Effects of transition metal doping in MgB$_2$ superconductor

Effects of chemical substitution of the divalent transition metals has been systematically investigated in Mg$_{1-x}M_x$B$_2$ (x=0.03; M=Mn, Fe, Co, Ni and Zn). Substitution of magnetic ions, i.e., Mn$^{2+}$, Fe$^{2+}$, Co$^{2+}$ and Ni$^{2+}$, for the Mg$^{2+}$ ion suppresses $T_{\rm c}$: d$T_{\rm c}$/d$x$ is the largest (= - 159 K) in the Mn-doped sample. We have found Zn-substitution increases $T_{\rm c}$ ($\Delta T_{\rm c} \approx$ 0.2 K at x=0.03), perhaps due to the enhanced density of state near the Fermi level.