Vortex phase diagram of Ba(Fe0.93Co0.07)(2)As-2 single crystals

Irreversible magnetic properties of Ba(Fe0.93Co0.07)(2)As-2 single crystals were studied by using several techniques including transport, magnetization, and magneto-optics. The mixed-state response was found to be very similar to high-T-c cuprates, such as Y-Ba-Cu-O and Nd-Ce-Cu-O. In particular, we find a nonmonotonic "fishtail" shape of the magnetization loops M(T=const,H) and a corresponding feature in M(T,H=const) scans, as well as a very large magnetic relaxation rate. The supercurrent density, j, in the critical state was evaluated from magnetization and from direct transport measurements close to T-c. At 5 K we estimate a moderate j approximate to 2.6 +/- 0.2x10(5) A/cm(2). Analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning and creep (weak pinning and fast creep) model and suggests a crossover from the collective to the plastic creep regime in fields exceeding the field at which the fishtail magnetization is maximum. While the zero-field anisotropy of the magnetic properties in the superconducting state of Ba(Fe0.93Co0.07)(2)As-2 is small, the vortex behavior changes for different orientations of magnetic field with respect to the crystal axes, implying field-dependent anisotropy of pinning properties.