Synthesis, Characterization and Hard Ferromagnetism in FePt/ZnO Nanocomposites with Ultra-Small Size

Multi-component hybrid nanostructures containing two nanoscaled components of FePt and ZnO were successfully fabricated through seed mediated growth. The preformed FePt nanoparticles, which were fabricated either by the reduction of Pt(acac)₂ and the decomposition of Fe(CO)₅ or by simultaneous chemical reduction of Pt(acac)₂ and Fe(acac)₃ by 1,2-hexadecanediol at high temperature, work as the hetero-nucleation seeds for the preparation of hybrid nanostructures. The end products can be either FePt@ZnO core/shell nanoparticle assembly or FePt/ZnO nanocomposites, depending on the seeding particle size. If the seeding particle size is larger than 3.5 nm, core/shell nanoparticle assembly was formed, while if the seeding particle is smaller than 2 nm, FePt/ZnO nanocomposites were formed. For the FePt@ZnO core/shell, HRTEM showed a quasi-epitaxial growth between the FePt core and the ZnO shell. The ZnO shell was highly deformed. The core/shell nanoparticle assembly exhibits both semiconducting and magnetic properties which is superparamagnetic at room temperature. For the nanocomposites, the as-synthesized ultra-small 1.9 nm FePt₃ nanoparticles are superparamagnetic. After embedding into the ZnO matrix, those superparamagnetic nanoparticles become magnetically hard with coercivity field of 650 Oe at room temperature. First-principles calculations indicate a giant interfacial anisotropic energy, induced by the strong spin-orbit interaction between Pt and O at interface, leading to room-temperature permanent ferromagnetism. The findings shed light on research for new material designs with giant interfacial anisotropy for various applications.