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 M

a

croscopic spectroscopy

vs.

m

i

croscopic magnetic characterization of Fe-based magnetic nanostructures; SQUID magnetometry, Mössbauer Isolated

vs.

interacting magnetic nanoparticles Conclusion Acknowledgements

Top

→

Down Synthesis by Physical Methods Up

←

Bottom Synthesis by Chemical Methods Bulk Classical behavior Macroscopic Metal and Metal-Alloy Nanoparticles

1. High Energy Ball Milling 2. Laser Ablation 3. Ion Sputtering 4. Thermal Evaporation 5. 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 H. Gleiter, Acta Mater. 48, 1 (2000) Abundance of interfaces

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

D

G n + 0

r c

Nucleation and Critical Radii

D

G n = 4

p

r 2

D

G s

-

(4/3)

p

r 3

D

G v

r 0 r

Variation of Gibb’s free energy of nucleation with cluster radius during synthesis. r

c

critical radius and r radius

0

is the kinetic the thermodynamic critical

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

~1 nm Fe 11 O 6 (OH) 6 (O 2 CPh) 15 .

6THF

Supramolecular Clusters

Controlled hydrolytic polymerization of iron. Iron-core growth arrested via surface passivation with benzoate ligands. Observation of novel magnetic behavior.

Fe 16 MnO 10 (OH) 10 (O 2 CPh) 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

COOH COOH FeCl 3 CoCl 2 Fe 3+

Cl 

COO



H + Co 2+ COO



H + Cl



Microphase Separation Film Formation Block Copolymer Solution in THF COO



H + Fe 3+ Cl



COO



H + Co 2+ Cl



Block Copolymer Matrix COO-Na CoFe 2 O 4 COO-Na COO-Na O 2 in H 2 O Block Copolymer Matrix COO-Na CoFe(OH) 4 COO-Na COO-Na NaOH Block Copolymer Matrix Fe 3+ COO-Na OH



COO-Na COO-Na Co 2+ OH



G.C. Papaefthymiou, S.R. Ahmed and P. Kofinas Rev. Adv. Mater. Sci. 10 (2005) 306

CoFe

2

O

4

Block Copolymer Films

Transmission Electron Microscopy

Morphology of block copolymer films: ensemble of

polydispersed

CoFe 2 O 4 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

→

←7nm → Ferritin

Gln-141

The Ferroxidase and Nucleation sites of Human H-chain Ferritin

→

2Fe 2+ 4Fe 2+ 2Fe 2+ + O + O + H 2 2 2 + 4H 2 O →2FeOOH (core) + H 2 O 2 + 6H 2 O →4FeOOH (core) + 8H + O 2 + 2H 2 O →2FeOOH (core) + 4H + + 4H + (1) (2) (3)

Glu-27 O His-65 O NH 2 Glu-107 O H O H N O Ferroxidation sites A & B Fe Fe O O O O HO N Glu-62 Glu-61 O

-

O Nucleation site C

-

O Tyr-34 O Glu-64 O HuHF Glu-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 (Fe 3 O 4 , Co 3 O 4 ), 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-Fe

2

O

3

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, (CH 3 ) 3 NO, as a mild oxidant produced highly crystalline γ-Fe 2 O 3 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-

Fe 2 O 3 , ~(11.8 ± 1.3) nm diameter are shown. XRD patterns confirm the presence of Fe 2 O 3 .

D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying, Chem. Mater. 18 (2006) 614

Schematic of the synthesis of MP/SiO

2

/MS nanoarchitectures

MP

= Magnetic Particle

SiO 2

=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-Fe

2

O

3

nanoparticles

TEM micrographs of ~12 nm γ-Fe 2 O 3 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-Fe 2 O 3 core solid 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 Fe 2 O 3 core solid silica shell nanocomposites were used as templating cores.

Fundamentals of Magnetic Ordering

H ex

 2

i

 

j J ij



S i



S



j

Direct Exchange Indirect Exchange Bethe-Slater Curve Magnetic ordering in solids is due to Quantum Mechanical Exchange and the Pauli Exclusion Principle

J ex

 2

n

 0 3

kT C S

(

S

 1 ) Curie temp

T c

in °C, Iron (Fe) 770, Cobalt (Co) 1130, Nickel (Ni) 358, Iron Oxide (Fe 2 O 3 ) 622

Magnetic Anisotropy

H ex

 2

i

 

j ij S



i



S



j

-

K



i S zi

) 2 Bulk Co in its demagnetized, multi-domain state Minimization of magnetostatic energy

U B

 1 2  0 

B

2

d



allspace

leads to domain wall formation ↘ Uniaxial Magnetic Anisotropy Anisotropy Field

H an

 2

K

 0

M 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

2

a

Process of Magnetic Saturation of a Multi-domain Particle

Hard process in a single domain system Easy process in a multi domain system The hysteresis loop defines the technological properties of the magnetic material

M s M r H c

   Saturation Magnetization Remnant Magnetization 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

3

R C



R SMD

 6

AK

 0

M s

2 ←

r

2 Below

R

c

the particle is a Single Magnetic Domain, and thus permanently magnetized. The demagnetized state cannot be formed.

R

c

~

100 nm

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

H c

SP Unstable

Maximum coercivity

H c

 2

K

 0

M s

0

D p D s

Particle Diameter D Single-magnetic domain particle Coherent spin rotation Multi-magnetic domain structure Magnetic wall movement Nanoparticle K ~ 10 5 J/m 3 Bulk K ~10 3 J/m 3

Origin of magnetic anisotropy enhancement in nanoparticles

K eff



K c

 6

K s

'

D

F. Bødker, S. Mørup, S. Lideroth, Phys. Rev. Lett. 72 (1994) 282

K eff



K c



K s



K

 

K sh c

= core

s σ

= surface = stress

sh

= shape

Nanoparticle coercivity for coherent spin rotation (Stoner and Wohlfarth model)

coherent moment rotation Maximum coercivity for coherent spin rotation of a single magnetic domain particle with uniaxial total effective anisotropy

H c

 2  0

K u M s

E.C. Stoner, E.P. Wohlfarth, Trans. Roy. Soc. Lond. A 240 (1948) 599

Spin Dynamics in Magnetic Nanoparticles

Easy axis

E a

(  ) 

K u V

sin 2  Temperature dependence of coercivity

H c

 2

K

 0

M u s

   1   25

kT K u V

  1 2   

(thermally assisted spin reversals)

Superparamagnetic relaxation time    0 exp

K u V kT

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 = K u V

where K u is the effective uniaxial magnetic anisotropy Energy density and V is the particle volume Magnetocrystalline Anisotropy Shape Anisotropy Surface effects Relaxation Time

t RELAX = t 0 exp (K u V/kΤ)

Observe net magnetic moment when t MEAS < t RELAX

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

τ

m

= τ

SQUID

= 10 s

Hysteresis Loops for CoFe

2

O

4

Copolymers Block

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

T B

for SQUID magnetometry.

Ahmed, Ogal, Papaefthymiou, Ramesh and Kofinas, Appl. Phys. Letts 80 (2002) 1616

Nuclear Hyperfine Interactions with Mössbauer Spectroscopy Observed Effect Illustration Observed Spectrum Isomer Shift Interaction of the nuclear charge distribution with the electron cloud surrounding the nuclei in both the absorber and source.

0 v 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) 0 v

Modeling Dynamical Spin Fluctuations in Isolated Nanostructures 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

T B

for Mössbauer 

m

Theoretically

T B

  0 exp  

K u kT V B

is defined by:   →

T B



k



n K u V

( 

m

/  0 )

Mössbauer spectra of lyophilized, in vitro reconstituted HoSF ferritin.

80 K 40 K 30 K

T B

= 40 K 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

25-nm thick protein shell

Zero-field cooled and field-cooled magnetization of lyophilized HoSF ferritin

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 K

u

for an ensemble of superparamagnetic nanoparticles

   0 exp  

K u kT V

  

m

  0 exp  

K u kT V B

  1. Determine average particle volume

by TEM 2. Determine

T B

with two different techniques, whose measuring response times lie in different time windows 3. Use the Arrhenius equation above to determine τ

0

and

K u

80 K

Surface Effects:Temperature Dependence of Mössbauer Magnetic Hyperfine Fields

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

T B

H hf

(

T

) 

H

0

hf

1 -

kT

2

K eff V

S. Mørup and H. Topsøe, Appl. Phys. 11 (1976) 63

Mössbauer Spectra of γ-Fe

2

O

3

/Solid Silica Nanoarchitectures

100.05

101 100 99 98 97 96 95 94 100 99 98 97 96

4.2 K 78 K

99.90

99.75

99.60

100.05

99.90

4.2 K 78 K

99.75

100 99 98 97 96 100.0

150 K

99.96

150 K

99.84

100.05

99.5

300 K

99.0

99.90

300 K

98.5

-10 -8 -6 -4 -2 0 2

Velocity (mm/s)

4 6 8 10 99.75

-12 -10 -8 -6 -4 -2 0 2

Velocity (mm/s)

4 6 8 10 12

Bare 12 nm particles 12 nm particles with 25 nm SiO 2 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 -Fe 2 O 3 nanoparticles

99 100.0

99.8

99.6

99.4

100.0

99.9

99.8

-10 -8 -6 -4 -2 0 2 4 6 8 10 -10 -8 -6 -4 -2 0 2 Velocity (mm/s) 4 6 8 10 Fig.3: MS at RT of the (a) 12.5nm bare particles dressed with (b) 4nm SiO2, (c) 25nm SiO2,

γ-Fe nm silica shell γ-Fe 2 2 O O 3 3 nanoparticles with 4 nanoparticles with 25 nm silica shell G.C. Papaefthymiou et. al. Phys. Rev. B 80 (2009) 024406

Magnetization of γ-Fe

2

O

3

/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.

1.0

1 0 3 2 9 8 5 4 7 6 0 0.8

0.6

A B 50 100 150 200 250 Temparature (Kelvin) 300 C D E 0.4

* 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 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-Fe 2 O 3 cores Dipole-dipole interaction ~   1 

r

3   2

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