Heusler alloys, such as ${\mathrm{Co}}_2\mathrm{MnSi}$ and ${\mathrm{Co}}_2\mathrm{MnGe},$ have been predicted from first–principles to be half metallic and potential candidates for spintronic applications. However, spin polarizations of only 50–60 % were experimentally obtained for these compounds—a decrease attributed to defects in the Mn and Co sublattices. Accurate ab initio full-potential linearized augmented plane wave calculations are performed in order to determine the effects of several types of defects (such as antisites and atomic swaps) on the electronic and magnetic properties of the bulk Heusler compounds. Our findings, in general agreement with experiments, show that Mn antisites have the lowest formation energy and retain the half-metallic character. On the other hand, Co antisites have a slightly higher formation energy and a dramatic effect on the electronic properties: the defect states that locally destroy half metallicity are energetically localized and are screened out in a couple of atomic shells. In this case, the spin polarization at the Fermi level is strongly reduced, and the spin polarization due to the s electrons, responsible for the tunneling current, is in excellent agreement with experiment. Finally, both Mn-Si and Mn-Co atomic swaps have very high formation energies, keeping however the half-metallic character.

• Received 23 October 2003

DOI:https://doi.org/10.1103/PhysRevB.69.094423