Microstructural evolution of perpendicular magnetization films with an ultra-thin Co2FeAl/MgAl2O4(001) structure

Abstract We investigated the structure of 1-nm-thick Co2FeAl (CFA)/MgAl2O4 (MAO) epitaxial heterojunctions in order to understand the origin of perpendicular magnetic anisotropy (PMA). The CFA/MAO stacks were fabricated by plasma oxidation of Mg-Al grown on the CFA layer and subsequent post-annealing process. Before post-annealing (as-oxidized), the stack showed in-plane magnetization with poor crystallinity of the MAO layer. After post-annealing, the stack showed PMA accompanied by the improved crystallinity in the MAO layer, leading to the formation of a lattice-matched CFA/MAO(001) epitaxial interface. These behaviors indicate that the crystallization of the MAO layer is strongly promoted by atomic templating effect from a lattice-matched underlayer. In addition, we also found that this MAO crystallization effectively enhanced PMA at the interface. At the same time, significant Al atomic diffusion from CFA to MAO was confirmed. This Al diffusion was observed in the as-oxidized stack, and this was further promoted by the post-annealing. Therefore, the strong PMA at the CFA/MAO interfaces is attributed to (i) the reduced in-plane magnetocrystalline anisotropy due to the lattice-matching and (ii) the promoted hybridization between Fe and O orbitals due to the Al re-distribution near the interface. These results include important findings for constructing high performance perpendicularly-magnetized magnetic tunnel junctions for non-volatile magnetic memory applications.