  We report the effect of 3d metal Cobalt (Co) and Nickel (Ni) doping on the
FeTe0.5Se0.5 superconductor with the nominal composition range
Fe1-xMxTe0.5Se0.5 (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 P4nmm. The
lattice parameters 'a', 'c' and volume decrease with increase in Co and Ni
content, although not monotonically. In fact the 'a' lattice parameter of Co
doped samples 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 FeTe0.5Se0.5 superconductor. It is
concluded that in-plane disorder on superconducting Fe-Se and Fe-Se-Se planes
directly affects superconductivity in FeTe0.5Se0.5 superconductor.
