Study on lattice symmetry of sputtered Co–Cu–Pt metastable ordered thin films

The lattice symmetry and atomic stacking sequence of Cu-doped metastable L11 Co50Pt50 (Co50−xCuxPt50) films with thicknesses varied from 5 to 50 nm were examined by X-ray diffraction along the (10.1) diffraction rod. Replacement of Co with Cu significantly decreases the linewidth of the (1 1 1) superlattice diffraction peak, revealing reduced defect density. The crystal symmetry of CoPt L11 is identified as face-centered. After the addition of Cu (x ≥ 23), it evolves into rhombohedral, which is the thermodynamically equilibrium structure of CuPt. Furthermore, the decrease in film thickness results in the modification of structure in CoPt binary films, forming pseudo-hexagonal phase. As the film thickness is decreased from 10 to 5 nm, the component of pseudo-hexagonal phase increases from 7.1 vol.% to 13.6 vol.%. However, such a phase evolution disappears in the films with x = 23. The stabilization of rhombohedral L11 promotes the alignment of magnetic easy-axis, yielding a high perpendicular remanence ratio and negligible in-plane magnetic component, even in the 5 nm-thick sample. The presented results provide detailed information for the understanding of the relation between structure and magnetic properties of metastable CoPt-based L11 films.

► Lattice symmetry of metastable ordered structure of sputtered Co–Pt L11 was confirmed as fcc, different from that of the films prepared by epitaxy.
► Partial replacement of Co with Cu (Cu ≥ 23) leads to a compression along c-axis, reducing the distribution of (1 1 1) alignment.
► Reduction of film thickness leads to the formation of pseudo-hcp structure in CoPt films.
► Rhombohedral symmetry retains even in the 5 nm-thick Co–Cu–Pt film.
► The addition of Cu stabilizes the rhombohedral phase.