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Recently half-metallic full-Heusler alloy films have attracted significant interests for spintronics devices. As these alloys have been known to have a high spin polarization, very large TMR ratio is expected in magnetic tunnel junctions. Among these alloys, Co2MnSi full-Heusler alloy with a high spin polarization and a high Curie temperature is considered a good candidate as an electrode material for spintronic devices. In this study, the magnetic and structural properties of Co2MnSi Heusler alloy films were investigated. TMR characteristics of magnetic tunnel junctions with a Co2MnSi/SiO2/CoFe structure were studied. A maximum MR ratio of 39% with SiO2 substrates and 27% with MgO(100) substrates were obtained. The lower MR ratio than expectation is considered due to off-stoichiometry and atomic disorder of Co2MnSi electrode together with oxidation of the electrode layer.