Synthesis and Characterization of Nanoparticulate Magnetic Materials

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Synthesis and Characterization of Nanoparticulate
Magnetic Materials

• Georgia C. Papaefthymiou

Villanova University, Villanova, PA 19085
BuildMoNa Workshop University of Leipzig Leipzig,
Germany October 28-29, 2010
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• Outline
• General concepts on the synthesis, stabilization
  and assembly of magnetic nanoparticles examples
• Fundamentals of nanoparticle magnetism
• Macroscopic vs. microscopic magnetic
  characterization of Fe-based magnetic
  nanostructures SQUID magnetometry, Mössbauer
  spectroscopy
• Isolated vs. interacting magnetic nanoparticles
• Conclusion
• Acknowledgements

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Top ? Down
Up ? Bottom Synthesis by Chemical Methods
Synthesis by Physical Methods
Molecules Quantum behavior
Nanoparticles Quantum-size effects
Bulk Classical behavior
Microscopic
Mesoscopic
Macroscopic
100 nm
1 nm
Metal and Metal-Alloy Nanoparticles
Metal and Metal-Oxide Nanoparticles

• High Energy Ball Milling
• Laser Ablation
• Ion Sputtering
• Thermal Evaporation
• etc.

1. Reduction of Metal Salts in Solution
2. Thermal Decomposition Reactions
3. Hydrolysis in Aqueous Solutions
4. Hydrolysis in Nonaqueous Solutions
5. etc..

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Nanoparticulate Magnetic Materials
Nanostructured
Nanocomposite
matrix
nanoparticle
Abundance of grain boundaries
Abundance of interfaces
H. Gleiter, Acta Mater. 48, 1 (2000)
Novel magnetic properties engineered through
tailoring of the grain boundary or interfacial
region and through interparticle magnetic
interactions. Particles can interact via
short-range magnetic exchange through grain
boundaries or long-range dipolar magnetic
interactions.
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General Concepts in Nucleation and Growth of
Magnetic Nanoparticles

Nucleation and Critical Radii
DGn
DGn 4pr2DGs - (4/3)pr3DGv
0
r0
rc
r
Variation of Gibbs free energy of nucleation
with cluster radius during synthesis. rc is the
kinetic critical radius and r0 the thermodynamic
critical radius
Stabilization of nanoclusters of various size
requires a competitive reaction chemistry between
core cluster growth and cluster surface
passivation by capping ligands that arrests
further core growth.
V. K. LaMer and R. H. Dinegar, J. Am. Chem. Soc.
72 (1950) 4847
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Supramolecular Clusters
Controlled hydrolytic polymerization of iron.
Iron-core growth arrested via surface passivation
with benzoate ligands. Observation of novel
magnetic behavior.
1 nm
Fe11O6(OH)6(O2CPh)15.6THF
Fe16MnO10(OH)10(O2CPh)20
G. C. Papaefthymiou, Phys. Rev. B 46 (1992) 10366
2 nm
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Block Copolymer Nanotemplates

• Principles of synthesis

Blocks of sequences of repeat units of one
homopolymer chemically linked to blocks of
another homopolymer sequence. Microphase
separation due to block incompatibility
or crystallization of one of the blocks.
Templates for synthesis and arraying of metal
oxide nanoclusters within space confined
nanoreactors
A-Block
B-Block
Chemical Link
0 - 21
21 - 34
34 - 38
38 - 50
Increasing Volume Fraction of Minority Component
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Cobalt Ferrite Nanocluster Formation within
Block Copolymers
G.C. Papaefthymiou, S.R. Ahmed and P. Kofinas
Rev. Adv. Mater. Sci. 10 (2005) 306
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Transmission Electron Microscopy
CoFe2O4 Block Copolymer Films
Morphology of block copolymer films ensemble of
polydispersed CoFe2O4 nanoparticles, oval in
shape and of average diameter of 9.6 2.8 nm.
Ahmed, Ogal, Papaefthymiou, Ramesh and Kofinas,
Appl. Phys. Letts 80 (2002) 1616
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Self-assembly within Protein Cages Ferritin
Ferritin
Apoferritin
24 amino acid subunits form a robust protein cage
?
?7nm ?
The Ferroxidase and Nucleation sites of Human
H-chain Ferritin
?
?
Iron Mineralization in Ferritin
ferrihydrite 2Fe2 O2 4H2O ?2FeOOH (core)
H2O2 4H (1) 4Fe2 O2 6H2O ?4FeOOH
(core) 8H (2) 2Fe2 H2O2 2H2O ?2FeOOH
(core) 4H (3) Demineralization followed by
metathesis mineralization leads to biomimetic
synthesis of various nanoscale particles. A large
number of nanostructures and mono-layer films on
various supports have been produced including
metal oxide (Fe3O4, Co3O4), iron sulfide,
metallic (Co, Mn, U, Co/Pt, Ni, Cr, Ag) and
semi-conducting (CdS, CdSe) structures, and
FeOOH(MO4)x, where MP, As, Mo or V.
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Two-dimensional Array of Ferritin
Ensemble of monodispersed magnetic nanoparticles
I. Yamashita Thin Solid Films, 391 (2001) 12
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Monodispersed ?-Fe2O3 nanoparticles
Thermal decomposition of iron pentacarbonyl,
Fe(CO)5, in the presence of oleic acid produced
monodispersed metal iron particles. Controlled
oxidation using trimethylamine oxide, (CH3)3NO,
as a mild oxidant produced highly crystalline
?-Fe2O3 particles. The particles were in the
size range 4 nm to 16 nm diameter depending on
experimental conditions. Highly uniform, oleic
acid covered, magnetic nanoparticles of ?-Fe2O3,
(11.8 1.3) nm diameter are shown. XRD
patterns confirm the presence of Fe2O3.
D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying,
Chem. Mater. 18 (2006) 614
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Schematic of the synthesis of MP/SiO2/MS
nanoarchitectures
MP Magnetic Particle SiO2 Solid Silica MS
Mesoporous Silica
D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying,
Chem. Mater. 18 (2006) 614
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Solid-silica coated ?-Fe2O3 nanoparticles
TEM micrographs of 12 nm ?-Fe2O3 particles
covered with solid silica shell. Shell thickness
from 1.8 nm to 25 nm was achieved. Scale bar 20 nm
D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying,
Chem. Mater. 18 (2006) 614
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TEM micrographs of ?-Fe2O3 core-solid silica
shell-mesoporous silica shell nanocomposites
Higher Nanoarchitectures

• 12 nm maghemite particles were used as
  templates
• A thick mesoporous layer (21nm) was obtained
  using a mixture of TEOS and C18TMS, 260 µl and
• a thinner mesoporous layer (10nm) was obtained
  using a mixture of TEOS and C18TMS, 120 µl.
• In both cases, (a) and (b), ca. 25 nm solid
  silica shell coated Fe2O3 core-solid silica shell
  nanocomposites were used as templating cores.

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Fundamentals of Magnetic Ordering
Indirect Exchange
Direct Exchange
Magnetic ordering in solids is due to Quantum
Mechanical Exchange and the Pauli Exclusion
Principle
Bethe-Slater Curve
Curie temp Tc in C, Iron (Fe) 770, Cobalt (Co)
1130, Nickel (Ni) 358, Iron Oxide (Fe2O3) 622
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Magnetic Anisotropy
Minimization of magnetostatic energy
Bulk Co in its demagnetized, multi-domain state
leads to domain wall formation
?
Uniaxial Magnetic Anisotropy
Anisotropy Field
Moment rotation at a Bloch Wall
Exchange energy per unit area of Bloch wall
where
for a simple cubic lattice with lattice constant
a.
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Process of Magnetic Saturation of a Multi-domain
Particle
Hard process in a single domain system
Easy process in a multi-domain system
Hysteresis Loop
The hysteresis loop defines the technological
properties of the magnetic material
Saturation Magnetization
Remnant Magnetization
Coercivity
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Critical Size for SMD Particles
Magnetostatic vs. wall energy as a function of
particle size for a spherical particle of radius r
?
Below Rc the particle is a Single Magnetic
Domain, and thus permanently magnetized. The
demagnetized state cannot be formed.
?
Rc 100 nm
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Coercivity as a function of particle size
F. E. Luborsky J. Appl. Phys.32 (1961) S171
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Nanomagnetism Coercivities and Spin Reversal
Mechanisms
S-D
M-D
SP
Maximum coercivity
Unstable
Hc
Dp
0
Ds
Particle Diameter D
Multi-magnetic domain structure Magnetic wall
movement
Single-magnetic domain particle Coherent spin
rotation
Bulk K 103 J/m3
Nanoparticle K 105 J/m3
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F. Bødker, S. Mørup, S. Lideroth, Phys. Rev.
Lett. 72 (1994) 282
Origin of magnetic anisotropy enhancement in
nanoparticles
c core s surface s stress sh shape
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Nanoparticle coercivity for coherent spin
rotation (Stoner and Wohlfarth model)
Maximum coercivity for coherent spin rotation of
a single magnetic domain particle with uniaxial
total effective anisotropy
coherent moment rotation
E.C. Stoner, E.P. Wohlfarth, Trans. Roy. Soc.
Lond. A 240 (1948) 599
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Spin Dynamics in Magnetic Nanoparticles
Easy axis
Temperature dependence of coercivity
Superparamagnetic relaxation time
(thermally assisted spin reversals)
Due to fast moment reversals at elevated
temperatures the internal magnetic order of the
particle escapes detection. You must either
lower the temperature or use ultrafast measuring
techniques that can record the moment before it
flips.
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Superparamagnetism of Small Magnetic Particles
Energy barrier ? E KuV where Ku is the
effective uniaxial magnetic anisotropy Energy
density and V is the particle volume
Relaxation Time tRELAX t0 exp (KuV/k?)
Observe net magnetic moment when tMEAS lt tRELAX
Magnetocrystalline Anisotropy
Shape Anisotropy
Surface effects
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Micro-magnetics and Spin Dynamics -Mössbauer
spectroscopic measurements Probe local magnetic
moments and internal magnetic fields, with a
response time of tm tMöss 10 ns -DC
Magnetization measurements Probe global
magnetic properties in an applied field, with a
response time of tm tSQUID 10 s
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Hysteresis Loops for CoFe2O4 Block Copolymers
Hysteresis due to particle moment rotation away
from the particles easy axis to the direction of
the applied magnetic field.
The temperature at which the coercivity vanishes
defines the blocking temperature TB for SQUID
magnetometry.
Ahmed, Ogal, Papaefthymiou, Ramesh and Kofinas,
Appl. Phys. Letts 80 (2002) 1616
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Nuclear Hyperfine Interactions with Mössbauer
Spectroscopy
Observed Spectrum
Observed Effect
Illustration
Isomer Shift Interaction of the nuclear charge
distribution with the electron cloud surrounding
the nuclei in both the absorber and  source.
v
0
Quadrupole Splitting Interaction of the nuclear
electric quadrupole moment with the EFG and the
nucleus
v
0
Zeeman Effect (Dipole Interaction) Interaction
of the nuclear magnetic dipole moment with the
internal magnetic field on the nucleus. 
I(v)
v
0
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Mössbauer spectra of lyophilized, in vitro
reconstituted HoSF ferritin.
Modeling Dynamical Spin Fluctuations in Isolated
Nanostructures
80 K
Determination of Blocking Temperature
Experimentally the temperature at which the
Mössbauer spectra pass from magnetic, six-line
spectra to paramagnetic or quadrupolar, two-line
spectra defines TB for Mössbauer
40 K
TB 40 K
Theoretically TB is defined by
30 K
Absorption (arb. units)
?
25 K
Spectrum Key Magenta spectral signature of
magnetic particle core (internal iron
sites) Green spectral signature of surface
layers (surface iron sites)
4.2 K
Velocity (mm/s)
G. C. Papaefthymiou, Biochim. Biophys. Acta 1800
(2010) 886
G. C. Papaefthymiou, et. al. MRS Symp. Proc. Fall
2007
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Zero-field cooled and field-cooled magnetization
of lyophilized HoSF ferritin
25-nm thick protein shell
FC
ZFC
Typical ZFC/FC behavior of an ensemble of
magnetically isolated superparamagnetic particles
Note Saturation magnetization is 0.05 emu/g,
weakly magnetic.
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Determination of Ku for an ensemble of
superparamagnetic nanoparticles

1. Determine average particle volume ltVgt by TEM
2. Determine TB with two different techniques, whose
   measuring response times lie in different time
   windows
3. Use the Arrhenius equation above to determine t0
   and Ku

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Surface EffectsTemperature Dependence of
Mössbauer Magnetic Hyperfine Fields
80 K
CME model, double potential well
40 K
30 K
25 K
4.2 K
complex potential energy landscape at the surface
Velocity (mm/s)
Collective magnetic excitations below TB
S. Mørup and H. Topsøe, Appl. Phys. 11 (1976) 63
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Mössbauer Spectra of ?-Fe2O3/Solid Silica
Nanoarchitectures
Bare 12 nm particles
12 nm particles with 25 nm SiO2 shell
Spectral Key Blue A-sites, Green B-sites of
spinel structure
G.C. Papaefthymiou et. al. Phys. Rev. B 80
(2009) 024406
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Effect of silica shell on the RT Mössbauer Spectra
Behavior typical of strongly interacting particles
Bare ?-Fe2O3 nanoparticles
?-Fe2O3 nanoparticles with 4 nm silica shell
?-Fe2O3 nanoparticles with 25 nm silica shell
G.C. Papaefthymiou et. al. Phys. Rev. B 80
(2009) 024406
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Magnetization of ?-Fe2O3/Solid Silica/Mesoporous
Silica Nanoarchitectures
A-bare B-4 nm (S) C-25 nm (S) D-25 nm (S) 10
nm (MS) E-25 nm (S) 21 nm (MS)
Typical behavior of strongly interacting magnetic
nanoparticles, spin-glass-like systems.
Bare particles are covered with a very thin
layer (1 nm) of oleic acid. Saturation
magnetization of the order of 8 emu/g,
strongly magnetic
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Conclusion
Ferrihydrite is an antiferro-magnet.
Magnetization of ferritin is due to uncompensated
spins at the surface ? Weak magnetism. Protein
coat of only 2.5 nm thickness sufficient to
magnetically isolate the ferritin iron cores
Maghemite is a ferri -magnet due to uncompensated
spin sublattices in its spinel structure. In
small particles uncompensated spins at the
surface also contribute ? Strong magnetism.
Silica coat of 23 nm thickness insufficient to
isolate the ?-Fe2O3 cores
Dipole-dipole interaction

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Acknowledgements
Steve Lippard, MIT Peter Kofinas, University of
Maryland Dennis Chasteen, University of New
Hampshire Jackie Ying, IBN Singapore Eamonn
Devlin, NCSR Demokritos, Greece NSF,
EU/Marie-Curie