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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!!!
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
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
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.
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
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.
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