1. Magnetic Domains Exchange Interactions Dipolar Interactions Atom Competition between exchange and dipolar interactions leads to domain formation

2. Magnetic Nanoparticle Arrays Vary Size Vary Spacing Vary Ordering

3. Nanoparticle Synthesis • Fe synthesized using air free solution chemistry methods • Thermal decomposition of Fe(CO)5 in octyl ether • Particles coated with surfactant • Washed with ethanol and dispersed in hexane D. F. Farrell, S. A. Majetich, and J. P. Wilcoxon, J. Phys. Chem.107, 11022-11030 (2003).

4. Heterogeneously Nucleated Fe 7.0 ± 0.8 nm 9.2 ±0.7 nm Seeded with Pt (Fe:Pt ~1000:1)

5. Homogeneously Nucleated Fe 9.1 ± 0.9 nm 11.2 ± 1.0 nm 19 nm No Pt salt, larger amount of oleic acid surfactant

6. Electron Diffraction HeterogeneousHomogeneous Fe oxide rings dominate

7. Blocking Temperatures Field-cooled, Zero field-cooled Magnetization 7.0 ± 0.8 nm 11.2 ± 1.0 nm 0.01vol. % H = 200 Oe Heterogeneously Nucleated Homogeneously Nucleated 2 Blocking T’s: Fe, Fe oxide

8. Particle Concentration Found from calibrated x-ray fluorescence of solutions* Use to determine total mass of Fe ss * Dr. Jess P. Wilcoxon, Sandia National Laboratories

9. Oxide Shell Thickness • Relate ss of particles to weighted average of ss of Fe core and Ms of oxide shell ss,partmpart = ss,coremcore + ss,shellmshell

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