Reduction of ordering temperature of self-assembled FePt nanoparticles by addition of Au and Ag

The [FePt]94Au6 and [FePt]90Ag10 nanoparticle arrays were synthesized on Si substrates by a reverse micellar method, combined with plasma treatment and in-situ deposition of a SiO2 overlayer, and the post annealing step was performed to drive the face-centered cubic to tetragonal phase transition. These FePt nanoparticles exhibit a quasi-hexagonal order with tailored inter-particle spacing and particle size. The effects of the Ag and Au on the structural and magnetic properties of FePt were investigated. The results indicate that both Au and Ag additives can remarkably enhance the coercivity and reduce the ordering temperature, however, the optimum composition is different for them. The optimum composition is determined to be [FePt]94Au6 and [FePt]90Ag10, respectively, for which the ordering temperature of FePt nanoparticles is reduced by -100 degrees C. After 600 degrees C annealing, the [FePt]94Au6 and [FePt]90Ag10 nanoparticles are totally ferromagnetic with apparent larger coercivities of -7.0 kOe, which is about 3.8 kOe larger than that of the pure FePt nanoparticles. The mechanism of the chemical ordering acceleration may be attributed to the defects and strains caused by the Au/Ag additives.