Sinter-free phase conversion and scanning transmission electron microscopy of FePt nanoparticle monolayers

Abstract

Thermally robust monolayers of 4–6 nm diameter FePt nanoparticles (NPs) were fabricated by combining chemical synthesis and atomic layer deposition. Spin-cast monolayers of FePt NPs were coated with thin, 11 nm-thick layers of amorphous Al₂O₃, followed by annealing to convert the FePt NPs from an alloy (A1) into intermetallic FePt (L1₀) and FePt₃ (L1₂) phases. The Al₂O₃ layer serves as a barrier that prevents sintering between NPs during annealing at temperatures up to 730 °C. Electron and X-ray diffraction in conjunction with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show that as-synthesized A1 FePt NPs convert into L1₀ and L1₂ phase NPs through annealing. HAADF-STEM measurements of individual NPs reveal imperfect ordering and show that the NP composition determines which intermetallic phase is obtained. Mixed-phase NPs with L1₀ cores and FePt₃ L1₂ shells were also observed, as well as a smaller number of unconverted A1 NPs. These results highlight the need for improved control over the compositional uniformity of FePt NPs for their use in bit-patterned magnetic recording.