Effect of nonstoichiometry on the half-metallic character of Co2MnSi 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 (alpha) dependence of the saturation magnetization per formula unit (mu(s)) of Co2Mn alpha Si beta thin films and the tunneling magnetoresistance (TMR) ratio of CMS/MgO/CMS magnetic tunnel junctions (CMS MTJs) having Co2Mn alpha Si beta 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 alpha dependence of mu(s) was well explained by density functional calculations with the coherent potential approximation based on the SSFU composition model for alpha up to a certain critical value (alpha(c)) > 1.0. The mu(s) data for Mn-deficient films deviated from the Slater-Pauling predicted data for half-metals due to Co atoms at the nominal Mn sites (Co-Mn). The theoretical spin polarizations, obtained from only the s- and p-orbital components, Pth(sp), were found to qualitatively explain the alpha dependence of the TMR ratio except for alpha > alpha(c). 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 CMSMTJs 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 Co-Mn atoms and Co atoms at the regular sites.