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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2014 Jan 30|