}$, and in magnetic fields up to 5.5 T. The results indicate that the critical current density in low-iron-composition samples has a small field dependence but a strong temperature dependence. The critical current density decreases as the Fe content increases. The model of twin boundaries acting as effective pinning centers in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{O}}_7$ systems is not supported by these results, at least for T>10 K.