Nanostructure-induced L10-ordering of twinned single-crystals in CoPt ferromagnetic nanowires

Abstract

L1₀-ordered ferromagnetic nanowires with large coercivity are essential for realizing next-generation spintronic devices. Ferromagnetic nanowires have been commonly fabricated by first L1₀-ordering of initially disordered ferromagnetic films by annealing and then etching them into nanowire structures using lithography. If the L1₀-ordered nanowires can be fabricated using only lithography and subsequent annealing, the etching process can be omitted, which leads to an improvement in the fabrication process for spintronic devices. However, when nanowires are subjected to annealing, they easily transform into droplets, which is well-known as Plateau–Rayleigh instability. Here, we propose a concept of “nanostructure-induced L1₀-ordering” of twinned single-crystals in CoPt ferromagnetic nanowires with a 30 nm scale ultrafine linewidth on Si/SiO₂ substrates. The driving forces for nanostructure-induced L1₀-ordering during annealing are atomic surface diffusion and extremely large internal stress at ultrasmall 10 nm scale curvature radii of the nanowires. (Co/Pt)₆ multilayer nanowires are fabricated by a lift-off process combining electron-beam lithography and electron-beam evaporation, followed by annealing. Cross-sectional scanning transmission electron microscope images and nano-beam electron diffraction patterns clearly indicate nanostructure-induced L1₀-ordering of twinned single-crystals in the CoPt ferromagnetic nanowires, which exhibit a large coercivity of 10 kOe for perpendicular, longitudinal, and transversal directions of the nanowires. Two-dimensional grazing incidence X-ray diffraction shows superlattice peaks with Debye–Scherrer ring shapes, which also supports the nanostructure-induced L1₀-ordering. The fabrication method for nanostructure-induced L1₀-ordered CoPt ferromagnetic nanowires with twinned single-crystals on Si/SiO₂ substrates would be significant for future silicon-technology-compatible spintronic applications.