We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe 0.94 at magnetic fields up to 14 T. It was 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, 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 FeSe 0.94 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.

The superconducting properties of mm-sized Fe1.02Se crystals grown by a flux method are investigated. The structural and morphological features are studied by x-ray diffraction (XRD) and by scanning electron microscopy-energy dispersive x-ray spectroscopy SEM-EDX analysis, which identified a co-growth of a dominant superconducting tetragonal phase, with the minority of a non-superconducting hexagonal phase. The ac magnetic response is analyzed using a combined method of the fundamental and the 3rd harmonic ac magnetic susceptibility as a function of the temperature at different ac magnetic field amplitudes and frequencies and with various superimposed dc fields. The variation of the ac magnetic field and frequency in different ranges especially affects the 3rd harmonic components, which are more sensitive to the changes in the flux dynamic regimes. This allows a fine observation of the evolution of the different linear and non-linear processes responsible for the ac magnetic response of the Fe1.02Se crystals. At low enough ac amplitudes and frequencies, and even in high imposed dc magnetic fields, the Fe1.02Se crystals show a typical critical state behavior, marking a high stability of the pinning, with very small influence of the vortex dynamical processes. With the change of ac field amplitude and frequency a gradual crossover is observed from the initial stable pinning state through the domination of the intermediate regimes as flux creep and finally to the complete dominance of flux flow. The ac magnetic response is also influenced by geometric edge barrier effects arising from the plate-like geometry of the Fe1.02Se crystals. The changes of the dominant irreversible (non-linear) mechanism from surface pinning to bulk pinning or to prevailing dynamical regimes is also identified by analyzing the behavior of the 3rd harmonic components.

We investigated the influence of different Ag additions (up to 10 wt %) on the superconducting properties of FeSe 0.94 . The structural investigations (XRD and SEM) indicated that Ag is present in three different forms. Ag at grain boundaries supports the excellent intergrain connections and reduces ΔT to values smaller than 1K at B=0 and ΔT ≤ 2.74 K at B=14 T. Ag insertion in the crystal lattice unit cell provides additional carriers and changes the electron hole imbalance in FeSe 0.94 . This results in an increase in the magnetoresistive effect (MR) and critical temperature (T c ). Reacted Ag forms a small amount (~1%) of Ag 2 Se impurity phase, which may increase the pinning energy in comparison with that of the undoped sample. The enhanced upper critical field (B c2 ) is also a result of the increased impurity scattering. Thus, unlike cuprates Ag addition enhances the T c , B c2 , pinning energy and MR making the properties of polycrystalline FeSe 0.94 similar to those of single crystals.