Effect of nonstoichiometry on the half-metallic character of Co₂ MnSi investigated through saturation magnetization and tunneling magnetoresistance ratio

We investigated the effect of nonstoichiometry on the half-metallic character of the Heusler alloy Co₂MnSi (CMS) through the Mn composition (α) dependence of the saturation magnetization per formula unit (μs) of Co₂MnαSiβ thin films and the tunneling magnetoresistance (TMR) ratio of CMS/MgO/CMS magnetic tunnel junctions (CMS MTJs) having Co₂MnαSiβ electrodes. As a basis for understanding the effect of nonstoichiometry in CMS, we developed a generalized form of the site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defects, not vacancies, to accommodate nonstoichiometry. The α dependence of μs was well explained by density functional calculations with the coherent potential approximation based on the SSFU composition model for α up to a certain critical value (αc)>1.0. The μs data for Mn-deficient films deviated from the Slater-Pauling predicted data for half-metals due to Co atoms at the nominal Mn sites (CoMn). The theoretical spin polarizations, obtained from only the s- and p-orbital components, Pth(sp), were found to qualitatively explain the α dependence of the TMR ratio except for α > αc. This is in contrast to the theoretical spin polarizations obtained from the s-, p-, and d-orbital components, Pth(spd). A decrease in the TMR ratio observed for CMS MTJs having Mn-deficient electrodes was ascribed to small s- and p-orbital components of the local density of minority-spin in-gap states at the Fermi level that appeared for both antisite CoMn atoms and Co atoms at the regular sites.