A doping dependent study of interplay between magnetic and superconducting properties in BaFe_2-xCo_xAs₂ single crystals

We show strong interplay between magnetic and superconducting order in three BaFe_2-xCo_xAs₂ single crystals with different x. Our study reveals the presence of magnetic fluctuations with superconducting order in our samples and the strength of the magnetic fluctuations as well as the pinning properties are found to be the strongest for the optimally doped sample and weakest for the overdoped sample. Using local magnetization measurements, we show that application of an external magnetic field in our samples suppresses the magnetic fluctuations and enhances the diamagnetic response. Further, we show presence of unusual superconducting fluctuations above T _c in our samples which we find strongly depends on the strength of the magnetic fluctuations. We believe that our data suggest the possible role of magnetic fluctuations in mediating superconducting fluctuations above T_c in our samples. (paper)