Effect of 3d Metal (Co and Ni) Doping on the Superconductivity of FeSe $_{0.5}$ Te $_{0.5}$

We report the effect of 3d metal Cobalt (Co) and Nickel (Ni) doping on the FeTe_0.5 Se_0.5 superconductor with the nominal composition range Fe _{1 - x}M _xTe _0.5Se _0.5 ( M = Co, Ni and x = 0.00, 0.01 0.02, 0.05 , and 0.10). Samples are synthesized through standard solid state reaction route and all are crystallize in single phase tetragonal structure with space group P4/nmm. The lattice parameters `a ´, `c ´ and volume decrease with increase in Co and Ni content, although not monotonically. In fact the ` a´ lattice parameter is nearly unaffected for Co doped samples. The superconducting transition temperature (Tc) is measured from both DC and AC magnetic susceptibility, which decrease with increase in Ni or Co content. Both Co and Ni suppress Tc and drive the system into the normal state. Interestingly, Ni suppresses the superconductivity much faster than the Co. This indicates less effect on in-plane Fe-Se distances and thus reduced disorder in case of Co when compared with Ni substation at Fe site in FeTe_0.5Se_0.5 superconductor. It is concluded that in-plane disorder on superconducting Fe-Se and Fe-Se-Se planes directly affects superconductivity in FeTe _0.5Se _0.5 superconductor.