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2D Nanoparticle Arrays and 3D Nanoparticle Crystals

2D Nanoparticle Arrays and 3D Nanoparticle Crystals. Thin layers (2D) of nanoparticles are formed by evaporating dispersions of nanoparticles on a solid substrate

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2D Nanoparticle Arrays and 3D Nanoparticle Crystals

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  1. 2D Nanoparticle Arrays and 3D Nanoparticle Crystals 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

  2. Layer Stacking TEM image Simulated phase contrast

  3. Preference for an Odd Number of Layers 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)

  4. 2D Array Structure Summary 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

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

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

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

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

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

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

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

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

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

  14. Magnetics Summary • Both the strength of dipolar forces and the structural coherence length Lcoh affect the magnetic properties of nanoparticle arrays • Stronger dipolar interactions slow the magnetic relaxation when Lcoh is short, and the arrays are spin glass-like • When Lcoh is long, magnetic relaxation is much faster, suggesting the presence of domain walls within coherent regions 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).

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