Few subjects are more difficult to understand than magnetism.

Encyclopedia Britannica

Presented By- R. UDAY KIRAN

Superparamagnetism and its Biological Applications

Nanotechnology Plays by Different Rules

Normal scale Nanoscale

3

Description of magnetic particles

MESOSCOPIC MAGNETISMmacroscale nanoscale

permanentmagnets

micronparticles

nanoparticles clusters molecularclusters

Individualspins

S = 1023 1010 108 106 105 104 103 102 10 1

multi - domain single - domain Single molecule nucleation, propagation andannihilation of domain walls

uniform rotation quantum tunneling,quantum interference

-1

0

1

-40 -20 0 20 40

M/M

S

m0H(mT)

-1

0

1

-100 0 100

M/M

S

m0H(mT)

-1

0

1

-1 0 1

M/M

S

m0H(T)

Fe8

1K0.1K

0.7K

Mn12-ac

Ferritin

1 nm10 nm100 nmsuperparamagnetism

Classical Quantum

size

Natural Nanomagnets:

• Ferritin

Man on average has 3-4 g of iron 30 mg per day are exchanged in plasma. Ferritin stores iron in mineral form; Ferritins are found in animals, vegetables, mushrooms and bacteria

The internal core, 7 nm, may contain up to 4,000 iron(III) ions Approximately FeO(OH) Magnetism depends on the number of ions Magnetic measurements provide information on the number of ions in the core

• Magnetosomes

Nanomagnets embedded in cell membranes

• Magnetotactic bacteria iron core

Magnetism in reduced dimensions

Intrinsic properties

Finite-size effects

Surface effects

Interparticle interactions

Nanomagnetism

Size, aspect ratio distribution

Magnetism in reduced dimensions

Surface effects

• lower coordination number• broken magnetic exchange bonds• frustrated magnetic interactions• surface spin disorder• reduced M in ferri-, antiferro-systems• enhanced M in metallic ferro-systems

Surface and core magnetic orders

spin glass?dead magnetic layer?

bulk-like?

• high-field irreversibilities

• high saturation fields

• shifted hysteresis loops

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Magnetic Moment vs. Cluster Size

Figure above from: Billas et al., J. Magn. Magn. Mater. 168 (1997) 64

Superparamagnetism• Superparamagnetism (SPM) is a type of magnetism that occurs in

small ferromagnetic or ferrimagnetic nanoparticles.• This implies sizes around a few nanometers to a couple of tenth of

nanometers, depending on the material.• Additionally, these nanoparticles are single-domain particles.• In a simple approximation, the total magnetic moment of the

nanoparticle can be regarded as one giant magnetic moment, composed of all the individual magnetic moments of the atoms which form the nanoparticle.

Tk

KV

B

exp0

Superparamagnetism

For a magnetic particle the magnetic energy with uniaxial anisotropy is given by

For particles with nanometric dimensions

Superparamagnetic relaxation is the spontaneous fluctuations of the magnetization direction such that it alternately is near θ=00 and θ=1800. The superparamagnetic relaxation time τ is given by

where τ0 is of the order of 10-10-10-13 s, kB is the Boltzmann’s constant and T is the temperature.

2sinKVE

KVTkB

Superparamagnetism (SPM)

τ=τ0exp(E / (kBT)) Neel-Arrhenius equation

τ – Average length of time that it takes for a ferromagnetic cluster to randomly flip directions as a result of thermal fluctuations

τ0 – Attempt period (characteristic of the material)

E – Anisotropic energy which is proportional to V

E=KV K is the anisotropy energy density constant

Superparamagnetism (SPM)

Blocking temperature Tb E=KV=25kBTb

T>Tb τ < <τ0 Behave like Paramagnetic particle

T<Tb τ > >τ0 Magnetic ordering and open loops

If V↓ then τ ↓ SPM limit of hard drives

REF: IEEE Transaction on Magnetics Vol 33, No. 1(1997)978-983 An upper bound of about 36 Gbit/in.2

τ=τ0exp(E / (kBT)) Neel-Arrhenius equation

• What are the implications of such superparamagnetic states? Without external magnetic field, the net moment is zero. As soon as

an external field is applied, the nanoparticles react similar to a paramagnet (hence the “paramagnetism” in the name) with the one exception that their magnetic susceptibility is much larger (hence the “super” in the name).• A word of clarification: Normally, any ferromagnetic or ferrimagnetic

material can behave paramagnetically. This is from a certain temperature on and upwards, the so called Curie temperature Tc • However, superparamagnetic behaviour is observed below the Cure

temperature and thus has to be explained differently.

New Properties of SPM

• Small size and larger magnetic moment for each particle like Ferromagnetism --Large MS

• Response to external field like paramagnetic response---No open loop

• Superparamagnetic relaxation

τ=τ0exp(E / (kBT)) Neel-Arrhenius equation

Paramagnet, Ferromagnet & Superparamagnet

Zero Magnetic Field

Magnetic Field Applied

Paramagnet Domain moments align randomly—no net moment.

Net moment appears; the applied magnetic field helps the domains “find” each other to become coupled.

Ferromagnet Domain moments coupled (below Curie temp.) to produce strong, permanent moment.

Even higher magnetic moment.

Superparamagnet

Domain moments that would couple as in Ferromagnet do not do so because of small size—boundary effect.

Domains “find” each other and now it generates a moment comparable to Ferromagnet.

Types of Magnetism

Application of Magnetic Nanoparticles in Biomedicine

• Their size is comparable to the targeted entities.• Nanoparticles can be magnetic. An external magnetic field gradient

can be applied to influence their movement. This way, they can either deliver certain drugs or tag certain entities.• Nanoparticles may also be resonantly excited. This allows heat

transfer to the surrounding tissue.

Radionuclide and Gene Delivery• Radionuclide Delivery: An advantage of radionuclide therapy is that

the radionuclides do not have to decouple from the magnetic carriers. The magnetic carriers can transport the radionuclides to the target area where they can destroy the cancerous tissue. After the desired result has been achieved, both the carriers and the radionuclides can be directed out of the circulatory system.• Gene Therapy: In gene therapies, the magnetic carriers are coated

with the therapeutical gene and transported to the target area. Thanks to the possibilitiy of holding the gene and carrier at the target for an extended time, the chances rise that the gene can get transfected. Applications in this field of study are only in their beginning

Ferrofluids: Suppose some particles do have magnetic moments.

N S N S N S N S

They will chain together!

The chain causes high viscosity.

Magnetorheological effect.

Magnetorheological Effect

A magnetic fluid.

Just pretty.

Hyperthermia:• Hyperthermia is usually an unwanted overheating of the body not to

be confused with common fever. In a hyperthermic state, the body absorbes or produces more heat than it can dissipate. However, hyperthermia can also be a wanted effect in order to destroy tumorous cells and hence is sometimes created artificially.• The magnetic particles first have to be brought to the target area,

where they can be caused to heat up by an AC magnetic field of sufficient strength and frequency. The heat should exceed the threshold of 42 degree Celsius and last for about 30 minutes in order to properly destroy the tumour.

Mechanism of heating process for MNPs Hyperthermia

1. Hysteresis loss

Hysteresis loss at different temp.

Applied field H(T)

Mag

netiz

ation

(em

u/g)

Tc

T2T1

2. Neel mechanism Rotation of the magnetization vector within the particles.

3. Brownian Mechanism Mechanical rotation of the magnetic particle

Intrinsic superparamagnetism(the particle magnetic moments aligns with external field)

Extrinsic superparamagnetism(the particle itself aligns with field)

H

H=0 H ≠ 0

H=0

Neel relaxation

H = 0

Brownian relaxation

Magnetic relaxation mechanisms

Drug Delivery The advantages of targeted drug delivery seem numerous: Most drugs are non-specific, i.e. they get distributed over the

whole body as soon as they get administered intravenously. Targeted delivery can ensure that only specific areas get

influenced by the (otherwise harmful) drugs and as little as possible of the drug needs to be administered. This method seems especially applicable, when the drug is very damaging to healthy tissue.

Fields of application:• Chemotherapy,• radionuclide therapy,• arthritis or• gene therapy.

Gene Delivery

Att tillföra en ny gen i en cell

• FeOfection is a solution of nanoparticles with an iron oxide core.

• The core is stable and the magnetic properties can be used e.g. in tracking of cells with MRI.

• The surface of the particles are modified to promote binding of DNA to the particles and facilitate transport of the resulting particle/DNA complexes into cells.

• FeOfection can be used for both transient (temporary expression) and stable (incorporated in the genome) transfection.

Imaging using magnetic nanoparticles

Marknaden drivs av ett medicinskt behov av effektivare och känsligare diagnostik

Iron Oxide

NH

2

Phospholipid

Amino-PEG

NHS-Alexa 647

Iron Oxide

NH2

U-2 OS cell incubated with Alexa-647 magnetic nanoparticles for 1 hour

FeOdots incubated with cells and exposed to a magnetic field

NH 2

NH2

Imaging - Regenerative medicine

Stamceller märks med Genovis magnetiska nanopartiklar ex vivo och injeceras i mus T2* Map Prussian blue

positive cells at edge of tumor

C6 glioma

FeOlabeled cells were injected i.v. in C6 glioma in mouse flank 14 days prior to 3T MRI

Cells labeled with FeOlabel can easily be visualised with MRI.

Mesenchymal stem cells were labeled with FeOlabel and then injected into a mouse with a C6 glioma. After 14 days the cells are visible with MRI. Particles can also be visualised by Prussian Blue iron staining.

Challenges in Nanomagnetism

100% spin-polarizedmaterials

MagneticlogicInstant boot-up

computerSpin-transistor

with gain

RT magneticsemiconductors

Nano-bioMag-sensors

Ultra-strongPermanent

Magnets

UltraHigh density

media

Opportunities in Nanomagnetism

Superparamagnetism

Superparamagnetism paramagnetism below Curie’s temperature large susceptibility superparamagnetism limit

Origin of superparamagnetism magnetism: result of spin alignment thermal excitation, ferromagnetism <-> paramagnetism small scale, below Tc:

thermal excitation destroys the ordering between the clusters thermal excitation cannot upset alignment within the cluster ferro~ inside & para~ outside => treated as a large spin as a whole

Experiment results stepped hysteresis can be found below certain temperature. frequency dependent AC susceptibility

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