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Epitaxial magnetic tunnel junctions (MTJ) using a Co-based full-Heusler alloy along with a MgO tunnel barrier were fabricated successfully using rf magnetron sputtering and electron gun deposition. The fabricated epitaxial MTJ layer structure was as follows: MgO buffer layer/Co2Cr0.6Fe0.4Al lower electrode/MgO tunnel barrier/Co50Fe50 upper electrode, grown on a MgO(001) single-crystal substrate. High tunnel magnetoresistance (TMR) ratios of 42% at room temperature and 74% at 55 K were measured for the MTJs. These results confirm the promise of epitaxial MTJs using Co-based Heusler alloys as a key device structure.