Growth and annealing effect of Co-doped BaFe2As2 single crystals

A series of Co-doped BaFe2−xCoxAs2 (0≤x≤0.32) single crystals have been grown by self-flux method. Study of X-ray diffraction indicates that the lattice parameter c of the crystals decrease linearly with the increase in Co-content added to the initial mixtures. The segregation coefficient of Co related to Fe is determined to be ∼0.78. The values of superconducting transition temperature Tc are found to increase to about 1–3 K by post growth annealing at high temperature, which should result from the improved crystallinity due to the release of residual strain in the samples. In addition, we demonstrate that the annealing atmosphere of Ar, air or vacuum has an identical effect on the enhancement of Tc. Due to inhomogeneous Co distribution, a broad transition temperature width ΔTc is observed and discussed for the over-doped samples.