  Fe3Si is a ferromagnetic material with possible applications in magnetic
tunnel junctions. When doped with Mn, the material shows a complex magnetic
behavior, as suggested by older experiments. We employed the
Korringa-Kohn-Rostoker (KKR) Green function method within density-functional
theory (DFT) in order to study the alloy Fe(3-x)Mn(x)Si, with 0 < x < 1.
Chemical disorder is described within the coherent potential approximation
(CPA). In agreement with experiment, we find that the Mn atoms align
ferromagnetically to the Fe atoms, and that the magnetization and Curie
temperature drop with increasing Mn-concentration $x$. The calculated spin
polarization P at the Fermi level varies strongly with x, from P=-0.3 at x=0
(ordered Fe3Si) through P=0 at x=0.28, to P=+1 for x>0.75; i.e., at high Mn
concentrations the system is half-metallic. We discuss the origin of the trends
of magnetic moments, exchange interactions, Curie temperature and the spin
polarization.
