  Alloying effects by T=Mn,Co,Ni-substitution on FeS$_2$ have been investigated
using density-functional calculations. The ferromagnetic alloys
Fe$_{1-x}$T$_x$S$_2$ have been investigated for concentrations
$x=\frac{1}{4},\frac{1}{2},\frac{3}{4}$ together with the ground states of the
pure compounds. The electronic structure is discussed with the goal to identify
candidates for half metals, which are of interest for spintronics applications.
We find interesting candidates at high concentration of the Mn-doped FeS$_2$
and at low concentrations for Ni-doped materials. For the Mn alloys we also
note the proximity to a low-spin to high spin transition. For Co-doped
materials we reproduce the well known finding of half metallicity over the
entire concentration range.
