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Effects of intrinsic defects and alloying with Fe on the half-metallicity of Co$_2$MnSi

The electronic structure and half-metallic gap of Co$_{2}$MnSi in the presence of crystallographic defects, partial Fe substitution for Mn, and thermal spin fluctuations are studied using the coherent potential approximation and the disordered local moment method. In the presence of 5\% Co or Mn vacancies the Fermi level shifts down to the minority-spin valence-band maximum. In contrast to NiMnSb, both types of Mn antisite defects in Co$_{2}$MnSi are strongly exchange-coupled to the host magnetization, and thermal spin fluctuations do not strongly affect the half-metallic gap. Partial substitution of Mn by Fe results in considerable changes in the Bloch spectral function near the Fermi level, which strongly deviate from the rigid-band picture. In particular, a light band with the Fe character crosses the Fermi level at about 50\% concentration. At room temperature, Fe substitution of up to 30\% slightly increases the spin polarization at the Fermi level.