EN
In this paper we report our experimental and theoretical studies on the effect of Tb impurity on the magnetic properties of the Heusler half metallic ferromagnet Co_2MnSi. The analysis of the band structures of the doped alloy shows that the half-metallic properties are completely conserved if Tb substitute Mn atoms, this effect being determined through the coupling between the R(4f) spin with the Mn(3d) itinerant electron spins. We evaluate the strength of such a coupling by calculating, in an ab initio fashion, the total energy of Co_{16}Mn_7TbSi_8 compound for a parallel and antiparallel f-d coupling. The experimental magnetic moments are in good agreement with the calculated ones in case of ferrimagnetic ordering.