  We study the two-impurity Anderson model for a semiconductor host using the
quantum Monte Carlo technique. We find that the impurity spins exhibit
ferromagnetic correlations with a range which can be much more enhanced than in
a half-filled metallic band. In particular, the range is longest when the Fermi
level is located above the top of the valence band and decreases as the
impurity bound state becomes occupied. Comparisons with the photoemission and
optical absorption experiments suggest that this model captures the basic
electronic structure of Ga_{1-x}Mn_xAs, the prototypical dilute magnetic
semiconductor (DMS). These numerical results might also be useful for
synthesizing DMS or dilute-oxide ferromagnets with higher Curie temperatures.
