National Institute for Research & Development of Isotopic and Molecular Technologies

Cluj-Napoca, Romania

AL V- LEA SEMINAR DE NANOSTIINTA SI NANOTEHNOLOGIE , 2 MARTIE 2006 , ACADEMIA ROMANA, BUCURESTI

SIZE DEPENDENT PHENOMEMA IN

NANOPARTICLES:

MAGNETIC RESONANCE DETECTION

LIVIU MIHAIL GIURGIU

RICHARD FEYNMAN

There is Planty of Room at the Bottom – An Invitation to Enter a New Field of Physics (Eng.Sci 23, 22, 1960)“Why canot we write the entire 24

volumes of the Encyclopaedia Britanica on the head of a pin ?”

OUR EPR INVESTIGATIONS

A. NANOMETER - SCALED CMR MANGANITES

Potential technological application due to their enhanced electronic and magnetic properties

- Influence of the grain size reduction on the spin dynamics

in nanometric La 0.67 Ca 0.33 Mn O3- manganites

Appl. Magn. Res. 27, 139, (2004) Acta Physica Polonica A 108, 113, (2005)

B. Metallic/ Magnetic nanoparticles - Size evaluation by ESR (Nanometrology)

Molec.Cryst.Liq.Cryst.415, 189, (2004)

Materials Letters, (in press, 2005)

OUTLINE OF THE TALK

II. Scenaries used to describe the spin dynamics in CMR manganites

DE interaction ; Polaronic model ;   Bottlenecked spin relaxation; Core / shell

III.  Effects of the grain size reduction in nanoscaled CMR on :

- Exchange coupling integral between Mn -spins, J

- Polaron activation energy , Ea

V. Discussions

VI. Conclusions

I. Introduction

IV. Magnetic/Metallic nanoparticle systems

- Size determination of core-shell Fe3O4/PPy nanoparticles

- Size dependencies of the g-shift and linewidth in metallic nanoparticles

Size of nanoparticles in context with other small particles

Size dependent phenomena rule the game

Parameters for the description I. Surface –to-volum ratio A/V

dRR

R

V

A 63

3/4

43

2

• Increased influence of the surface of smaller particles

II. Dispersion F - fraction of atoms in the surface shell of a material

3/1

6

NF N number of atom in a system

• Packing density at the surface is lower

III. Coordination number C

• Atoms at and near the surface have fewer neighbours• They are less strongly bound than those in the bulk• Surface has a higher energy

A/V d-1

PEROVSKITE CRYSTAL STRUCTURE OF SUBSTITUTED

RARE EARTH MANGANITES La1-x Cax Mn O3

Chemical doping at La site Mn 3+ / Mn 4+ pairs

GRAIN SIZE REDUCTION

Bulk (ceramic)

micro range

d > 1000 nm

Nanograins

nano range

d 90 – 150 nm

La0.67 Ca0.33 Mn O3- D

( nm )

d

( nm )

Mn4+

( % )

TC

( K )

( K )

TC /

ceramic

TA = 1723 K > 1000 30 263 339 0.78

nanosized

TA = 1373 K 150 59 32 245 306 0.80

TA = 973 K 90 24 40 201 271 0.74

Annealing temperature TA, mean grain size D, average crystallite size d, percentage of Mn4+,

critical temperature TC, Curie-Weiss temperature and the ratio TC / for

La0.67 Ca0.33 Mn O3- samples

DIAGRAM OF THE DOUBLE –EXCHANGE MECHANISM

Basic feature : Simultaneous transfer (hopping) of an eg electron between neighbouring Mn3+ and Mn4+ ions

through the Mn – O – Mn path

The configurations 1 : Mn3+ - O 2- - Mn4+ ; 2 : Mn4+ -O2- - Mn3+

are degenerate if the core spins of t2g electrons are parallel

DOUBLE - EXCHANGE IS ALWAYS FERROMAGNETIC

POLARONIC MODEL

Jahn – Teller ( JT ) polaron : the mobile eg electron carries with it an anisotropic

local distortion which removes the degeneracy of the electronic ground state.

The displacement pattern of a polaron associated with the Jahn – Teller effect of a Mn3+ O6 octahedron

eg - JT polarons could be involved in the spin - lattice relaxation

mechanism of CMR manganites

INFLUENCE OF THE Mn3+ CONTENT ON THE

POLARON ACTIVATION ENERGY, Ea, IN CMR

De Teresa et al : Phys. Rev. B58, R5928, 1998

Linear relationship between Ea and Mn3+ content

Ea is also proportional to the Mn - O distorsion (C. H. Booth et al : Phys. Rev. Lett. 80, 853, 1998)

SCENARIO FOR NANOMETRIC CMR PARTICLES

Decreasing grain size :

INNER CORE -        Physical properties similar to the bulk (magnetic and transport, oxygen stoichiometry) - First – order magnetic transition at TC

OUTER SHELL ( SURFACE LAYER)

 -     M agnetically disorded state -        Oxygen non-stoichiometry and vacancies, superficial stress, faults in the structure -        Width t 3 nm- Second - order magnetic transition

(i) increased surface contact between grains(ii)   influence of the outer shell increases 

COMPARISON OF THE TEMPERATURE

DEPENDENCIES OF IESR * T FOR NANOSIZED

La0.67 Ca0.23 Mn O3- MANGANITES

300 350 400 450 500

0

2

4

6

8

10

12

14

16

I E

PR *

T

( a.u

. )

T ( K )

D = 150 nm D = 90 nm

Exchange coupling integral J between

Mn spins as function of grain size

La0.67 Ca0.33 Mn O3- J ( K )

ceramic 116

nanosized

D = 150 nm 87

D = 90 nm 39

EXCHANGE COUPLING INTEGRAL J BETWEEN Mn SPINS IN NANOSIZED La0.67Ca0.33MnO3-

Reasons for the degradation of DE interaction

Two similar contributions ( inner core , outer shell )

J decreases with D

A) OUTER SHELL OS

Increased influence of OS in smaller grains • J in OS much weaker than in IC

- TC with D

B) INNER CORE IC

Exchange coupling J in OS decreases when D

Exchange coupling J in IC decreases with D

IINFLUENCE OF STRUCTURAL CHANGES ON J

Increase Mn – O bond

     Decrease of Mn – O - Mn bond angle J ( de Gennes : Phys. Rev. 118, 141, 1960)

TEMPERATURE DEPENDENCIES OF THE LINEWIDTH H1/2 FOR La0.67 Ca0.33 Mn O3- SAMPLES FITTED WITH

THE SMALL POLARON MODEL

200 250 300 350 400 450 500 550100

200

300

400

500

600

700

800

H

1 / 2

(

G )

T ( K )

D = 90 nm D = 150 nm Ceramic sample

H1/2 ( T ) = H0 + AT -1 exp ( -Ea / kBT )

Polaron activation energy Ea and the residual

linewidth H0 as function of grain size in the

paramagnetic regime of

nanostructured La0.67 Ca0.33 Mn O3-

La0.67 Ca0.33 Mn O3- Ea

( meV)

H0

( G )

ceramic 120 24

nanosized

D = 150 nm 104 42

D = 90 nm 83 70

POLARONIC EFFECTS IN NANOSTRUCTURED La0.67Ca0.33MnO3-

Ea decreases with D

Two opposite contributions

( inner core ,

outer shell )

Inner-core contribution to Ea is dominant

B) OUTER SHELL OS

Mn 3+ and Mn 4+ spins are disordered defects, oxygen vacancies higher energy barrier for eg polarons to hope over

Ea as disorder in OS in smaller grain

A) INNER CORE IC

Mn4+ content ( Mn3+ ) with D

Ea when Mn3+ content

Ea with Mn4+ content or D

IRON OXIDE / POLYPYRROLE (PPy)

NANOCOMPOSITES

MAGNETITE

B. Core-shell Fe3O4 / PPy nanoparticles

MAGNETIC NANOPARTICLES

POLYPYRROLE ( PPy ) – Fe3O4 NANOPARTICLES WITH CORE-SHELL STRUCTURE

EPR BEHAVIOR

- ESR spectra of PPy-Fe3O4

nanoparticles

- Synthesis by the oxidative polymerization of PPy in aqueous solution containing an oxidant and water based magnetic Fe3O4 nanofluid ( MF )- MF / PPy = 20 ( v / v )

STRUCTURE

TEM IMAGE OF PPy-Fe3O4 CORE-SHELL NANOPARTICLES

Dm 16 nm

TEMPERATURE DEPENDENCE OF THE RESONANCE FIELD FOR ISOLATED MAGNETIC NANOPARTICLE

Bulk & surface contributions

Keff = KB + KS KS – surface anizotropy

Keff ( T ) = KB + keff T

TABTM

k2

M

K2H

γ

ωH

S

eff

S

BD

RR

S

BD

R

M

K2H

γ

ωB

S

eff

M

k2A A

2

Mk S

eff

P.C. Morais et. all. IEEE Trans. Mag. 36, 3038 (2000)

ADR

R HHγ

ωH

HD – demagnetization field HA – anizotropy field

S

effA M

K2H MS – saturation magnetization

Keff – magnetocrystalline anisotropy density

0 50 100 150 200 250 300

2200

2300

2400

2500

2600

2700

2800

2900

H0

( G

)

T ( K )

PPy-Fe3O

4

the fit

HR = f (T) for PPy – Fe3O4 CORE-SHELL – NANOPARTICLES

HR = B – A T

keff = 224 G 2K -1 A = 0.95 G K -1

MS = 528 G experimental

TEMPERATURE DEPENDENCE OF THE ESR LINEWIDTH

P.C. Morais et. all. Phil. Mag. Letters 55, 181 (1987)

T2k

EΔtanhHΔHΔ

B

0R

VTkKEΔ effB

V

k2

kV

T2k

KtanhHΔHΔ

B

eff

B

B0R

3m π

V6D V – nanoparticle’s volume

Dm – mean diameter

0 10 20 30 40 50

450

600

750

900

H

1/2

( G

)

1000 / T ( 1 / K )

PPy-Fe3O

4

ΔH = f ( T ) FOR PPy-Fe3O4 CORE-SHELL NANOPARTICLES

Mean diameter of PPy-Fe3O4 nanoparticles

KB = 6.4 x 10 4 erg cm -3

keff = 224 G 2 K -1 from HR = f ( T )bulk value

Dm (ESR) 12 nm compared with Dm (TEM) 16 nm

Size dependencies of the g-shift and linewidth

in metallic nanoparticles

HEAVY METALLIC NANOPARTICLES-

Au – nanoparticles ( ZnO/MgO/Al2O3 supported gold catalyst)

Pt – nanoparticles ( porous Al2O3 membranes)

OUANTUM SIZE EFFECTS (QSE) Kubo (1962); Kawabata (1970)

CONDUCTION ELECTRON BAND

BULK Metal

• Very short relaxation times, • Broad CESR line

CONTINUUM

NANOSIZED Particles

= (4 EF/3N) 1 / d3

• Quenching effect on the relaxation process

• Longer relaxation times, • Narrowing of the CESR line

DISCRETE ENERGY LEVELS

Kawabata conditions under which d is small (QSE) :

1/ z1)(/ s

Energy spacing δ becomes larger than the Zeeman energy WZ

Weak Spin-orbit coupling

CESR LINEWIDTH

E.Roduner – Lecture Notes 2004

Re

pp

gH

3

)(2 2

Bulk metallic elements

g(∞) – bulk g-shift

R = f (T) - rezistivity relaxation timeHpp= f (T-1)

d

vTf

gH F

e

pp )(3

)(2 2

)(/1 TfdvFr

Classical small particles (d < skindepth) Surface contribution to r

-1

Hpp = f ( d-1) - classical scatering, size dependent term dominant

dv

f(T)δγ3

hγ)Δg(2ΔH F

e

e2

pp

Metallic nano - particles (Kawabata) Quantum regime

d-3 ; f (T) neglected at low T

Hpp = f ( d2 )

Quantum regime Classical behavior

H as function of Li particle sizes (Saiki: J.Phys.Soc.Jpn , 1972)

SIZE DEPENDENCE OF THE g –FACTOR IN METALLIC NANOPARTICLES

CESR

g < 0

Bulk

)(1)(

g

L

cLg

Diffuse of electron density outside the quantum sphere

g > 0

L – edge length of a cubic box/ diameter of a spherical particle

Nanosized particles

Buttet (1982); Myles (1982)

-Cubic particles -Ortogonalized standing waves- Surface effect included

Kawabata (1970)

Nanosized particles

g > 0

g = g(∞) - ħ / s

-ħ / s dissapears proportional to – L2 for small particle size

g = g(∞)

g(∞)

0 1 2 3 4 5 6 7 8 9

0

100

200

300

400

500

600

700

800

900

Hp

p (

mT

)

d ( nm )

g ( bulk Au ) = 0.11

Hpp = f (d) - Kawabata

CESR THEORETICAL MODELS FOR NANO-Au

0,5 1,0 1,5 2,0 2,5 3,0

0

20

40

60

80

100

120 g ( bulk Au ) = 0.11

Hp

p (

mT

)

d (nm)

Hpp = f (d)- Kawabata

g = f(d) - Buttet, Myles

0,0 0,5 1,0 1,5 2,0 2,5 3,0

0,00

0,02

0,04

0,06

0,08

0,10

0,12g (bulk Au)

c ( Au-bulk) = 0.4078 nm c ( Au-cluster) = 0.285 nm

g

d ( nm )

)(1)(

g

L

cLg

Hpp = f ( d2 )

c – lattice constant

SPIN SUSCEPTIBILITY (ESR INTEGRAL INTENSITY )

Halperin, 1986

Au-PARTICLES

Odd number of electrons/ atom

• S expected to be constant down to Kubo gap T = /kB

• For T < /kB the onset of the Curie dependence should be seen

QUANTUM LIMIT FOR Au – PARTICLES

Kawabata conditions (QSE) :

1/ z1)(/ s

/ kB >> 0.5 K

Quantum narrowing (X-band)

d << 0.5 nm for T < 10 K

(Monod, Janossy , 1977)

Quantum narrowing is not consistent with experimental results

d (nm) g H (G)

Au/NaCl 2-3 0.22-0.27 100-200 Dupree, 1967

Au/ZrO2 1.4 0.0613 69 Hoffmeister,2000

The application of Kawabata conditions is excesivelly stringent in defining an upper bound on sample diameters.

3 5 4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5 8 0 8 5

( 3 1 1 )( 2 2 0 )

( 2 0 0 )( 1 1 1 )

8

9

1 0

1 1

7

Re

lati

ve

In

ten

sit

ies

( 2 o)

Au/ZnO/MgO/Al2O3 catalyst

XRD investigations

Dm 6.3 nm for Au crystallites (sample 2 )

0 50 100 150 200 250 3001

2

3

4

5

6

7

Hp

p /

Gau

ss

Temperature / K

Au/Al2O

3

Deff=6.3 nm

3300 3400 3500

Au/Al2O

3

Deff

=6.3 nm

Inte

nsi

ty /

a. u

.

Magnetic Field / Gauss

4 K

10 K

20 K

50 K

150 K200 K

100 K

RT

0 50 100 150 200 250 3000

1

2

3

4

5

6

1 /

(

a.u

. )

T ( K )

CESR of NANO – Au in ZnO/MgO/Al2O3

(Deff = 6.3 nm)

Results :

Hpp = 3.8 G – temperature independent

g = 2.0054 - temperature independent

g = 3.1 x 10-3

IESR (spin) – follows a Curie dependence

d = 0.17 nm with g (bulk Au) - Kawabatad = 6.43 nm with g experimental

Al2O3 / Pt NANOCOMPOSITES

L

d2a

2a = 12 nm

d = 30 nm

Al2O3

Pt – nanoparticles electrodeposited in thechannels of porous Al 2O3

QUANTUM SIZE EFFECTS IN Al2 O3 / Pt

0 10 20 30 40 507

8

9

10

11

Pt / Al2O

3

T = 6 K

I ES

R

( a.

u. )

T ( K )

1. IESR follows a Curie law

3. Observation of Pt –ESR signals : large spin-orbit coupling

2. g – factor and ESR linewidth are temperature independent

g = 2.092 H1/2 = 242 (G)

Parameters for Pt-metalVF = 14.49 x 105 m / s; M = 195 g mol-1; = 21.472 g / cm3

H1/ 2 0.135 x 105 ( g )2 d2 ( nm )

The estimate of g for Pt-metalg / E; - spin-orbit coupling constant

The percentage change in g is similar to that exhibited by spin orbit coupling in going from

H1/2 ( G ) 7.776 dm2 (nm )

11 cm2239Pttocm1769Ag

Experimental H1/2 = 242 (G) for Al2O3 / Pt

dm ( ESR ) 6 nm

Compared with crystallite size Dm = 9.7 nm

CONCLUSIONS

- Core-shell PPy-Fe3O4 nanoparticles Dm ≈ 12 nm mean diamater

- CESR of Au – nanoparticles is very difficult to experimentally observe and theoretical models need refinement - Pt – nanoparticles electrodeposited in porous Al2O3 are characteristic of the quantum size effect Dm = 6 nm

Degradation of the DE interaction as the grain size decreases

Polaron activation energy decreases with decreasing grain size

Core-shell effects explain the results

COWORKERS

Dr. M.N.Grecu National Institute for Material Physics ,Bucharest

National Institute for R&D of Isotopic and Molecular Technologies, Cluj – Napoca, Romania

Drd. O. Raita

Drd. D.Toloman

Dr. R. Turcu

Dr. X. Filip

Dr. A. Popa

Dr. A.Nan

Dr. Al. Darabont Faculty of Physics, University of Cluj

Dr. L. Vekas LLM-CCTFA, Timisoara

International Cooperations

Institute of Materials Science, NCSR “Demokritos” Athens, Greece

Institute for Problems of Materials Science, Kiev

Institut fur Physikalische Chemie, Universitat Stuttgart, Germany

National Projects

2. Spin dynamics and  size reduction  effects  of metallic/magnetic particles inserted in oxidic and polymeric matrixes, PNCDI-CERES

1.Nanometrology  on oxidic composite nanostructures and metallicnanoclusters investigated by X ray and magnetic resonance, PNCDI - MATNANTECH

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