1. Thin layers (2D) of nanoparticles are formed by evaporating dispersions of nanoparticles on a solid substrate Three-dimensional assemblies are prepared by slowly diffusing a poorly coordinating solvent into the liquid dispersion of nanoparticles With Fe nanoparticles the 2D and 3D assemblies have different structural and magnetic behavior 2D Nanoparticle Arrays and 3D Nanoparticle Crystals

2. Simulated phase contrastTEM image Layer Stacking

3. Found for hexagonal close packed arrays of larger Fe nanoparticles Not seen with nonmagnetic particles S. Yamamuro, D. Farrell, and S. A. Majetich, Phys. Rev. B65, 224431 (2002) Preference for an Odd Number of Layers

4. Dilute solutions form hexagonal monolayers Concentrated solutions form thicker cubic or hexagonal arrays BCC structure entropically stabilized for small diameters Slower formation increases the coherence length 2D Array Structure Summary

5. Use very slow precipitation (hours, weeks, months) by diffusion of “bad” solvent Can make 3D array crystals up to 10 microns in size Particles dispersed in toluene Ethanol Propanol 3D Nanoparticle Arrays

6. For standard surfactants, edge-to-edge interparticle separation ≥ 2.5 nm  Expect magnetostatic interactions to dominate Learn about interactions from M r (H), M relax (t), M ZFC (T) Dipolar Interactions

7. Magnetization with H perpendicular harder to saturate, decays faster Interactions shape anisotropy in 2D arrays H H=0 Field Orientation M r (H)

8. Dipolar energy per pair of particles At T = 10 K Vary the Particle Size

9. Larger particles have: slightly faster approach to saturation slower decay in M(t) higher T B and broader M ZFC (T) Particle Size Effects

10. Same batch of 6.7 nm Fe particles with different surfactants Oleic Acid/Oleyl Amine Hexanoic Acid/Hexyl Amine Avg. spacing 2.5±0.3 nm 1.2±0.3 nm At T = 10 K Varying the Particle Spacing

11. Smaller spacing leads to: more gradual saturation slower decay in M(t) a slightly higher Blocking T Interparticle Spacing Effect

12. 3D arrays have: slower approach to saturation higher T B and broader M ZFC (T) faster decay in M(t) not explained by demagnetization field due to different shape 2D and 3D Arrays

13. 5 minutes; x = -0.67 2 weeks: x = -1.17 4 weeks: x = -1.89 x = -2 Ferromagnet x = -1/2 amorphous magnet (spin glass-like) Remanent magnetization 10 K Small L coh like spin glass Large L coh FM Approach to Saturation

14. Both the strength of dipolar forces and the structural coherence length L coh affect the magnetic properties of nanoparticle arrays When L coh is long, magnetic relaxation is much faster, suggesting the presence of domain walls within coherent regions Stronger dipolar interactions slow the magnetic relaxation when L coh is short, and the arrays are spin glass-like D. Farrell, Y. Ding, S. A. Majetich, C. Sanchez-Hanke, and C.-C. Kao, J. Appl. Phys. 95, 6636 (2004). D. Farrell, Y. Cheng, Y. Ding, S. Yamamuro, C. Sanchez-Hanke, C.-C. Kao,and S. A. Majetich, J. Magn. Magn. Mater. 282, 1-5 (2004). Magnetics Summary

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