  Doping of the band insulator FeS$_2$ with Co on the Fe site introduces a
small density of itinerant carriers and magnetic moments. The lattice constant,
AC and DC magnetic susceptibility, magnetization, and specific heat have been
measured over the $0\le x\le 0.085$ range of Co concentration. The variation of
the AC susceptibility with hydrostatic pressure has also been measured in a
small number of our samples. All of these quantities show systematic variation
with $x$ including a paramagnetic to disordered ferromagnetic transition at
$x=0.007\pm 0.002$. A detailed analysis of the changes with temperature and
magnetic field reveal small power law dependencies at low temperatures for
samples near the critical concentration for magnetism, and just above the Curie
temperature at higher $x$. In addition, the magnetic susceptibility and
specific heat are non-analytic around H=0 displaying an extraordinarily sharp
field dependence in this same temperature range. We interpret this behavior as
due to the formation of Griffiths phases that result from the quenched disorder
inherent in a doped semiconductor.

Strongly Correlated Electrons;Materials Science