In this paper, we perform Fe-site substitution experiments onK 0.8 Fe 2？y Se 2 superconductor by choosing Cr, Mn, Co, and Zn as dopants. It is found that the doping of Cr, Co, and Zn leads to a drastic depression of superconductivity, while the introduction of Mn does not decrease the superconducting transition temperature (Tc) when x≤0.067. Electron-spin-resonance measurements reveal the existence of strong magnetic fluctuation in both the parent compound and the doped samples. In the Cr-, Co-, and Zn-doped samples, the magneticpair-breaking effect severely affects the superconductivity, resulting in drastic depression of superconductivity.On the other hand, in the Mn-doped case, it does not break the cooper pairs and thus hardly affects the Tc value.
