Upper critical field, critical current density and thermally activated flux flow in CaFFe _0.9Co _0.1As superconductor

In this paper, we report the synthesis, structure, transition temperature, upper critical field, critical current density and thermally activated flux flow in the CaFFe _0.9Co _0.1As superconductor. Superconductivity arises at 23K by Co substitution at the site of Fe atoms and the upper critical field is estimated as 102T using the WerthamerHelfandHohenberg formula. The flux-flow activation energy (U ₀/k _B) varies from 3230K and 4190K in a field of 9T and 1T, respectively. At 2K, the J _c is found to be approximately 4×10 ³Acm ² and 0.3×10 ³Acm ² in zero and 6T field, respectively. Transmission electron microscopy analysis shows an amorphous region surrounding most of the grains which is likely to be present in the form of amorphous and weak link grain boundaries in this compound. It seems that most of the current is hindered by mis-aligned grains, amorphous grain boundaries and impurities, which are invariably found between the grains. The presence of the weakly linked granules and their weakly pinned intergranular Josephson vortices are responsible for both low J _c and the Arrhenius temperature dependence of resistivity.