Flux creep in iron-doped Y-Ba-Cu-O single crystals

We report the zero-field-cooled magnetic relaxation and thermal activation energy in various ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{O}}_7$ single crystals. No subgranularity effects were observed in our single crystals. The phenomenon of giant flux creep is evident in these systems. The magnetic relaxation rate dM/d(lnt) is strongly dependent on temperature and applied magnetic field. We utilize the Anderson-Kim thermally activated flux-creep model to analyze these systems.