A. Sundaresan

Magnetism in nanostructured materialsNanomagnetism

Chemistry and Physics of Materials Unit

Jawaharlal Nehru Centre for Advanced Scientific Research

Bangalore 560 064

Lodestone or Loadstone, Magnetite Fe3O4

Lightening!

Langmuir lab

New Mexico Institute of Technology

The Chinese would certainly not have invented the magnetic compass. Magnetism would have been discovered much later, and one wonders how.

Lacking the compass, the great voyages of discovery could hardly have taken place--Columbus, De Gama, Magellan and the rest.

The history of the world might have been quite different!

What if no lodestones existed?

Outline

• Introduction to – Types of magnetism– Domains– anisotropy

• Magnetism – In nanoparticles

• Surprising magnetism!!!

Diamagnetism

No unpaired electrons

Paramagnetism

Curie law

Unpaired electron

Ferromagnetism

Exchange energy ~ 1000 T

Earth’s field ~ 50 µT Moments are aligned in direction

Antiferromagnetism

Antiparallelarrangement

Ferrimagnetism

• Fe3O4

• Spinel structure

• [Fe3+][Fe3+,Fe2+]O4

Antiparallel with fine net moment

Magnetic field, magnetic induction and magnetization

Oersted’s field

H = i/2πr

B = µ0 (H + 4πM)

UnitsQuantity SI CGS conversion factor

B T G 104

H Am-1 Oe 4πx 10-3

M Am-1 emu/cc 10-3

m Am2 emu 103

Parameters from Hysteresis loop

Losses

Parameters are not solely intrinsic but dependent on grain size, domain state, stresses and temperature

Soft and Hard magnetic materials

Transformers

Flux guides

Magnetic shielding

Permanent magnets

Motors

Magnetic recording

Hard

Soft

Origin of domains

Magnetic anisotropy• Are the magnetic properties same in all directions?• No• It depends on the crystallographic direction in which the

magnetic dipoles are aligned

– Crystal anisotropy (Spin Orbit Coupling)– Shape anisotropy– Stress anisotropy– Externally induced anisotropy– Exchange anisotropy

• E = KVsin2θ (simplest form)– K the effective uniaxial anisotropy energy per unit volume– V particle volume – θ angle between moments and easy axis

Magnetocrystalline anisotropy in magnetite

Magnetocrystalline anisotropy of cobalt

K = 4.1 x 105 J/m3

Uniaxial anisotropy

• For a permanent magnet, the easy direction of mangnetization must be uniaxial, however, it is also possible to have materials with multiple easy axes or where the direction can lie anywhere on a certain plane or on the surface of a cone.– It means that it is difficult to demagnetise as it

is resistant to rotation of the direction of magnetization

Surface or shape anisotropy –demagnetizing field

Hi = He – HD

HD = N M

N = demagnetizing factor

Geometry N

Long cylinder 0

Cylinder l/d=1 0.27

Sphere 0.333

Stress anisotropy

• In addition to magntocrystalline anisotropy there is another effect related to spin orbit coupling called magnetostriction arises from the strain dependence of the anisotropy constants. It is measure of strain as a function of field. Magnetic materials changes its dimension when magnetized.

Weiss theory of ferromagnetism• There is an exchange

field (molecular or atomic field) through which the individual magnetic moments interact postively and therefore order ferromagnetically

• If so, why a ferromagnetis not spontaneously magntized?

Magnetic domain

• Weiss suggested the existence of magnetic domains in ferromagnets based on the work of Ampere Weber and Ewing. In each domain, the magnetic moments were aligned parallel, however, the direction of alignment varies from domain to domain in a more or less random manner.

• The net vector sum of all the domains therefore produce a total magnetization of near zero.

Break up of magnetization into domains due to energy minimization

Single domain

Four domains

Two domains

Closure domains

Large magnetostaticenergy

Uniformly magnetized specimen

180o domain wall

e = A (dθ / dx)2 + K sin2θ

Exchange J/m Anisotropy J/m3

λB

Exchange energy

small

large

Change in domains during magnetization process

Partial magnetization

E = - µ0M.H

Domain wall movement

Irreversible rotation of domain magnetization

Coherent rotation of moments from the easy axes lying close to the field direction

Single-domain sampleSpontaneous magnetization

Néel walls• In the case of Bloch wall,

the magnetization lie normal to the plane of the materials that results in large demagnetization energy whereas in the Néel wall the moments rotate within the plane of the specimen results in lower energy. It occurs only in thin films.

Ferromagnetic domain pattern on single crystal platelet of Nickel

Bitter method

Other methods

• Barkhausen effect (indirect)

• Faraday and Kerr effects– In this method, the

axis of polarization of linearly polarized light beam is rotated by the action of a magnetic field

B

H

Discontinuous domain boundary motion

Domain rotation

Critical length scales

• Resistivity – mean free path• Strenth – dislocation Burgers vector• Absorption – penetration depth• Superconductivity – coherence length• Magnetism – exchange length, domain

wall width

Preparation methods of nanostructured materials

Mechanical alloying Plasma or Vapor Deposition

Dip Pen Lithography: Transport of molecules to the surface via water meniscus

Production methods• Sputtering• Pulsed Laser Deposition• Chemical precipitation from solution• Electrodeposition• Filling of nanopores• Dip Pen Lithography• Mecahnical alloying• Self assembled monolayers• Sol gel chemistry• Rapid solidification• Electrodeposition• and ……

Classification of magnetic structures

Isolated particles

Bulk materials with nanoscalestructure

Only size effects

Ferrofluid

Core shell

composite

Example for exchange anisotropy

Motivation • The dramatic change in magnetic properties that

occurs when the fundamental magnetic lengths are comparable to nanoparticle or nanocrystalsize

• Magnetization occur via activation over an energy barrier– Each mechanism has length scale

• Crystalline anisotropy length lK = (J/K)1/2

• The applied field length, lH = (2J/HMS)1/2

• Magnetostatic length, lS = (J/2πMS2)1/2

• For most materials, 1 – 100 nm

Magnetic nanoparticles behavior

Assembly of magnetic clusters (each comprised of many ferromagentically aligned elemental moments of magnitude (µ) acting independently – Superparamagnetic material

Superparamagnetismin nanoparticles

Applied field, H (T)

Mag

netiz

atio

n (e

mu)

Applications:Magnetic Inks

Magnetic separation

Vacuum sealing

Magnetic marking

Magnetic refrigeration

MRI

No remanent magnetism upon field removal

300 K

Coercivity vs particles size

Single domain Mulltidomain

Hc

Dc D

Super-paramagnetic

D ~ λB

stable

unstable

Thermal fluctuationsDp

Spin rotation

Potential applications of magnetic nanoparticles

• Active component ferrofluids• Recording tape• Flexible disk recording media• Biomedical materials• Catalysts• Hard disk recording media • Permanent magnets• Magnetic refrigerators• Research tools in materials physics, geology,

biology and medicine

Fe nanoparticles

• Fe nanoparticles below 20 nm are in the superparamagnetic regime

• It has potential applications in bioseparation, biosensing, drug delivery, and MRI contrast enhancement

• Preparation method– Fe(CO)5 + Olylamine +Octadecene

Fe180oC

JACS

128, 10676 (2006)

Deocompositionof the hexane dispersion of the Fe nanoparticleson an amorphous carbon-coagedCu grid under N2 lead to self assembled superlatticearray

FerrofluidAudio speaker

Domain detection

Optical pick-up

Biomedical applications

NiO nanoparticles

Moumita et al J. Mater. Chem. 16, 106 (2006)

Ferromagnetism in Au nanoparticles

Hard/soft ferromagnetics• Composites of nm thick Hard &

soft ferromagnets are thought to be the way to the future “Hardest” ferromagnets

• This is because the magnetic exchange interactions at the interface causes the magnetization to change by the more difficult process of the coherent rotation of all the spins at one time, rather than by nucleation & growth of domains. This results in a large corercivity

• Substitution of soft FMs increase the saturation M, resulting in an increase in hysteresis loop area.

Surprising magnetism

• Ceria, CeO2 Ce4+ (4f0) • Has flourite structure• A strong oxidizing agent• Applications include, metallurgy, glass

polishing, phosphors and catalysts• Prepared by adding hexamine to a

solution of cerium nitrate under constant stirring.

X-ray pattern

CeO2 nanoparticles

CeO2 powder and bar heated at 500oC

-4000 -2000 0 2000 4000

-6.0x10-4

-4.0x10-4

-2.0x10-4

0.0

2.0x10-4

4.0x10-4

6.0x10-4

Magnetic Field ( Oe )

Mag

netiz

atio

n ( e

mu/

g )

CeO2 nanoparticles

Annealed at 5000 C in O2 Powder Bar

5 10 15 20 25 301.0x10-3

2.0x10-3

3.0x10-3

4.0x10-3

5.0x10-3

6.0x10-3

(b) Magnetization Vs Particle Size

Mag

netiz

atio

n (e

mu/

g)Particle Size (nm)

Saturation Magnetization at H = 5 kOe

-4000 -2000 0 2000 4000-6.0x10-3

-4.0x10-3

-2.0x10-3

0.0

2.0x10-3

4.0x10-3

6.0x10-3

Mag

netis

atio

n (e

mu/

g)

Magnetic Field (H)

3-5 nm 5 nm 7 nm 10 nm 12 nm 20 nm 25-30 nm cubes bulk

CeO2 nanoparticles

Magnetization versus particles size

Al2O3

-3000 -2000 -1000 0 1000 2000 3000-4.0x10-3

-2.0x10-3

0.0

2.0x10-3

4.0x10-3

(b)

Magnetic field (Oe)

Al2O3

500oC (300 K) 500oC (390 K) 1400oC (300 K)

Mag

netiz

atio

n (e

mu/

g)

ZnO

-2000 -1500 -1000 -500 0 500 1000 1500 2000

-3.0x10-4

0.0

3.0x10-4

As prepared 1200 oC

Mag

netiz

atio

n (e

mu/

g)

Magnetic Field (Oe)

Solid state commun. 138, 136 (2006)

In2O3

-2000 -1500 -1000 -500 0 500 1000 1500 2000

-3.0x10-4

0.0

3.0x10-4

As prepared 1200 oC

Mag

netiz

atio

n (e

mu/

g)

Magnetic Field (Oe)

SnO2

-6000 -4000 -2000 0 2000 4000 6000-4.0x10-3

-3.0x10-3

-2.0x10-3

-1.0x10-3

0.0

1.0x10-3

2.0x10-3

3.0x10-3

4.0x10-3

Mag

netis

atio

n (e

mu/

g)

Magnetic field (Oe)

as prepared 1200oC

SnO2

Possible origin or ferromagnetism

• Surface defects due to oxygen deficiency

• Possibility of orbital contribution as suggested for thiol capped Au nanostructures

Magnetism: past and the present

De Magnete

William Gilbert 1600

Vortex structure of submicron ferromagnetic dot of permalloy