Comparative study of pure and Co doped-BaFe2As2

We present a comparative study of the high critical temperature superconductor $Co$ doped-$BaFe_{2}As_{2}$ at the electron correlation level by the embedded cluster method; the electron correlation is calculated through the second order M{\o}ller Plesset perturbation theory. We study successively the pure compound, the $Co$ doped-compound in the antiferromagnetic state and in the non-magnetic state. The $Co$ doping introduces a strong modification of the spin distribution in its neighboring atoms. The analysis of the orbital population reveals that the spin density of the $Co$ impurity becomes 3 times greater than that of the central $Fe$ of the pure compound, increase attributed to a corresponding increase of the $d_{z^{2}}$ orbital population; a local antiferromagnetic order along the {\bf b} axis of the crystal structure appears. The formation mechanism of the local magnetic moments implies a spin transfer from the (n.n.) and (n.n.n) atoms to the central $Co$ and is relevant to the $J_{1}-J_{2}$ Heisenberg model. The orbital population analysis reveals also that, in the doped compound and in both magnetic cases, the electron charge is associated to a singlet state and may correspond to a holon.