Prevention of coalescence during annealing of FePt nanoparticles assembled by convective coating.

FePt nanoparticle suspension was synthesized by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine. The composition of the synthesized nanoparticles was Fe40Pt60. To prevent the coalescence during annealing of FePt nanoparticles we tried two steps of convective coating, where first coating was for silica particle assembly on a silicon substrate and second one was for FePt nanoparticles on the silica layers. It was observed by scanning electron microscopy that FePt nanoparticles were dispersed on the silica surface. After being annealed at 700 degrees C for 30 minutes under nitrogen atmosphere, the particle size of FePt nanoparticles increased slightly from 4 nm to 6 nm but dispersity of the nanoparticles was maintained. Magnetic hysteresis of Fe40Pt60 nanoparticles coating on silica layer after annealing showed typical characteristics of hard magnetic materials, and no incorporation of soft magnetic materials. It was suggested that convective self-assembly with the parallel use of pre-coating that offers solvent flux weakening capillary force between FePt nanoparticles was an effective method to prevent coalescence of nano-sized particles under high temperature annealing.