  Chemical doping has recently become a very important strategy to induce
superconductivity especially in complex compounds. Distinguished examples
include Ba-doped La$_2$CuO$_4$ (the first high temperature superconductor),
K-doped BaBiO$_3$, K-doped C$_{60}$ and Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O. The
most recent example is F-doped LaFeAsO, which leads to a new class of high
temperature superconductors. One notes that all the above dopants are
non-magnetic, because magnetic atoms generally break superconducting Cooper
pairs. In addition, the doping site was out of the (super)conducting structural
unit (layer or framework). Here we report that superconductivity was realized
by doping magnetic element cobalt into the (super)conducting-active
Fe$_2$As$_2$ layers in LaFe$_{1-x}$Co$_{x}$AsO. At surprisingly small Co-doping
level of $x$=0.025, the antiferromagnetic spin-density-wave transition in the
parent compound is completely suppressed, and superconductivity with $T_c\sim $
10 K emerges. With increasing Co content, $T_c$ shows a maximum of 13 K at
$x\sim 0.075$, and then drops to below 2 K at $x$=0.15. This result suggests
essential differences between previous cuprate superconductor and the present
iron-based arsenide one.
