  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.
