Transport and pinning properties of Ag-doped FeSe_0.94

We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe_0.94 at magnetic fields up to 14 T. We established that, due to Ag addition, the hexagonal phase formation in melted FeSe_0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp, with a transition width below 1 K. T_c and the upper critical field were found to increase, while the normal-state resistivity was significantly reduced, becoming comparable with that of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe_0.94 is dominated by a thermally activated flux flow. From the activation energy U versus H dependence, we found a crossover from single-vortex pinning to a collective-creep pinning behavior by increasing the magnetic field. (paper)