  We investigated the superconducting transition and the pinning properties of
undoped and Ag-doped FeSe0.94 at magnetic fields up to 14 T. It was established
that due to Ag addition the hexagonal phase formation in melted FeSe0.94
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), Tc and the upper critical field were found to increase,
whereas the normal state resistivity significantly reduces becoming comparable
with those of FeSe single crystals. In addition, a considerable
magnetoresistance was observed due to Ag doping. The resistive transition of
undoped and Ag-doped FeSe0.94 is dominated by thermally activated flux flow.
From the activation energy U vs H dependence, a crossover from single-vortex
pinning to a collective creep pinning behavior was found with increasing the
magnetic field.
