Using density-functional theory within the generalized gradient
approximation, we show that Si-based heterostructures with 1/4 layer
δ-doping of {\em interstitial} Mn (Mnint) are
half-metallic. For Mnint concentrations of 1/2 or 1 layer, the
states induced in the band gap of δ-doped heterostructures still display
high spin polarization, about 85% and 60%, respectively. The proposed
heterostructures are more stable than previously assumed δ-layers of
{\em substitutional} Mn. Contrary to wide-spread belief, the present study
demonstrates that {\em interstitial} Mn can be utilized to tune the magnetic
properties of Si, and thus provides a new clue for Si-based spintronics
materials.Comment: 5 pages, 4 figures, PRL accepte