We investigated the effect of nonstoichiometry on the half-metallic character of the Heusler alloy Co${}_2$MnSi (CMS) through the Mn composition (α) dependence of the saturation magnetization per formula unit (${\mu }_s$) of Co${}_2$Mn${}_{\alpha }$Si${}_{\beta }$ thin films and the tunneling magnetoresistance (TMR) ratio of CMS/MgO/CMS magnetic tunnel junctions (CMS MTJs) having Co${}_2$Mn${}_{\alpha }$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 α dependence of ${\mu }_s$ 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 (${\alpha }_c$)>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${}_{\mathrm{Mn}}$). The theoretical spin polarizations, obtained from only the $s$- and $p$-orbital components, $P_{\mathrm{th}}$($sp$), were found to qualitatively explain the α dependence of the TMR ratio except for α > ${\alpha }_c$. This is in contrast to the theoretical spin polarizations obtained from the $s$-, $p$-, and $d$-orbital components, $P_{\mathrm{th}}$(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 Co${}_{\mathrm{Mn}}$ atoms and Co atoms at the regular sites.

• Received 9 July 2013
• Revised 29 December 2013

DOI:https://doi.org/10.1103/PhysRevB.89.014428