High-field and low-temperature magnetization and flux creep in untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$

We report on the magnetization and flux creep in untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ in a function of field up to 20 T and temperature between 0.4 and 4.2 K. We observe a field-dependent magnetization down to 0.4 K, exhibiting a hump at intermediate fields. The normalized relaxation rate decreases with field before going through a minimum at intermediate fields. Extrapolation to zero temperature indicates the existence of a nonzero relaxation at all magnetic fields. We compare our data to relaxation measurements on an organic superconductor. The data analyses within the framework of the collective pinning theory fail. We suggest the existence of an additional pinning mechanism in the sample. © 1996 The American Physical Society.