magnetic nanocomposites

CHAPTER 1

INTRODUCTION

Materials with features on the scale of nanometer often have properties dramatically different

from their bulk scale counterparts. Nanocrystalline materials are single phase or multiphase

polycrystals, the crystal size of which is of the order of few nanometers so that about 40 to 80

% of the atoms are in the grain boundaries . Nanostructure science and technology is a broad

and interdisciplinary area of research and development activity that has been growing

worldwide in the past decades. Important among these nanoscale materials are

nanocomposites, in which the constituents are mixed at nanometer length scale. They often

have properties that are different compared to conventional microscale composites and can be

synthesized using simple and inexpensive techniques. The study of nanocomposite materials

requires a multidisciplinary approach with impressive technological promise, involving novel

synthesis techniques and an understanding of physics and surface science .

During the last decade, the development of magnetic nanocomposite materials has been the

source of discovery of spectacular new phenomena, with potential applications in the fields of

information technology, telecommunication or medicine . Magnetic nanocomposite materials

are generally composed of ferromagnetic particles (grain size in nanometer scale) distributed

either in a non-magnetic or magnetic matrix . The shape, size and distribution of the magnetic

particles play an important role in determining the properties of such materials. The matrix

phase separates the magnetic particles and changes the magnetic exchange interaction. This

affects the transport and magnetic properties. Therefore, understanding and controlling the

structure of materials is essential to obtain desired physical properties.

Department of Mechanical Engineering

CHAPTER 2

BRIEF HISTORY

Nanocomposite magnetic materials have their origins in the amorphous alloys that were

brought to market in the 1970's. Amorphous materials are characterized by a lack of long

range atomic order, similar to that of the liquid state. Production techniques include rapid

quenching from the melt and physical vapor deposition is another. The lack of crystallinity

causes amorphous materials to have a very low magnetic anisotropy. METGLAS 2605™

Fe78Si13B9 is a common amorphous magnetic alloy, in which B acts as a glass forming

element. The importance of anisotropy suggests searching for other materials with isotropic

magnetic properties. In magnetic materials the ferromagnetic exchange length expresses the

characteristic distance over which a magnetic atom influences it's environment, and has

values on the order of 100 nm. If the magnet has a structure with grain diameters smaller than

the exchange length, it becomes possible to "average" the anisotropy of the grains to a low

bulk value. Such a material then realizes the high saturation magnetisation (Ms) of the

crystalline state and low coercivity (Hc) due to randomized anisotropy.

In 1988 Y.Yoshizawa developed the FINEMET™ alloy based on Fe73.5 Si13.5B9Nb3Cu1.

This was an extension of the common Fe-Si-B alloy with Cu as a nucleation agent and Nb as

a grain refiner. The material is produced in the amorphous state and then crystallized by

annealing. Nb that segregates to the grain boundaries acts a diffusion barrier preventing grain

growth. The structure is a nanocomposite of 10- 100 nm diameter bcc- FeSi grains embedded

in an amorphous intergranular matrix.

In 1990 K.Suzuki reported the development of the Fe88Zr7B4Cu1 alloy which was named

NANOPERM™. Zr and B act as glass forming agents in this alloy and the microstructure

consists of α-Fe grains embedded in an amorphous matrix. By eliminating Si, higher

saturation inductions are achieved than in FINEMET, but the Hc are also higher. The

amorphous intergranular phase in both FINEMET and NANOPERM have Curie temperatures

lower than that of the nanocrystalline grains.


In 1998 M.A. Willard reported the development of HITPERM, an alloy based on the

composition Fe44Co44Zr7B4Cu1. The key distinction is the substitution of Co for Fe.

HITPERM forms α'- FeCo grains in a Co enriched amorphous matrix. The amorphous matrix

has a Curie temperature higher than the primary crystallization temperature of the alloy. This

allows the α'-FeCo grains to remain exchange coupled at high operating temperatures. Due to

the presence of Co, HITPERM alloy has an Ms higher than FINEMET or NANOPERM as

well as a higher Hc.


CHAPTER 3

RECENT DEVELOPMENT AND UPDATED INFORMATION

In the field of magnetic nanocomposites,there has been a lot of progress in the preparation of

functional magnetic nanocomposites and hybrid materials. Some of the latest magnetic

nanocomposite materials will be briefly explained. The preparation methods , few properties

and applications will be explained in short since there are a lot of new and hybrid

materials.The main focus will be on the different types of the functional nanocomposites and

hybrid materials. There are a number of categories according to which functional

nanocomposites are classified. Some of them that will be dealt with are given below-

Core Shell type Multicomponent Magnetic Nanoparticles

Colloidal crystals

Mesoscale magnetic nanocomposites

Functional magnetic polymers

3.1 Multicomponent magnetic NPs: core–shell type NPs

The combination of two nanoscaled entities into a single hybrid particle has recently attracted

much attention due to the numerous possibilities of application. Hybrid NPs may provide a

platform with dual imaging capabilities for medical diagnosis (e.g., simultaneous magnetic

and optical imaging), dual action combining magnetic imaging and therapy, and multiplexing

in sensors. By this approach, the respective properties of the components may be combined

and optimized independently. In addition, cooperatively enhanced performances due to

collective interactions between the constituents have been achieved. Otherwise, however, the

direct combination of the different entities may lead to undesired effects such as

luminescence quenching by direct contact of magnetic NPs and quantum dots (QDs). To date,

several morphologies of multicomponent, magnetic hybrid NPs have been reported, including

core–shell and heterodimeric NPs.

The general strategy for multicomponent nanostructures is to first prepare NPs of one

material, and then use them as nucleation seeds to deposit the other material. This strategy

has been well established for the synthesis of semiconductor QDs with epitaxial shells, while


the controlled synthesis of uniform NPs that combine materials with different

crystallographic structures, lattice dimensions, chemical stabilities and reactivities still faces

many challenges. To date, a number of heterostructures has been synthesized by applying a

seedmediated approach. Coating has been routinely applied for magnetic core stabilization

and surface functionalization in view of biomedical and technical applications.

One of the simplest methods for preparing core–shell type NPs has been the partial oxidation

of magnetic metal NPs to form a shell of the native oxide on the particle surface.

Polycrystalline Fe3O4 shells, e.g., which were generated by chemical oxidation on Fe

particles, were shown to successfully protect and stabilize Fe NPs against full oxidation.For

Co-CoO NPs, additionally to their stabilization, an exchange bias effect was observed as a

result of a strong interaction between the nanometre scale antiferromagnetic CoO layer and

the ferromagnetic Co core. Bimagnetic core–shell systems such as FePt-Fe3O4 or FePt-

CoFe2O4, where both core and shell are strongly magnetic (ferro- or ferrimagnetic), show

effective exchange coupling phenomena and facilitate the fabrication of magnetic materials

with tunable properties. The magnetic properties, e.g., magnetization and coercivity, can be

readily controlled by tuning the chemical composition and the geometrical parameters of the

core and the shell (Fig. 1).

Fig. 1 FePt-Fe3O4 NP assembly: (a) TEM image, (b) magnetization curve measured at 10 K

(Fe3O4 shell thickness 1 nm), and (c) normalized coercivity hc as a function of the Fe3O4

volume fraction.


3.2 Colloidal crystals

The assembly of small building blocks (e.g., atoms, molecules, and NPs) into ordered

macroscopic superstructures has been an important issue in various areas of chemistry,

biology, and material science. Self-assembly of NPs into two-dimensional and three-

dimensional superlattices with a high degree of translational order has attracted a lot of

attention since the early observation of iron oxide super crystals by Bentzon.

More recently, self-assembled super crystals of iron oxide nanocubes by a drying-mediated

process, applying a magnetic field at the initial stage of the process was developed. These

super crystals did not only reveal a translational order but further an orientational order with a

crystallographic alignment of the nanocubes. The assembly of NPs of different materials into

defined colloidal crystals or quasi crystals provides a general path to a large variety of

composite materials (metamaterials) with new collective properties arising from the

interaction of the different Nanocrystals(NCs) in the assembly.

The formation of three-dimensionally ordered binary superlattices with a large structural

diversity, by combining two sets of NCs, e.g., magnetic NCs with semiconductor

QuantumDots(QDs) or metal particles was achieved. In a model system, PbSe semiconductor

QDs and superparamagnetic g-Fe2O3 NCs with independently tuneable optical and magnetic

properties were co-assembled by slow solvent evaporation.The PbSe NCs displayed a size-

dependent, near-infrared (NIR) absorption and emission, whereas the as-synthesized,

superparamagnetic g-Fe2O3 NCs revealed a weak absorption in the NIR at 1400 nm. It was

shown that electrical charges on sterically stabilized NCs determine the stoichiometry of the

superlattices together with entropic, van der Waals, steric and dipolar forces. The charge state

of the NCs could be tuned by adding small amounts of ligands, e.g., carboxylic acids, TOPO,

or dodecylamine. The addition of carboxylic acid to solutions of PbSe–Fe2O3 NC mixtures

resulted in the growth of AB or AB2 superlattices, whereas the addition of TOPO to the same

mixtures favoured growth of AB13 or AB5 structures (Fig. 2). The single domain regions of

the AB2 and AB13 superlattices ranged from 0.16 to 2 mm2. As there are a growing number

of monodisperse NC systems available, the use of NCs with independently tuneable

properties will enable the synthesis of divers materials with material responses which can be

fine-tuned to magnetic, electrical, optical, and mechanical stimuli.


Fig. 2 TEM micrographs and sketches of AB13 superlattices of 11 nm g- Fe2O3 and 6 nm PbSe NCs.

(a) Cubic subunit of the AB13 unit cell. (b) AB13 unit cell built up of eight cubic subunits. (c) Projection of a {100}SL plane at high magnification. (d) As (c) but at a low magnification (inset: small-angle electron diffraction pattern). (e) Depiction of a {100} plane. (f) Projection of a {110}SL plane. (g) As (f) but at a high magnification. (h) Depiction of the projection of the {110} plane. (i) Small-angle electron diffraction pattern. (j) Wide-angle electron diffraction pattern of an AB13-superlattice .


3.3 Mesoporous Magnetic Nanocomposites

A mesoporous material is a material containing pores with diameters between 2 and 50 nm.

In recent years, the synthesis of functional mesoporous magnetic microspheres with a defined

size and narrow size distribution has attracted increased attention as promising materials for

various applications. The following figure displays a typical four step procedure for the

synthesis of mesoporous superparamagnetic microspheres consisting of:

(1) Synthesis of superparamagnetic NPs .

(2) Development of a dense, nonporous SiO2 layer.

(3) Templated growth of the porous SiO2 shell.

(4) Template removal by calcination or solvent extraction.

The supermagnetic nanoparticles used was Fe3O4.Etching of the magnetic cores in harsh

media is typically prevented by introduction of an intermediate, nonporous SiO2 layer in step

(2). Particles (500 nm) with magnetic core and an ordered, mesoporous SiO2 shell with

perpendicular oriented accessible pores were obtained by such a four-step procedure using

cetyltrimethylammonium bromide (CTAB) as mesopore template. The template was finally

removed by extraction with acetone.

Schematic illustration of a typical four-step procedure for the synthesis of superparamagnetic

mesoporous SiO2 spheres.


After obtaining Fe3O4-SiO2 particles, by adding other compounds, it can be used in various Biomedical applications Some of them are given below-

The Fe3O4- SiO2 particles could be further loaded with fluorescing dyes (fluorescein isothiocyanate (FITC) and rhodamine B isothiocyanate (RITC)) and doxorubicin (DOX) and were tested for MR and fluorescence imaging as shown in figure (a).

(a) Uniform Fe3O4-SiO2 particles with a single Fe3O4 core

2-bromo-2-methylpropionic acid-modified Fe3O4 NPs were reacted with amine-functionalized, dye-doped mesoporous SiO2 spheres.The pores of the nanocomposite could be further loaded with the anti-cancer drug doxorubicin and thus served as a multimodal platform for optical imaging, MR contrast enhancement, and drug delivery.

(b) mesoporous SiO2 particles decorated with multiple Fe3O4 NPs


3.4 Functional Magnetic Polymers

Polymer coatings have been formed on magnetic NPs to simply change the surface properties

of superparamagnetic NPs. The polymer then acts as a stabilizer or improves the

biocompatibility of the NPs. However, magnetic NPs are also able to couple their physical

properties with those of the polymer matrix. For example, magnetic NPs can be used to

transfer forces applied by an external magnetic field to a surrounding polymer matrix,

resulting in a change of shape or movement. This can be utilized for a variety of applications,

such as actuators, switches, or magnetic separation. Moreover, magnetic NPs have been

combined with polymer matrices which are sensitive to temperature changes induced by an

AC magnetic field. Inductive heating of thermoresponsive polymers has been exploited for

temperature- responsive flocculation of NPs, drug delivery, and shape transition

applications.The following two functional magnetic polymers will be explained further-

Au-shell NPs with amphiphilic diblock copolymers Ferrogels

3.4.1 Au-Shell NPs with Amphiphilic Diblock Copolymers

Thermo-responsive γFe2O3-Au NPs have been prepared by using amphiphilic organic

diblock copolymer chains (Fig. 3).127 The diblock copolymer chains included a thermally

responsive poly- (N-isopropylacrylamide) (pNIPAAm) block and an amine-containing

poly(N,N-dimethylaminoethylacrylamide) (DMAEAm) block. An additional –C12H25

hydrocarbon tail drived the formation of micelles. The micelles were loaded with Fe(CO)5,

followed by subsequent thermolysis. The amine of the pDMAEAm block further served as

electron donor for reducing AuCl4_ to form a Au shell. Thermal aggregation of the particles

above their lower critical solution temperature leads to dielectric coupling and to changes in

the surface plasmon spectra.


Fig. 3 Schematic illustration of the synthesis of magnetic-core, Au-shell NPs with

amphiphilic diblock copolymers.

3.4.2 Ferrogel

Matrix-dispersed composite materials of rather rigid polymer matrices filled with magnetic

particles, viz. magnetic elastomers or magnetoelasts, have been known for many years. These

materials are used as permanent magnets, magnetic cores, connecting and fixing elements in

many areas. They display a low flexibility and do not change their size, shape, and elastic

properties in the presence of an external magnetic field.

More recently, a new generation of magnetic elastomers, consisting of mainly nanosized,

superparamagnetic particles dispersed in a highly elastic polymer matrix, has attracted

increasing interest in basic research as well as in certain applications. ‘‘Smart’’ ferrogels

show unique magneto-elastic properties, i.e., they undergo a quickly controllable change in

shape upon exposure to a magnetic field. These peculiar magnetoelastic properties may be

used to create a wide range of motion and allow a smooth change in shape and movement.

Ferrogels are a promising class of materials for many applications, including actuators,

switches, artificial muscles, and drug delivery systems. Ferrogels usually consist of a

crosslinked polymer forming the gel matrix, and magnetic NPs dispersed in the matrix.

Owing to interactions between the NPs and the polymer chains, the incorporated magnetic

NPs connect the shape and physical properties of the gel to an external magnetic field.


A ferrogel composed of crosslinked poly(N-tert-butylacrylamide-co-acrylamide) and Fe3O4

NPs, e.g., has been prepared by a two-step procedure.

First, the hydrogel was synthesized by free-radical crosslinking copolymerization of the

corresponding monomers, followed by subsequent co-precipitation of Fe2+ and Fe3+ in

alkaline medium. A cylinder of the ferrogel was placed in a nonuniform magnetic field

switching on and off, where the average magnetic field gradient was perpendicular to the axis

of the ferrogel. Fig. 4 shows the reversible bending process of this ferrofluid cylinder due to

the magnetic field.

Fig. 4 Bending process of a ferrogel cylinder due to a magnetic field


3.5 Applications of Magnetic NanoComposites

The combination of nanotechnology and medicine has yielded a very promising offspring that is bound to bring remarkable advance in fighting cancers. In particular, nanocomposite materials based novel nanodevices with bi- or multi- clinical functions appeal more and more attention as such nanodevices could realize comprehensive treatment for cancers. Because it can provide an effective multimodality approach for fighting cancers, cancer comprehensive treatment has been fully acknowledged. Among the broad spectrum of nano-biomaterials under investigation for cancer comprehensive treatment, magnetic nanocomposite (MNC) materials have gained significant attention due to their unique features which not present in other materials. For instance, gene transfection, magnetic resonance imaging (MRI), drug delivery, and magnetic mediated hyperthermia can be effectively enhanced or realized by the use of magnetic nanoparticles (MNPs). Therefore, MNPs are currently believed with the potential to revolutionize the current clinical diagnostic and therapeutic techniques.

3.5.1 Destruction of tumour cells by action of NanomagneticComposites

The treatment involves getting the nanoparticles inside the target cell, then applying a strong enough magnetic field to orient them within the cell. Indeed, nanoMag nanoparticles have an iron oxide core carrying a magnetic moment. During activation, the magnetic moments, which were initial randomly oriented within the cell, line up with the external magnetic field, transforming the magnetic energy into rotational kinetic energy. The forced orientation of these particles throughout the period of exposure induces directional forces which strain the cell. When the nanoparticle concentration is high enough within the cell, the tumour cell is destroyed. Depending on the level of stress in the cell and/or the resulting damage, the tumour cells enter into apoptosis or necrosis. When the field is switched off, the nanoparticles adopt once again a random orientation and their anti-tumour activity ceases instantaneously.

Rotation Time-dependent binding of Cell components Action of nanoMag on tumour cells


Cell Stresss

Apoptosis

Necrosis

Cell Stresss

Repair & Survival

Apoptosis

Necrosis

3.5.2 Transformers

Miniaturization and efficiency requirements demand the reduction of size and mass of core

materials in transformer. Increasing the Ms and μ will allow less magnetic material to be used

for a given transformer application. Decreasing Hc will reduce loss in AC applications,

improving efficiency. Operating temperatures may increase as power electronic systems

become more densely populated with components. This creates a need for magnetic materials

with increased operating temperatures. This can be achieved with nanocrystalline materials

with high crystalline and amorphous Tc to prevent particle decoupling.

3.5.3 DC-DC power converters

DC-DC power converters offer the advantage of reduced size and weight over conventional

line frequency transformer based power supplies. These converters are high frequency

devices that use magnetic transformers and inductors, along with active circuit elements, to

convert voltage levels. Ferrite materials are presently used to meet the frequency

requirements. The low Ms and Tc of ferrite materials limits the miniaturization potential of

converters. A magnetic material that had the Ms of iron and an operating frequency of 1 MHz

could result in a factor of 50 reduction in weight and volume. Nanocomposite magnetic

materials already have this Ms and have operating frequencies of 100 kHz. New, more

resistive nanocomposite structures have been conceived that will increase the operating

frequency above 1 MHz.


CHAPTER 4

CONCLUSION

The field of Magnetic nanocomposites is indeed very vast and still growing at a very fast

pace.It has great advantages and applications as discussed in the previous chapters. As it is

small in size it has great advantages like higher surface area which can carry drugs to the

biological systems. Also because of the smallness in the size of the particles it can be

transported to various parts of the body and can be detected by advanced technological

systems.

On the other hand ,synthesis of high-quality magnetic nanoparticles in a controlled manner,

and detailed understanding of the synthetic mechanisms are still challenges to be faced in the

coming years. Synthesis of oxide or metallic magnetic nanoparticles often require the use of

toxic and/or expensive precursors, and the reaction is often performed in an organic phase at

high temperature at high dilution. These conditions to be maintained is a great challenge in

itself and the safety aspect of humans involved is to be considered.

One of the biggest challenges in biomedical applications of magnetic nanoparticles lies in

dealing with the issue of technology transfer. There are opportunities in this respect for more

interdisciplinary approaches, for example, to ensure that the laboratory based experiments

can more explicitly emulate the expected conditions that would be encountered in real life

situations. There is also scope for significant contributions via the mathematical modelling of

complex systems, with the objective of understanding more specifically the full gamut of

physical phenomena and effects that together determine whether, in the final analysis, a

given application will be successful.

Magnetic nanocomposites offer to open new vistas in the area of drug delivery and they

promise as a prudent tactic to overcome the drug delivery related problems when the

problems of toxicity,localization and cost are addressed. If once the safety and hazardous

aspects of the materials is clearly understood and overcome, this field will certainly offer

much more benefits to mankind than it has already done.


CHAPTER 5

REFERENCES

Magnetic Nanocomposite Materials for High Temperature Applications by Frank

Johnson, Amy Hsaio, Colin Ashe, David Laughlin, David Lambeth, Michael E.

McHenry - Department of Materials Science and Engineering, Carnegie Mellon

University,Pittsburgh.

Magnetic Nanocomposite Materials by Bibhuthi Bhusan Nayak(Doctor of

philosophy).

Preparation of functional magnetic nanocomposites and hybrid materials:recent

progress and future directions.- Silke Behrens

Activatable Nanoparticles for Cancer Treatment. Nanobiotix by V. Simon, A.

Ceccaldi, and L. L´evy.


magnetic nanocomposites

Education

magnetic matrix

nanocrystalline materials

magnetic atom

thestructure of materials

amorphous matrix

low magnetic anisotropy

finemet alloy

magnetic exchange interaction