Fabrication of Heusler-type Co2MnAl epitaxial films by using sputtering method

This paper presents an experiment performed to determine the growth condition of an ideal Co₂MnAl epitaxial films as a bottom electrode of magnetic tunnel junctions (MTJ). A high-quality epitaxial films of Heusler-type alloys Co₂MnAl was fabricated using magnetron sputtering in ultrahigh vacuum. On a single crystal MgO(001) substrate, a 40 nm-thick Cr(001) buffer layer was deposited at room temperature before it was annealed to achieve better substrate morphology. A 30 nm-thick Co₂MnAl bottom electrode was then subsequently grown at various temperatures. From X-ray diffraction (XRD) results, a perfect [001]-preferred growth from the θ-2θ mode was confirmed and the Co₂MnAl lattice is rotated by 45° relative to the MgO lattice in the plane. Atomic force microprobe measured the substrate temperature dependence of surface roughness, Ra, of samples with and without Cr buffer layer. It was observed that Ra was improved using Cr buffer layer. The film deposited at room temperature was found to have the flattest surface. However, from magnetization measurement by SQUID, it showed that Co-Mn type disorder causing a reduction of spin polarization could occur in the Co₂MnAl film, which is unsuitable for an electrode of MTJ. The films were post-annealed at various temperatures and obtained a high spin-polarization in the fabricated Co₂MnAl film.