X-ray studies of magnetic nanoparticles assemblies

Monodisperse FePt nanoparticles were prepared using high temperature solution phase synthesis. Polymer-mediated layer-by-layer growth leads to precise control of the particle self-assembly. The narrow particle size distribution (σ ≤ 5%) offers the potential for increased data storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. We have studied self-assembled multilayers of magnetic FePt nanoparticles. The L10 phase of FePt has a very high magnetic anisotropy which allows the magnetization of particles of about 4 nm diameter to be thermally stable at room temperature. Magnetic measurements using vibrating sample magnetometer were combined with X-ray diffraction (XRD) and Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy to study the annealed FePt nanoparticle assemblies and to optimize annealing conditions. NEXAFS spectra showed that a fraction of the iron in the as-deposited particles was oxidized, and this fraction was reduced by annealing in inert or reducing atmospheres. A very thin layer (<0.4 nm) of oxide surrounding the particle is sufficient to explain the observed spectra. Structural analysis using XRD showed that a minimum temperature of 450°C was required to start the formation of the ordered ferromagnetic phase. Annealing for longer times and a