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<sub>0.5</sub> Se<sub>0.5</sub> superconductor with the nominal composition range Fe <sub>1 - x</sub>M <sub>x</sub>Te <sub>0.5</sub>Se <sub>0.5</sub> ( 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 `<i>a</i> ', `<i>c</i> ' and volume decrease with increase in Co and Ni content, although not monotonically. In fact the ` <i>a</i>' lattice parameter is nearly unaffected for Co doped samples. The superconducting transition temperature (<i>Tc</i>) is measured from both DC and AC magnetic susceptibility, which decrease with increase in Ni or Co content. Both Co and Ni suppress <i>Tc</i> 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<sub>0.5</sub>Se<sub>0.5</sub> superconductor. It is concluded that in-plane disorder on superconducting Fe-Se and Fe-Se-Se planes directly affects superconductivity in FeTe <sub>0.5</sub>Se <sub>0.5</sub> superconductor.