Abstract
The influence of off-stoichiometry on the half-metallic character of quaternary Heusler alloy thin films of Co2(Mn,Fe)Si (CMFS) was investigated by studying the composition dependence of the tunnelling magnetoresistance (TMR) ratio of fully epitaxial CMFS/MgO/CMFS magnetic tunnel junctions (CMFS MTJs) having Co2(Mnα′Feβ′)Si0.84 electrodes with various Mn and Fe compositions. It was found that with (Mn + Fe)-rich electrodes had higher TMR ratios than ones with (Mn + Fe)-deficient electrodes at 4.2 and 290 K. These results indicate that the suppression of Co antisites at nominal Mn/Fe sites is critical to obtaining half-metallic quaternary Co2(Mn,Fe)Si in a similar way as in ternary alloy Co2MnSi. CMFS MTJs with Mn-rich and lightly Fe-doped CMFS electrodes showed giant TMR ratios of 2610% at 4.2 K and 429% at 290 K. These results suggest that Co-based Heusler alloy thin films would be highly applicable to spintronic devices because of their half-metallicity and material diversity arising from not only ternary alloy but also quaternary alloy systems.
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