A magnetic tunnel junction with a perpendicular magnetic easy-axis (p-MTJ) is a key device for spintronic non-volatile magnetoresistive random access memory (MRAM). Co-Fe-B alloy-based p-MTJs are being developed, although they have a large magnetisation and medium perpendicular magnetic anisotropy (PMA), which make it difficult to apply them to a future dense MRAM. Here, we demonstrate a p-MTJ with an epitaxially strained MnGa nanolayer grown on a unique CoGa buffer material, which exhibits a large PMA of more than 5 Merg/cm3 and magnetisation below 500 emu/cm3; these properties are sufficient for application to advanced MRAM. Although the experimental tunnel magnetoresistance (TMR) ratio is still low, first principles calculations confirm that the strain-induced crystal lattice distortion modifies the band dispersion along the tetragonal c-axis into the fully spin-polarised state; thus, a huge TMR effect can be generated in this p-MTJ.

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Tetragonal ultrathin (1–5 nm) ordered MnGa films on a CsCl-type CoGa buffer layer were fabricated by a sputtering method. The (001)-CoGa layer was first deposited on a Cr-buffered MgO substrate and then annealed in-situ at 500 °C. The ultrathin MnGa film deposited on the CoGa buffer layer formed the L10 structure with very small roughness even when grown at room temperature. In addition, the films showed well-squared perpendicular magnetization hysteresis curves even when the film thickness was as little as 1 nm. The obtained results are important for the development of the MnGa-based spin-transfer torque devices for Gbit class magnetic random access memory and high frequency applications.