Synthesis, self-assembly, and magnetic properties of [FePt]1-xAux nanoparticles

[FePt]_1-xAu_x nanoparticles were prepared by the simultaneous polyol reduction of platinum acetylacetonate and gold acetate and the thermal decomposition of iron pentacarbonyl, giving 3.5-nm-diameter FePt particles with gold atoms substituted in the lattices. The addition of gold promoted the face-centered cubic to tetragonal phase transition, thereby reducing the temperature required for this transition by more than 150°C compared with FePt nanoparticles with no additives. This effect is even more significant than adding silver to FePt nanoparticles. For a given annealing temperature, the coercivity increases with the content of gold up to 24%, above which the coercivity starts to decrease. The mechanism for the chemical ordering acceleration may relate to the defect and strains introduced by gold atoms. Upon annealing, gold atoms leave the FePt lattice, leaving lattice vacancies that increase the mobility of the FePt atoms to rearrangement. Dynamic coercivity measurements yield thermal stability factors that are slightly higher than would be expected for noninteracting particles.