thin layers (2d) of nanoparticles are formed by evaporating dispersions of nanoparticles on a solid...
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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
Simulated phase contrastTEM image
Layer Stacking
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
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
Evaporating droplet
2D Array Structure Summary
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
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)
Dipolar Interactions
-60
-40
-20
0
20
40
60
σr
(emu/g)
-4000 -2000 0 2000 4000
H (Oe)
T = 10 K
6.7 nm Fe cores, OA/OY
Arrays, H parallel
Arrays, H perpendicular
Magnetization with H perpendicular harder to saturate, decays faster
Interactions shape anisotropy in 2D arrays
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
0.84
Normalized M
6543210
ln(t/t0
)
H = 0 Oe
T = 10 K
6.7 nm Fe cores
2.5 nm separation
H parallel to substrate
H perpendicular
H
H=0
Field Orientation Mr(H)
€
Φmag =μ0
4πr3
r μ •
r μ [ ]
€
Φmag =1.4kT
€
Φmag=2.4kT
Dipolar energy
per pair of particles
At T = 10 K
Vary the Particle Size
1.0
0.8
0.6
0.4
0.2
Normalized M
zfc
30025020015010050
T (K)
H= 100 Oe
2.5 nm spacing
6.7 nm Fe cores
8.5 nm cores
-1.0
-0.5
0.0
0.5
1.0
Normalized M
r
-4000 -2000 0 2000 4000
H (Oe)
T = 10 K
OA/OY
6.7 nm Fe cores
8.5 nm Fe cores
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
0.84
Normalized M
6543210
ln(t/t0
)
H = 0
T = 10 K
OA/OY
6.7 nm Fe cores
8.5 nm Fe cores
Larger particles have:
• slightly faster approach to saturation
• slower decay in M(t)
• higher TB and broader M ZFC(T)
Particle Size Effects
Same batch of 6.7 nm Fe particles with different surfactants
Oleic Acid/Oleyl Amine Hexanoic Acid/Hexyl AmineAvg. spacing 2.5±0.3 nm 1.2±0.3 nm
€
Φmag=2.8kT
€
Φmag=1.4kTAt T = 10 K
Varying the Particle Spacing
1.0
0.8
0.6
0.4
0.2
Normalized M
zfc
30025020015010050
T (K)
H = 100 Oe
6.7 nm Fe cores
OA/OY (2.5 nm spacing)
HA/HY (1.2 nm spacing)
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
0.84
Normalized M
543210
ln (t/t0
)
H = 0
T = 10 K
6.7 nm Fe
OA/OY (2.5 nm spacing)
HA/HY (1.2 nm spacing)
-1.0
-0.5
0.0
0.5
1.0
Mr
/Ms
-20000 -10000 0 10000 20000
H (Oe)
Surfactants Spacing
Oleic Acid/Oleyl Amine 2.5 nm
Heanoic Acid/Hexyl Amine 1.2 nm
Smaller spacing leads to:
• more gradual saturation
• slower decay in M(t)
• a slightly higher Blocking T
Interparticle Spacing Effect
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
-1.0
-0.5
0.0
0.5
1.0
M/Ms
-40x103
-20 0 20 40H (Oe)
T = 10 K 3D arrays, 8.5 nm Fe 2D arrays, 8.2 nm Fe
1.00
0.98
0.96
0.94
0.92
0.90
M/M(t
0)
6543210ln (t/t0)
H = 0T = 10 K
2D arrays, 8.2 nm Fe 3D arrays, 8.5 nm Fe
1.4
1.2
1.0
0.8
0.6
0.4
0.2
M/M(T
B)
30025020015010050T (K)
solvent melting
H = 200 Oe 3D arrays, 8.5 nm Fe 2D arrays, 8.2 nm Fe
2D and 3D Arrays
-1.0
-0.5
0.0
0.5
1.0
M/M (50 kOe)
-30 -20 -10 0 10 20 30
H (kOe)
5 minutes; x = -0.672 weeks: x = -1.174 weeks: x = -1.89
€
ΔM(H)∝H x
x = -2 Ferromagnet
x = -1/2 amorphous magnet (spin glass-like)
Remanent magnetization 10 K
Small Lcoh like spin glass
Large Lcoh FM
Approach to Saturation
• Both the strength of dipolar forces and the structural coherence length Lcoh affect the magnetic properties of nanoparticle arrays
• When Lcoh is long, magnetic relaxation is much faster, suggesting the presence of domain walls within coherent regions
• Stronger dipolar interactions slow the magnetic relaxation when Lcoh 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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