  We consider ferromagnetism in spatially randomly located magnetic moments, as
in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect
exchange RKKY interaction. We obtain via Monte Carlo the magnetic phase diagram
as a function of the impurity moment density $n_{i}$ and the relative carrier
concentration $n_{c}/n_{i}$. As evidenced by the diverging correlation length
and magnetic susceptibility, the boundary between ferromagnetic (FM) and
non-ferromagnetic (NF) phases constitutes a line of zero temperature critical
points which can be viewed as a magnetic percolation transition. In the dilute
limit, we find that bulk ferromagnetism vanishes for $n_{c}/n_{i}>.1$. We also
incorporate the local antiferromagnetic direct superexchange interaction
between nearest neighbor impurities, and examine the impact of a damping factor
in the RKKY range function.
