Enhanced half-metallicity of off-stoichiometric quaternary Heusler alloy Co2(Mn, Fe)Si investigated through saturation magnetization and tunneling magnetoresistance

We investigated the factors that critically affect the half-metallicity of the quaternary Heusler alloy Co2(Mn, Fe)Si (CMFS) by examining the film composition dependence of the saturation magnetization per formula unit, mu(s), of CMFS thin films and the tunneling magnetoresistance (TMR) ratio of CMFS/MgO/CMFS magnetic tunnel junctions (MTJs). We also investigated the origin of the giant TMR ratio of up to 2610% at 4.2 K (429% at 290 K) obtained for CMFS MTJs with Mn-rich, lightly Fe-doped CMFS electrodes. Co antisites at the nominal Mn/Fe sites (Co-Mn/Fe antisites) can consistently explain the mu(s) for (Mn + Fe)-deficient CMFS thin films being lower than the half-metallic Z(t) - 24 value and the TMR ratio for MTJs with (Mn + Fe)-deficient CMFS electrodes being lower than that for MTJs with (Mn+ Fe)-rich CMFS electrodes. It was revealed that the Co-Mn/Fe antisite is detrimental to the half-metallicity of the CMFS quaternary alloy, as it is in the Co2MnSi (CMS) ternary alloy. It was also shown that (Mn + Fe)-rich compositions are critical to suppressing these harmful antisites and to retaining the half-metallic electronic state. In addition, a relatively small Fe ratio, rather than a large one, in the total (Mn + Fe) composition led to a more complete half-metallic electronic state. Half-metallicity was more strongly enhanced by increasing the Mn composition in Mn-rich, lightly Fe-doped CMFS than in Mn-rich CMS. This phenomenon is the cause of the giant TMR ratio recently reported for CMFS MTJs. Our findings indicate that the approach to controlling off-stoichiometry and film composition is promising for fully utilizing the half-metallicity of quaternary CMFS thin films as spin source materials.