Effects of Cu substitution by Fe on the magnetic properties of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ single crystals

We report magnetization measurements on single crystals in the system ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$, with Fe concentration x up to 0.24. The relation between the superconducting properties and twin boundaries is discussed. Our results indicate that a model of higher-${\mathit{T}}_{\mathit{c}}$ regions in the twin boundaries seems incorrect. An anti-Meissner effect and crossover in the low-temperature inductive ${\mathit{J}}_{\mathit{c}}$ measurements are observed, which indicate a possible pinning mechanism due to twin boundaries. A positive curvature to the ${\mathit{H}}_{\mathit{c}1}$-versus-temperature data suggests a strong temperature dependence of the Ginzburg-Landau parameters. No single pinning mechanism is dominant over the entire temperature and magnetic-field range in these oxide superconductors.