X-ray Studies of Magnetic Nanoparticle Assemblies

S. Anders,, M. F. Toney, T. Thomson, J.-U. Thiele, and B. D. Terris IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120 Shouheng Sun, and C. B. Murray IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 a) Electronic mail: [email protected] 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 Xray 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 at SLAC-PUB-9994 June 2003