Paper NM+PS+AS-WeA12
Size Distributions and Agglomeration Effects in FePtAu Nanoparticles

Wednesday, October 22, 2008, 5:20 pm, Room 309

We have investigated the effect of thermal treatment on the microstructure and positional ordering of chemically synthesized (Fe₄₉Pt₅₁)₈₈Au₁₂ nanoparticles using small angle neutron scattering (SANS). The as made FePtAu particles have an average diameter of 3 nm. Thin films of nanoparticles were prepared by spincoating the dispersion of FePtAu nanoparticle on Si wafers. The samples were annealed for 30 minutes in a flowing nitrogen atmosphere in order to promote FCC-L1₀ phase transformation. The annealing temperature of the samples was varied from 300 °C to 550 °C in steps of 50 °C. The crystal structure and the lattice constant of the samples have been determined by x-ray diffraction. The magnetization hysteresis has been measured in a vibrating sample magnetometer. SANS measurements were performed at room temperature with an incident neutron wavelength of 6 Å using the 30 m NG3 SANS instrument at NIST. The scattering vector magnitude (q) dependence of the SANS intensity, I(q) shows strong dependence on the sample annealing temperature. The q dependence of the SANS intensity in the range of 0.2 to 3 nm^-1 could be fitted by modeling the nanoparticles as polydisperse spheres with Schultz distribution for the diameters and a hard sphere interaction between the particles. In this q range, the agglomerates are assumed to be polydisperse spheres. The results indicate that significant agglomeration occurs in all the samples. The average size of the agglomerates is found to increase from 18 nm at 300 °C to 53 nm at 550 °C. The data also seem to indicate the sintering of particles in the temperature range of 500-550 °C. These results will be compared with the particle size distributions in off-situ annealed Fe₄₉Pt₅₁ nanoparticles and in-situ annealed (Fe₄₉Pt₅₁)₈₈Au₁₂ nanoparticles.