Abstract. We have fabricated Fe_1+xTe_0.6Se_0.4 high quality bulk single crystals with different composition of x = 0, 0.005, 0.01, -0.005, and -0.01 by the melting method with two stage heat treatment. The crystal structure and superconducting properties were studied. Temperature dependence of magnetization showed that low-T_c region exists inside the crystals with x = 0, 0.005, -0.005. Critical temperature T_cwas the highest value of 14.0 K for the crystal with x = -0,005. Critical current densities J_cunder the magnetic fields parallel to the c-axis at 4.2 K achieved 0.98 and 0.44 × 10⁵ A / cm² at 0 T and 5 T respectively for the crystal with x = -0.005. The crystal lattice parameters were examined by Rietveld refinement. The anion height from Fe layer was calculated using the obtained parameters. We investigated the anion height dependence of T_cand J_c, and mentioned an important role of excess Fe about the magnetic flux pinning.

Keywords: Single crystal, Fe1+xTe0.6Se0.4, Anion height, Flux pinning