Density functional theory is used to investigate how atomic substitutions modify the magnetization and anisotropy of L1_0-ordered ferromagnets. Our VASP supercell calculations focus on two classes of materials: Mn-Al-(Fe) and Fe-Co-Pt. We find that the Mn and Al moments in pure MnAl are 2.420 μ_B and -0.61 μ_B per atom, respectively. The calculated zero temperature anisotropy is 1.77 MJ/m~3. Replacing 50% of Mn by Fe enhances the anisotropy from 1.77 MJ/m~3 to 2.50 MJ/m~3 but reduces the magnetization. We have also calculated the magnetic moments of L1_0-ordered Fe_(1-x)Co_xPt with various degrees of Fe-Co disorder. Configurational supercell averaging shows that the net moment decreases systematically with Co concentration, but the individual Fe and Co moments depend on the number of Fe-Co bonds.
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