Vortex phase diagram of Ba(Fe 0.93 Co 0.07 ) 2 As 2 single crystals

Irreversible magnetic properties of $\text{Ba}{({\text{Fe}}_{0.93}{\text{Co}}_{0.07})}_{2}{\text{As}}_{2}$ 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 $M(T=\text{const},H)$ and a corresponding feature in $M(T,H=\text{const})$ 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 $j\ensuremath{\approx}2.6\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{A}/{\text{cm}}^{2}$. 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 $\text{Ba}{({\text{Fe}}_{0.93}{\text{Co}}_{0.07})}_{2}{\text{As}}_{2}$ 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.