Origin of large moments in Mn${}_x$Si${}_{1-x}$ at small x

Recently, the magnetic moment/Mn, $M$, in Mn${}_x$Si${}_{1-x}$ was measured to be 5.0 ${\mu }_B$/Mn, at $x$ =0.1%. To understand this observed $M$, we investigate several Mn${}_x$Si${}_{1-x}$ models of alloys using first-principles density functional methods. The only model giving $M=5.0$ was a 513-atom cell having the Mn at a substitutional site, and Si at a second-neighbor interstitial site. The observed large moment is a consequence of the weakened d-p hybridization between the Mn and one of its nearest neighbor Si atoms, resulting from the introduction of the second-neighbor interstitial Si. Our result suggests a way to tune the magnetic moments of transition metal doped semiconductors.