Fabrication of epitaxial, magnetic multiple-barrier-structures and development of spin-resonant-tunneling -device basic technology

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Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys, and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co_2Cr_<0.6>Fe_<0.4>Al (CCFA) and Co_2MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600℃. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.25-nm rms. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier, and a Co_<50>Fe_<50> (CoFe) upper electrode. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios of 42% at RT and 74% at 55K. These results confirm the promise of epitaxial MTJs as a key device structure for utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films. We also fabricated epitaxial double-barrier-structures consisting of CCFA/MgO/Co_<50>Fe_<50>/MgO/Co_<50>Fe_<50>. The fabricated double-barrier-structures showed TMR ratios comparable with that of single-barrier structures consisting of CCFA/MgO/Co_<50>Fe_<50> and V_<half> (the bias voltage at which the TMR ratio fell to half the zero-bias value) twice as high as that of the single-barrier structures. In summary, we have developed basic fabrication technologies for epitaxial, magnetic multiple-barrier-structures.