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
Outline
1.
2.
3.
4.
5.
6.
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
Top → Down
Synthesis by Physical Methods
Up ← Bottom
Synthesis by Chemical Methods
Bulk
Classical behavior
Macroscopic
1.
2.
3.
4.
5.
Metal and Metal-Alloy
Nanoparticles
High Energy Ball Milling
Laser Ablation
Ion Sputtering
Thermal Evaporation
etc…….
Nanoparticles
Quantum-size effects
Mesoscopic
Molecules
Quantum
behavior
Microscopic
100 nm
1 nm
Metal and Metal-Oxide
Nanoparticles
1. Reduction of Metal Salts in Solution
2. Thermal Decomposition Reactions
3. Hydrolysis in Aqueous Solutions
4. Hydrolysis in Nonaqueous Solutions
5. etc……..
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.
General Concepts in
Nucleation and Growth of Magnetic Nanoparticles
+
Nucleation and Critical Radii
DGn
0
DGn = 4pr2DGs - (4/3)pr3DGv
rc
r0
r
Variation of Gibb’s 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
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
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
Cobalt Ferrite Nanocluster Formation
within Block Copolymers
Cl
Fe
COOH
COO
F eC l 3
C oC l 2
COOH
-
3+
M icrophase
Separation
-
H
COO
+
Co
2+
F ilm F orm ation
-
H
+
Cl
-
B lock C opolym er Solution in T H F
Fe
COO
-
H
3+
Cl
-
COO
+
H
B lock C op o lym e r M atrix
O 2 in
H 2O
C O O -N a
C o F e2O 4
B lock C op o lym e r M atrix
+
Co
2+
Cl
-
N aO H
B lock C op o lym e r M atrix
Fe
C O O -N a
C O O -N a
C O O -N a
-
C o F e (O H ) 4
C O O -N a
C O O -N a
3+
C O O -N a O H
-
C O O -N a
2+
C O O -N a C o
-
OH
G.C. Papaefthymiou, S.R. Ahmed and P. Kofinas Rev. Adv. Mater. Sci. 10 (2005) 306
CoFe2O4 Block Copolymer Films
Transmission Electron Microscopy
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
Self-assembly within Protein Cages: Ferritin
Apoferritin
24 amino acid subunits
form a robust protein cage
→
Ferritin
←7nm →
G ln -141
O
NH2
G lu -107
O
H
The Ferroxidase and
Nucleation sites of
Human H-chain Ferritin
H
O
→
O
O
O
Ferroxidation
sites A & B
HO
Fe
Fe
G lu -27
T yr-34
O
O
N
O
O
-
O
Iron Mineralization in Ferritin
→
N
ferrihydrite
H is-65
G lu -62
G lu -61
-
(1)
(2)
(3)
G lu -64
O
O
H uH F
2Fe2+ + O2 + 4H2O →2FeOOH (core) + H2O2 + 4H+
4Fe2+ + O2 + 6H2O →4FeOOH (core) + 8H+
2Fe2+ + H2O2 + 2H2O →2FeOOH (core) + 4H+
O
N ucleation
site C
G lu -67
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 M=P, As, Mo or V.
Two-dimensional Array of Ferritin
Ensemble of
monodispersed
magnetic
nanoparticles
I. Yamashita Thin Solid Films, 391 (2001) 12
Monodispersed γ-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 γ-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 γ-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
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
Solid-silica coated γ-Fe2O3 nanoparticles
TEM micrographs of
~12 nm γ-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
Higher Nanoarchitectures
TEM micrographs of γ-Fe2O3 coresolid silica shell-mesoporous silica
shell nanocomposites
~ 12 nm maghemite particles were used
as templates
(a) A thick mesoporous layer (~21nm)
was obtained using a mixture of
TEOS and C18TMS, 260 μl and
(b) 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 coresolid silica shell nanocomposites
were used as templating cores.
Fundamentals of Magnetic Ordering
H ex  - 2 
i j
Direct Exchange
Bethe-Slater Curve
 
J ij S i  S j
Indirect Exchange
Magnetic ordering in solids
is due to Quantum
Mechanical Exchange and
the Pauli Exclusion Principle
J ex 
3 kT C
2 n  0 S ( S  1)
Curie temp Tc in °C, Iron (Fe) 770, Cobalt (Co) 1130, Nickel (Ni) 358, Iron Oxide (Fe2O3) 622
Magnetic Anisotropy
H ex  - 2 
i j
 
2
J ij S i  S j - K  ( S zi )
i
Minimization of
magnetostatic energy
Bulk Co in its
demagnetized,
multi-domain
state
UB 
1
20
 B d
2
allspace
leads to domain
wall formation
↘
Uniaxial Magnetic Anisotropy
2K
H

Anisotropy Field
an
0M
s
Exchange energy per unit area of Bloch wall  BW  p
for a simple cubic lattice with lattice constant a.
Moment rotation at a Bloch Wall
AK
where A 
2 J ex S
a
2
Process of Magnetic Saturation of a Multi-domain Particle
Hard process in a
single domain system
Easy process in a multidomain system
The hysteresis loop defines the
technological properties of the
magnetic material
M s  Saturation Magnetization
M
r
 Remnant Magnetization
H c  Coercivity
Hysteresis Loop
Critical Size for SMD Particles
Magnetostatic vs. wall energy as a function of
particle size for a spherical particle of radius r
←r
←r 2
3
R C  R SMD 
6
AK
0M s
Below Rc the particle is a
Single Magnetic Domain,
and thus permanently
magnetized. The
demagnetized state cannot
be formed.
Rc ~ 100 nm
2
Coercivity as a function of particle size
F. E. Luborsky J. Appl. Phys.32 (1961) S171
Nanomagnetism: Coercivities and Spin
Reversal Mechanisms
S-D
M-D
SP
Maximum coercivity
Unstable
Hc 
Hc
0
Dp
2K
0M s
Ds
Particle Diameter D
Single-magnetic domain particle
Coherent spin rotation
Multi-magnetic domain structure
Magnetic wall movement
Nanoparticle
Bulk
K ~ 105 J/m3
K ~103 J/m3
Origin of magnetic anisotropy enhancement in
nanoparticles
'
K eff  K c 
6K s
D
K eff  K c  K s  K   K sh
F. Bødker, S. Mørup, S. Lideroth, Phys.
Rev. Lett. 72 (1994) 282
c = core
s = surface
σ = stress
sh = shape
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
Hc 
2Ku
0M s
E.C. Stoner, E.P. Wohlfarth, Trans. Roy. Soc. Lond.
A 240 (1948) 599
Spin Dynamics in Magnetic Nanoparticles
E a ( )  K u V sin 
2
Easy axis
Temperature dependence of coercivity
Hc 
2Ku
0M
s

 25 kT 
1 - 




 K uV 

1
2




(thermally assisted
spin reversals)
Superparamagnetic relaxation time
 K uV 

 kT 
   0 exp 
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.
Superparamagnetism of Small Magnetic Particles
Energy barrier
Δ E = Ku V
where Ku is the
effective uniaxial
magnetic anisotropy
Energy density and V is
the particle volume
Magnetocrystalline Anisotropy
Shape Anisotropy
Surface effects
Relaxation Time
tRELAX = t0 exp (KuV/kΤ)
Observe net magnetic moment
when
tMEAS < tRELAX
Micro-magnetics and Spin Dynamics
-Mössbauer spectroscopic measurements
Probe local magnetic moments and internal magnetic
fields, with a response time of
m = Möss = 10 ns
-DC Magnetization measurements
Probe global magnetic properties in an applied field, with
a response time of
τm = τSQUID = 10 s
Hysteresis Loops for CoFe2O4 Block
Copolymers
Hysteresis due to particle
moment rotation away from
the particle’s 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
Nuclear Hyperfine Interactions with Mössbauer Spectroscopy
Observed Effect
Observed Spectrum
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)
I(v)
Interaction of the nuclear
magnetic dipole moment
with the internal magnetic
field on the nucleus.
v
0
Modeling Dynamical Spin
Fluctuations in Isolated
Nanostructures
Mössbauer spectra of lyophilized, in
vitro reconstituted HoSF ferritin.
80 K
Determination of Blocking Temperature
Experimentally the temperature at which the
Mössbauer spectra pass from magnetic, sixline spectra to paramagnetic or quadrupolar,
two-line spectra defines TB for Mössbauer
Theoretically TB is defined by:
K V 
 m   0 exp  u 
 kT B 
→ TB 
40 K
TB = 40 K
30 K
K uV
k  n ( m /  0 )
25 K
Spectrum Key
Magenta: spectral signature of magnetic
particle core (internal iron sites)
Green: spectral signature of surface
layers (surface iron sites)
G. C. Papaefthymiou, Biochim.
Biophys. Acta 1800 (2010) 886
4.2 K
Velocity (mm/s)
G. C. Papaefthymiou, et. al. MRS Symp. Proc. Fall 2007
Zero-field cooled and field-cooled
magnetization of lyophilized HoSF
ferritin
25-nm thick protein shell
FC
ZFC
Note: Saturation magnetization is ~ 0.05
emu/g, weakly magnetic.
Typical ZFC/FC behavior of
an ensemble of magnetically
isolated superparamagnetic
particles
Determination of Ku for an ensemble of
superparamagnetic nanoparticles
 Ku V
   0 exp 
 kT




m
 Ku V
  0 exp 
 kT
B





1. Determine average particle volume <V> 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 τ0 and Ku
Surface Effects:Temperature 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
H hf (T )  H
0
hf

kT
1 
2 K eff V





S. Mørup and H. Topsøe, Appl. Phys. 11 (1976) 63
101
100
99
98
97
96
95
94
Mössbauer Spectra of γ-Fe2O3/Solid Silica
Nanoarchitectures
100.05
99.90
4.2 K
4.2 K
99.75
99.60
100.05
100
Transmission (%)
Transmission (%)
99
98
97
78 K
96
100
99
98
99.90
78 K
99.75
99.96
150 K
150 K
97
96
100.0
99.84
100.05
99.5
99.0
300 K
99.90
300 K
98.5
-10
-8
-6
-4
-2
0
2
4
6
8
99.75
10
-12 -10
Velocity (mm/s)
-8
-6
-4
-2
0
2
4
6
8
10
12
Velocity (mm/s)
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
Effect of silica shell on the RT Mössbauer Spectra
Behavior typical of strongly interacting particles
100
Bare γ-Fe2O3 nanoparticles
Transmission (%)
99
γ-Fe2O3 nanoparticles with 4
nm silica shell
100.0
99.8
99.6
99.4
-10 -8
-6
-4
-2
0
2
4
6
8
10
γ-Fe2O3 nanoparticles with 25
nm silica shell
100.0
99.9
99.8
-10 -8
-6
-4
-2
0
2
4
Velocity (mm/s)
6
8
10
G.C. Papaefthymiou et. al. Phys. Rev. B
80 (2009) 024406
Magnetization of γ-Fe2O3/Solid
Silica/Mesoporous Silica Nanoarchitectures
9
A-bare *
8
7
M (emu/g)
B-4 nm (S)
C-25 nm (S)
D-25 nm (S) + 10 nm (MS)
6
5
4
3
A
B
2
E-25 nm (S) +21 nm (MS)
1
0
0
50
1.0
* 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
150
200
250
300
Temparature (Kelvin)
C
D
E
0.8
M (emu/g)
Typical behavior of strongly
interacting magnetic nanoparticles,
spin-glass-like systems.
100
0.6
0.4
0.2
0.0
0
50
100
150
200
250
Temparature (Kelvin)
300
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 γ-Fe2O3 cores

Dipole-dipole interaction ~

1   2
r
3
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