Report - Nanomagnetism

7/29/2019 Report - Nanomagnetism

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A

REPORT

ONNANOMATERIALS & NANOMAGNETISM

Submitted

By

GAGANJOT SINGH

University roll no. - 601202004

Under the esteemed guidance of

Dr. POONAM UNIYAL

(Assistant Professor, SPMS, Thapar University.)

School of Physics and Materials Science,

Thapar University, Patiala [Pb.]


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Contents

1. Introduction to materials..........3

1.1 General.......3

1.2 Classification of materials .3

1.3 Selection of materials ....4

2. Nano Materials.....5

2.1 Introduction...............5

2.2 Improved properties ofnano over bulk.6

2.3 Characterization of nano materials...7

2.3.1. Structural Characterization7 2.3.2. Chemical Characterization7

2.4 Properties of nano materials .....9

2.5 Applications of nano materials ......10

3. Nano Magnetism........11

3.1 Introduction.....11

3.2 Challenges in nano magnetism.......11

3.3 Dimensionality in nano magnetism ...........13

3.4 Magnetic Anisotropy .....15

3.5 Single Domain....16

3.6 Domain walls..........19

3.7 Applications....19

3.8 Future......21

3.9 Bismuth Ferrite.......213.10 Summary...........23

References.....24


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Chapter 1

Introduction to materials

1.1General

Material is anything made of matter, constituted of one or more substances. Wood,

cement, hydrogen, air, water and any other matter are all examples of materials. Materials

science and engineering plays a vital role in this modern age of science and technology.

Various kinds of materials are used in industry, housing, agriculture, transportation, etc.

to meet the plant and individual requirements. The rapid developments in the field of

quantum theory of solids have opened vast opportunities for better understanding and

utilization of various materials. The spectacular success in the field of space is primarilydue to the rapid advances in high - temperature and high-strength materials. The selection

of a specific material for a particular use is a very complex process. However, one can

simplify the choice if the details about (i) operating parameters, (ii) manufacturing

processes, (iii) functional requirements and (iv) cost considerations are known.

1.2. Classification of materials

The factors which form the basis of various systems of classifications of materials inmaterial science and engineering are: (i) the chemical composition of the material, (ii) the

mode of the occurrence of the material in the nature, (iii) the refining and the

manufacturing process to which the material is subjected prior it acquires the required

properties, (iv) the atomic and crystalline structure of material and (v) the industrial and

technical use of the material.

Common engineering materials that fall within the scope of material science and

engineering may be classified into one of the following six groups:

1) Metals (ferrous and non-ferrous) and alloys

2) Ceramics

3) Organic Polymers

4) Composites


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5) Semi-conductors

6) Biomaterials

7) Advanced Materials

Table 1: Different classes of materials

1.3. Selection of Materials

One of the most challenging tasks of an engineer is the proper selection of the material

for a particular job, e.g., a particular component of a machine or structure. An engineer

must be in a position to choose the optimum combination of properties in a material at the

lowest possible cost without compromising the quality. The properties and behavior of a

material depends upon the several factors, e.g., composition, crystal structure, conditions

during service and the interaction among them. The performance of materials may be

found satisfactory within certain limitations or conditions. However, beyond these

conditions, the performance of materials may not be found satisfactory.

Table 2: Factors affecting the material selection


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Chapter 2

Nano Materials

2.1. Introduction

Nanomaterialsis a term that includes all nano sized materials, including

engineered nanoparticles, incidental nanoparticles and other nano-objects, like those that

exist in nature.

The European Commission adopted the definition of nano materials as given below.

A natural, incidental or manufactured material containing particles, in an unbound state

or as an aggregate or as an agglomerate and where, for 50% or more of the particles in

the number size distribution, one or more external dimensions is in the size range 1 nm

100 nm. In specific cases and where warranted by concerns for the environment, health,

safety or competitiveness the number size distribution threshold of 50% may be replaced

by a threshold between 1 and 50%. The European Commission adopted the above

definition of a nano material.

Nanomaterials are a field that takes materials science-based approach on nanotechnology.

It studies materials with morphological features on the nanoscale, and especially those

that have special properties stemming from their nanoscale dimensions. Nanoscale isusually defined as smaller than a one tenth of a micrometer in at least one dimension [1],

though sometimes includes up to a micrometer. An important aspect of nanotechnology is

the vastly increased ratio of surface area to volume present in many nanoscale materials,

which makes possible new quantum mechanical effects. One example is the

quantum size effect where the electronic properties of solids are altered with great

reductions in particle size. Nanoparticles, for example, take advantage of their

dramatically increased surface area to volume ratio. Their optical properties,

e.g. fluorescence, become a function of the particle diameter. This effect does not come

into play by going from macro to micro dimensions. However, it becomes pronounced

when the nanometer size range is reached.

A certain number of physical properties also alter with the change from macroscopic to

nanoscopic system. Novel mechanical properties of nanomaterials are a subject of nano


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mechanics research. When brought into a bulk material, nanoparticles can strongly

influence the mechanical properties of the material, like stiffness or elasticity. For

example, traditional polymers can be reinforced by nanoparticles resulting in novel

materials which can be used as lightweight replacements for metals. Such nano

technologically enhanced materials may enable a weight reduction accompanied by an

increase in stability and improved functionality. Catalytic activities also reveal new

behavior in the interaction with biomaterials.

2.2. Improved Properties of Nanomaterials over Bulk Materials

Nanomaterials offer better properties over their bulk counterparts. It has been observed

that changes in particle properties can be observed when particle size is less than a

particular level, called the critical size. Table presents the feature sizes for significant

changes in properties in nanomaterials.Table 3: Sizes for changes in properties in nano composites

Additionally, as dimensions reach the nanometer level, interactions at phase interfaces

become largely improved, and this is important to enhance material properties. In this

context, the surface area / volume ratio of reinforcement materials employed in the

preparation of nanomaterials is crucial to understand their structure-property

relationships. One of the major factors which alter the properties in nanomaterials is the

increase in ratio of surface area to volume. The surface area of particle increases

exponentially, creating more sites for bonding, catalysis or reaction with surroundingmaterial, resulting in improved properties such as increased strength or chemical or heat

resistance. Hence due to the high surface to volume ratio associated with nanometer sized

particles, it is possible to control the fundamental properties of materials through

surface/size effect.


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2.3. Characterization of nanomaterials

Materials in the nanometer scale, such as colloidal dispersions and thin films, have been

studied over many years and many physical properties related to the nanometer size, such

as coloration of gold nanoparticles, have been known for centuries. One of the critical

challenges faced currently by researchers in the nanotechnology and nano science fields

is the inability and the lack of instruments to observe, measure and manipulate the

materials at the nanometer. In the past, the studies have been focused mainly on the

collective behaviors and properties of a large number of nano structured materials. A

better fundamental understanding and various potential applications increasingly demand

the ability and instrumentation to observe measure and manipulate the individual

nanomaterials and nanostructures.

Characterization and manipulation of individual nanostructures require not only extremesensitivity and accuracy, but also atomic-level resolution. It therefore leads to various

microcopies that will play a central role in characterization and measurements of nano

structured materials and nanostructures. The development of novel tools and instruments

is one of the greatest challenges in nanotechnology.

In this chapter, various structural characterization methods that are most widely used in

characterizing nanomaterials and nano structures are mentioned. These include: X-ray

diffraction (XRD) [2] [3], various electron microscopy (EM) including scanning electron

microscopy (SEM) and transmission microscopy (TEM) [4] [5] [6] [7], and scanning probe

microscopy (SPM) [8].

2.3.1. Structural CharacterizationCharacterization of nanomaterials and nanostructures has been largely based on the

surface analysis techniques and conventional characterization methods developed for

bulk materials. For example, XRD has been widely used for the determination of

crystallinity, crystal structures and lattice constants of nanoparticles, nano wires and thin

films; SEM and TEM together with electron diffraction have been commonly used in

characterization of nanoparticles; optical spectroscopy is used to determine the size of

semiconductor quantum dots. SPM is a relatively new characterization technique and has

found wide spread applications in nanotechnology. The two major members of the SPM

family are scanning tunneling microscopy (STM) and atomic force microscopy (AFM).


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Although both STM and AFM are true surface image techniques that can produce

topographic images of a surface with atomic resolution in all three dimensions,

combining with appropriately designed attachments, the STM and AFM have found a

much broadened range of applications, such as nano indentation, nanolithography (as

discussed in the previous chapter), and patterned self-assembly. In the following, we

mentioned characterization techniques in nanotechnology.

1. X-ray diffraction (XRD)

2. Scanning electron microscopy (SEM)

3. Transmission electron microscopy (TEM)

4. Scanning probe microscopy (SPM)

5. Gas adsorption

2.3.2. Chemical CharacterizationChemical characterization is to determine the surface and interior atoms and compounds

as well as their spatial distributions. As mentioned in the introduction section, many

chemical analysis methods have been developed for the surface analysis or thin films, but

are readily applicable to the characterization of nanostructures and nanomaterials.

2.3.2.1.Optical Spectroscopy

1. Absorption and transmission spectroscopy

2.

Photoluminescence

3. Infrared spectroscopy

4. Raman spectroscopy

2.3.2.2.Electron spectroscopy

1. Energy dispersive X-ray spectroscopy

2. Auger electron spectroscopy

3. X-ray photo electron spectroscopy

2.3.2.3.Ionic spectroscopy1. Rutherford back scattering spectroscopy2. Secondary ion mass spectroscopy


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2.4.Properties of nano materials

Between the dimensions on an atomic scale and the normal dimensions, which

characterize bulk material is a size range where condensed matter exhibits some

remarkable specific properties that may be significantly different from the physical

properties of bulk materials. Some such peculiar properties are known, but there may be a

lot more to be discovered. Some known physical properties of nanomaterials are related

to different origins: for example, (i) large fraction of surface atoms, (ii) large surface

energy, (iii) spatial confinement, and (iv) reduced imperfections. The following are just a

few examples:

(1)Nano materials may have a significantly lower melting point or phase transition

temperature and appreciably reduced lattice constants, due to a huge fraction of

surface atoms in the total amount of atoms.(2)Mechanical properties of nanomaterials may reach the theoretical strength, which are

one or two orders of magnitude higher than that of single crystals in the bulk form.

The enhancement in mechanical strength is simply due to the reduced probability of

defects.

(3)Optical properties of nanomaterials can be significantly different from bulk crystals.

For example, the optical absorption peak of a semiconductor nano particle shifts to a

short wavelength, due to an increased band gap. The color of metallic nanoparticles

may change with their sizes due to surface Plasmon resonance.

(4)Electrical conductivity decreases with a reduced dimension due to increased surface

scattering. However, electrical conductivity of nano materials could also be enhanced

appreciably, due to the better ordering in microstructure, e.g. in polymeric fibrils.

(5)Magnetic properties of nano structured materials are distinctly different from that of

bulk materials. Ferromagnetism of bulk materials disappears and transfers to super

paramagnetism in the nanometer scale due to the huge surface energy.

(6)

Self-purification is an intrinsic thermodynamic property of nanostructures and

nanomaterials. Any heat treatment increases the diffusion of impurities, intrinsic

structural defects and dislocations, and one can easily push them to the nearby

surface. Increased perfection would have appreciable impact on the chemical and

physical properties. For example, chemical stability would be enhanced.


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2.5.Applications of Nanomaterials

1. Nano electronics

2. Nano medicines

3. Biological applications of nano particles

4. Catalysis by Gold particles

5. Band gap engineered quantum devices

6. Quantum well and quantum dot devices

7. Carbon nano tubes emitter

8. Photo-electro chemical cells


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Chapter 3

Nano Magnetism

3.1. Introduction

Nanomagnetism describes the science and technology underlying the magnetic behavior

of nano structured (1100 nm) systems. It focuses on the magnetic behavior of individual

building blocks of nano structured systems as well as on combinations of individual

building blocks that display collective magnetic phenomena. A fundamental

understanding of nanomagnetism can be exploited to yield integrated systems with

complex structures and architectures that possess new functionalities. A nanomagnet is a

sub micro metric system that presents spontaneous magnetic order (magnetization) atzero applied magnetic fields (remanence). The small size of nano magnets prevents the

formation of magnetic domains.

The magnetization dynamics of sufficiently small nano magnets at low temperatures,

typically single-molecule magnets, presents quantum phenomena, such as macroscopic

spin tunneling. At larger temperatures, the magnetization undergoes random thermal

fluctuations (super paramagnetism) which present a limit for the use of nano magnets for

permanent information storage.

Canonical examples of nano magnets are grains [9] [10]of ferromagnetic metals

(iron, cobalt, and nickel) and single-molecule magnets. [11]The vast majority of nano

magnets feature transition metal (titanium, vanadium, chromium, manganese, iron, cobalt

or nickel) or rare earth (Gd, Eu, and Er) magnetic atoms.

3.2.Challenges in nano magnetism

The grand challenges in nanomagnetism can be expressed in a number of ways

depending on the audience.[12] At the most basic level the challenge is to create, explore

and understand new materials that exhibit unexpected collective behavior, also known as

emergent behavior. The creation of emergent matter energizes the synthesis community,

and high-quality synthesis of new materials is critical to the future of all of condensed

matter and materials physics. Nano science offers exciting new routes to create advanced

materials and hierarchically assemble systems in a non-Edisonian manner. Utilizing the


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three basic tools of nano science - geometric confinement, physical proximity and self-

organization - one can create materials by design. Confinement creates new properties

from known materials via manipulation of dimensionality. Proximity effects allow multi-

component composites to behave as new materials that embrace properties that are often

mutually exclusive and thus not found in single component systems. Self-organization

utilizes rules of Nature to segregate and/or assemble systems on ultra small length scales

that transcend the limits of present-day Litho graphic definition. Self-assembly shows

promise of becoming a powerful approach to miniaturize structures in an affordable

manner with very high uniformity. A challenge is to utilize self-organization, as is

accomplished in biological systems, to create not just component materials, but fully

functional, integrated systems. One can imagine pulling an entire computer processor out

of a test tube in the future. Achieving such goals certainly encompasses non-equilibriumsynthetic routes, and the creation of multifunctional materials. Competing interactions

and the preponderance of low-lying energetic states and quantum fluctuations help create

the complexity that gives rise to unanticipated phenomena in magnetic nano systems. We

note that, during the exploration of magnetic surfaces, interfaces and multi layers

undertaken a few decades ago, the discoveries of spin-valve read heads or magnetic

random access memory was completely unanticipated. This realization underscores the

fact that fundamental studies involving the creation of emergent matter is an important

stimulant of advanced technologies. The second grand challenge in nanomagnetism,

beyond the creation of emergent matter, is the exploration of its magnetic properties. This

task requires all levels of characterization tools. Essential in this is the utilization of the

advanced user facilities that are available to the research community. This underscores an

important synergy between materials creation and exploration that requires national

investments both in synthesis and in major user facilities. Additionally, understanding

emergent behavior requires the talents of the theory community and the utilization of

high-performance computing to simulate complex behavior. This challenge has many

facets. It includes: (i) defining the rules that govern self-organization; (ii) understanding

spin dynamics and equilibrium states at the spatial and temporal limits of interest; (iii)

understanding spin transport phenomena, spin-torque effects, spin accumulation, spin

diffusion, etc.; and (iv) relating such effects to the detailed structure of the material. This


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challenge also involves understanding how to create entangled spin systems, and how to

control the process as well as read the states. A longer-term aspect of the challenge is to

foster communication of information via spin currents and excitations without the flow of

charge. This would be a novel approach to complement existing electronic circuitry that

depends on charge flow.

Why create, explore and understand nano magnetic emergent matter? An important

reason is that the more detailed understanding which is the goal of basic science will

enable the community to address the strategic needs of our society at large. It is vital to

stimulate the economy and create new jobs by ushering in new technologies and

innovations. New diagnostic tools and treatments for diseases will preserve health and

wellbeing, as well as heal the ill and help the impaired. New sensors will help ensure

homeland security, and advanced materials and approaches will provide for nationaldefense needs. It is important to create materials and harness phenomena that will

advance transportation requirements, and it is imperative to take leadership in meeting

national goals of energy independence. Additionally, all of these initiatives must be

advanced in a manner that is respectful of the environment. These are goals for the

nanomagnetism community, for the nano science and nanotechnology communities, and

for the larger materials research communities. The nanomagnetism community is in a

particularly advantageous position to rise to these challenges because of the enormous

potential derived from recent breakthroughs. Although magnetism is one of the oldest

sciences known, it is a driving force in the new scientific era of nanotechnology. This

statement is highlighted quite succinctly by examining the evolution of the Giant

Magneto Resistance (GMR) effect, which made the transition from a laboratory curiosity

to a technology driver within the short span of about ten years. From GMR, a host of new

phenomena, materials, and potential technologies are blossoming. [13]

3.3.Dimensionality in nano magnetismDimensionality is a general term which is used to describe the effect of size confinement

on the physical properties of matter. It becomes very important, when the properties of

matter are discussed on the nanometer scale. In general, one divides nanostructures into

three different groups including: thin films (2D structures), nano wires (1D structure),


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and nano dots or nanoparticles (0D structures). The magnetization of these different

groups is significantly influenced by confinement.

3.3.1. Thin magnetic filmsThe electronic structure at the surface of a given material differs considerably from the

bulk. Early studies of nickel mono layers on copper substrates show significant changes

in the density of states as a function of surface energy. The different coordination number

of atoms and the number of incomplete bonds at the surface modifies the local density of

states. The d orbital is more localized and interacts more strongly with spin magnetic

moments giving a higher net magnetization at the surface. This is an example of size

effect in one dimension. The magnetization reversal in thin ferromagnetic films is

affected by such size confinement. The nucleation and propagation of magnetic domains

which involve Bloch walls in the bulk is more likely to involve Nel walls when thethickness decreases. Thickness confirms the tendency of the formation of Nel walls in

the thinner film. In thin films with uniaxial in-plane anisotropy, magnetic domains form

stripes and are separated by Nel walls for thicknesses below 50 nm. In thin films with

perpendicular anisotropy, circular magnetic domains form which are highly mobile.

These magnetic bubbles hold potential for magnetic storage technology.[14]

3.3.2. Nano wires or one dimensional magnetsFundamental interest in arrays of ferromagnetic nano wires lies in the emergence of novel

magnetic and transport properties as the dimension approaches the length scale of a few

nano meters to a few tens of Nano meters. Current interest in research on ferromagnetic

nano wires is stimulated by the potential application to future ultra-high-density magnetic

recording media.[15]The controlled production of magnetic nano wire arrays with

outstanding characteristics is important to control the magnetization process. Free

standing nanowires can be fabricated by different methods such as lithography and

template electrodeposition [16] [17] [18] [19] [20]. The smaller diameter wires have high out-of-

plane remanence while larger diameter wires have low remanence. This is consistent with

the predictions of a micro magnetic model, which indicates a change from a flower (or

single domain) to a vortex remanent state with increasing diameter. The flowervortex

transition occurs at a diameter of 3.5 times of exchange length for cylinders without any

magneto crystalline anisotropy [21]. However, it is important to note that the remanence


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only decreases slowly as the vortex develops, and high aspect ratio vortex state particles

can still have significant remanence.

3.3.3. Nanodots and superparamagnetismCompetition between exchange and anisotropy constants [22] induces a critical size of

nanodots. The state of lowest free energy of a ferromagnetic particle is one of uniform

magnetization below a certain critical size. For larger particles, the magnetization is non-

uniform. These two modes are known to correspond to single domain and vortex states

which have been studied extensively over last decade [23] [24]. Arrays of magnetic

nanodots have been fabricated, the magnetization curves obtained for the arrays of

circular dots exhibiting the two reversal modes. The effect of diameter and thickness is

quite important. As the size approaches the critical radius, the single domain mode is

predominant [25]. Nanodots smaller than the critical radius, tend to give up ferromagneticalignment, becoming superparamagnetic.

Figure: 1 M vs. H curve for superparamagnetism

This behavior takes place when the fluctuations of magnetic moments caused by the

thermal energy distort the intrinsic alignment of the ferromagnet. Considering a single

domain particle with two distinct magnetic states, when the thermal energy becomes

comparable with the energy barrier between the two states, superparamagnetism is

established.


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3.4.Magnetic anisotropy

The term magnetic anisotropy is used to describe the dependence of the internal energy

on the direction of the spontaneous magnetization, creating easy and hard directions of

magnetization. The total magnetization of a system will prefer to lie along the easy axis.

The energetic difference between the easy and hard axis results from two microscopic

interactions: the spin-orbit interaction and the long-range dipolar coupling of magnetic

moments. The anisotropy energy arises from the spin-orbit interaction and the partial

quenching of the angular momentum. The spin-orbit coupling is responsible for the

intrinsic (magneto crystalline) anisotropy, surface anisotropy, and magneto striction,

while the shape anisotropy is a dipolar contribution and is calculated e.g. by assuming a

uniform distribution of magnetic poles on plane surfaces. Anisotropy energies are usually

in the range 102-107 Jm-3. This corresponds to energy per atom in the range 10-8-10-3eV. The anisotropy energy is larger in lattices (of magnetic ions) of low symmetry and

smaller in lattices of high symmetry. In bulk materials, magneto crystalline and magneto

static energies are the main source of anisotropy whereas in fine particles, thin films and

nano structures, other kinds of anisotropies such as shape and surface anisotropy are

relevant in addition to these usual anisotropies. In the following we will discuss four

different contributions to magnetic anisotropy: magneto crystalline anisotropy, shape

anisotropy, strain anisotropy and surface anisotropy.

3.4.1. Magneto crystalline anisotropyMagnetic anisotropy is meant as the dependence of the internal energy on the direction of

spontaneous magnetization. An energy term of this kind is called as magnetic anisotropy

energy. Generally the magnetic anisotropy energy term possesses the crystal symmetry of

the material, and known as crystal magnetic anisotropy or magneto crystalline anisotropy[26]. The simplest forms of crystal anisotropies are the uniaxial anisotropy in the case of a

hexagonal and the cubic anisotropy in the case of a cubic crystal. For example, hexagonal

cobalt exhibits uniaxial anisotropy, which makes the stable direction of internal

magnetization (or easy direction) parallel to the c axis of the crystal at room temperature.

3.5. Single domain

In magnetism, it refers to the state of a ferromagnet [27] in which the magnetization does

not vary across the magnet. A magnetic particle that stays in a single domain state for all


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magnetic fields is called a single domain particle [28]. They are very important in a lot of

applications because they have a high coercivity. They are the main source of hardness in

hard magnets, the carriers of magnetic memory in tape drives, and the best recorders of

the ancient Earth's magnetic field.

3.5.1. History & Definitions of a single-domain particleEarly theories of magnetization in ferromagnets assumed that ferromagnets are divided

into magnetic domains and that the magnetization changed by the movement of domain

walls. However, as early as 1930, Frenkel and Dorfman predicted that sufficiently small

particles could only hold one domain, although they greatly overestimated the upper size

limit for such particles [29]. The possibility of single domain particles received little

attention until two developments in the late 1940s:

(1)

Improved calculations of the upper size limit by Kittel and Nel, and(2)A calculation of the magnetization curves for systems of single-domain particles by

Stoner and Wohlfarth [30] [31].

Early investigators pointed out that a single-domain particle could be defined in more

than one way [32]. Perhaps most commonly, it is implicitly defined as a particle that is in a

single-domain state throughout the hysteresis cycle, including during the transition

between two such states. However, it might be in a single-domain state except during

reversal. Often particles are considered single-domain if their saturation remanence is

consistent with the single-domain state. More recently, it was realized that a particle's

state could be single-domain for some range of magnetic fields and then change

continuously into a non-uniform state [33]. Another common definition of single-domain

particle is one in which the single-domain state has the lowest energy of all possible

states.

3.5.2. Limits on the single-domain sizeExperimentally, it is observed that though the magnitude of the magnetization is uniform

throughout a homogeneous specimen at uniform temperature, the direction of the

magnetization is in general not uniform, but varies from one region to another, on a scale

corresponding to visual observations with a microscope. Uniformity of direction is

attained only by applying a field, or by choosing as a specimen, a body which is itself of

microscopic dimensions (a fine particle). The size range for which a ferromagnet become


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single-domain is generally quite narrow and first quantitative results in this direction are

due to William Fuller Brown, Jr. who, in his fundamental paper [34]rigorously proved (in

the framework of Micro magnetic. This paper states the existence of a critical radius such

that the state of lowest free energy is one of uniform magnetization if the existence of a

critical size under which spherical ferromagnetic particles stay uniformly magnetized in

zero applied field.

Although pure single-domain particles (mathematically) exist for some special geometry

only, for most ferromagnets a state of quasi-uniformity of magnetization is achieved

when the diameter of the particle is in between about atomic meters and nanometers. The

size range is bounded below by the transition to super paramagnetism and above by the

formation of multiple magnetic domains.

1) Lower limit: super paramagnetismThermal fluctuations cause the magnetization to dance around in a random manner. In the

single domain state, the moment rarely strays far from the local stable state. Energy

barriers prevent the magnetization from jumping from one state to another. However, if

the energy barrier gets small enough, the moment can jump from state to state frequently

enough to make the particle super paramagnetic. The frequency of jumps has a strong

exponential dependence on the energy barrier, and the energy barrier is proportional to

the volume, so there is a critical volume at which the transition occurs. This volume can

be thought of as the volume at which the blocking temperature is at room temperature.

2) Upper limit: transition to multiple domainsAs size of a ferromagnet increases, the single-domain state incurs an increasing energy

cost because of the demagnetizing field. This field tends to rotate the magnetization in a

way that reduces the total moment of the magnet, and in larger magnets the

magnetization is organized in magnetic domains. The demagnetizing energy is balanced

by the energy of the exchange interaction, which tends to keep spins aligned. There is a

critical size at which the balance tips in favor of the demagnetizing field and the multi

domain state is favored. Most calculations of the upper size limit for the single-domain

state identify it with this critical size [35] [36] [37].


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3.5.3. Domain formationNext is to focus on how domain formation occurs in an initially saturated specimen. In

general this process constitutes a very considerable resistance to the process of

demagnetization in many specimens. Saturation is expected in a magnetic specimen when

the demagnetizing fields can be overcome in certain magnetic fields. However, the real

demagnetizing fields are non-uniform over the volume of the specimen. Usually the end

regions are much more difficult to saturate than the bulk of the crystal, and residual

domains persist near the ends until external magnetic fields can completely make

saturation [38]. The demagnetizing effect of the end surfaces can be eliminated assuming a

ring-shaped specimen; however residual domains are still expected to be stabilized by

pores, inclusions and grain boundaries. Considerable demagnetizing fields can arise from

grain boundaries and this requires higher fields which are considerably higher than bulk

saturation. A critical field designated as Hn (nucleation fields) may be needed some time

to start nucleation of domains. However it is quite possible that critical fields may

represent the initiation of wall motion rather than the nucleation of the walls, and in this

case they may be designated as starting fields (Hs).

3.6.Domain walls

Domain walls are interfaces between regions in which the spontaneous magnetization has

different directions. At or within the wall the magnetization must change direction. A

simplistic picture of a domain wall which makes an abrupt change between two domains.

For this ferromagnetic specimen the easy axis is y and a row of atoms is shown parallel

to x-axis, with the 180 domain wall lying in the y-z plane. In this case the domain wall

will have a large exchange energy associated with it because the spins adjacent to the

wall are anti-parallel and the exchange energy in a ferromagnet is a minimum only when

adjacent spins are parallel.

3.7.Applications3.7.1. Applications in R&DMagnetic nano-particle applications are being investigated in multiple disciplines from

biomedical sensors, drug delivery, magnetic resonance imaging, data storage, nano-

electronics, etc. Two applications, which are being heavily investigated by several

research groups, are: detoxification of contaminated personnel for the military and drug


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delivery and treatment. Current techniques in both of these fields are very limited and

minimally robust and nano-technology has the potential to enhance the capabilities in

both fields immensely.

3.7.2. Detoxification of Contaminated Personnel MilitaryBiochemical warfare is steadily becoming a greater threat worldwide. In the case of a

conflict where a soldier is exposed to toxins there are very few options for survival.

Current medical detoxification treatment methods available are dialysis and blood

purification, both of which have low rates of success and take extensive periods of time

to undergo. Magnetic nano-particles are being investigated for their capability to bind

with toxins within the body. The various synthesis techniques including the thermal

decomposition of metal-carbonyl complexes have shown that magnetic nano-particles are

stabilized by an organic surfactant which acts as a capping layer. If a chemically activecapping layer, which will functionally bind to the toxins within the body, can be utilized,

then magnetic nano-particles can be sent into the bloodstream of a contaminated person.

Then, by utilizing a magnetic field gradient, toxins can be extracted from the body. In

research this methodology has been labeled tag and drag and is depicted in Figure.

Figure 2: Tag and Drag Detoxification of Contaminated Personnel via Magnetic Nano-particles.

Current research has shown success in this tag and drag approach using gold coated iron,

nickel, and cobalt ferromagnetic nano-particles. The next phase of research is identifying

pre-dominant toxins, functional organic capping layers, and bio-compatibility.

3.7.3. Drug Delivery and EfficacyDrug delivery and effective application of drugs within the body is another heavily

investigated application for magnetic nano-particles. Researchers are particularly


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investigating ferromagnetic nano-particles with respect to treatment of various cancers.

The goal is to have medicines functionally bind to magnetic nano-particles and utilize a

magnetic gradient to guide the nano-particle to the affected region. Once at the affected

region, the hysteretic behavior of ferromagnetic particles is utilized to provide thermal

activation. Hysteresis occurs in ferromagnets because when an external field is applied

and removed, a portion of the field is absorbed by the ferromagnet, called the remnant

magnetization. When a reverse field is applied, this remnant magnetization has to be

overcome. This effectively yields an energy loss in the form of heat which is

proportional to the area within the hysteresis loop. Researchers are utilizing this thermal

energy for localized heating within the body. Once the medicine has been guided via a

magnetic gradient to a desired location, the magnetic field is varied to create hysteresis.

This in effect causes localized heating that helps in the activation and completion of achemical reaction between the applied medicine and the adversely affected area. Current

research is attempting to develop magnetic nano-particles with large magnetic moments

and resistance to physical breakdown within the body.

3.7.4. Information storageResearch is going into the use of using MNPs for magnetic recording media. The most

promising candidates for high-density storage are the face-centered tetragonal phase Fe

Pt alloy. Grain sizes can be as small as 3 nanometers. If it is possible to modify the MNPs

at this small scale, the information density that can be achieved with this media could

easily surpass 1 Terabyte per square inch.

3.8.Future Research and Development

For magnetic nano-particles, current and future research is geared towards size

distribution control. For most applications it is critical to have a specific size as well as

very tight distribution. Current research is pushing to synthesize super-lattices of

magnetic nano-particles which show size distribution of less than 5%. For theapplications discussed in this paper, future research is being geared towards discovering

biocompatible capping layers. This is essential in order to utilize magnetic nano-

particles in vivo.


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3.9. Bismuth ferrite

3.9.1. Introduction toBiFeO3BiFeO3 (BFO) is an inorganic chemical compound with aperovskite structure. It is one

of the most promising lead-free piezoelectric materials by exhibiting multi ferroic

properties at room temperature. Multiferroic materials exhibit ferroelectric or anti

ferroelectric properties in combination with ferromagnetic (or antiferromagnetic)

properties in the same phase. BiFeO3 is the only prototype among all other multi ferroic

oxides which shows both ferromagnetism and ferro electricity in a single crystal above

room temperature. It has ferroelectric Curie temperature Tc = 1143K and

antiferromagnetic Nel temperature TN= 643K. The ions responsible for the production of

ferro electricity and magnetism are Bi3+ and Fe+3 ions. Ferro electricity is produced due to

Bi3+ and anti ferromagnetism is due to Fe+3 ions [39].

3.9.2. Crystal structure of BiFeO3 It is having rhombohedrally distorted perovskite structure with R3c space group at

room temperature.

Figure 3: BiFeO3 in Perovskite structure

Bi3+ ion occupy the corner position, Fe3+ in the body centered position, and O2- in all

face centered position. The lattice parameters are a = 5.587 , b = 5.587 and c =

13.867 with = = 900and =1200. The hexagonal unit cell contains 6 formulas.

Generally ferroelectricity is produced by vacant d0 orbital and ferromagnetism is

observed due to partially filled dn orbital. In BiFeO3, ferroelectricity is produced due to

stereo chemical activity of Bi3+ ion and ferromagnetism is due to Fe+3ions[39].
http://en.goldenmap.com/Inorganichttp://en.goldenmap.com/Chemical_compoundhttp://en.goldenmap.com/Perovskitehttp://en.goldenmap.com/Piezoelectrichttp://en.goldenmap.com/Multiferroichttp://en.goldenmap.com/Multiferroichttp://en.goldenmap.com/Ferroelectrichttp://en.goldenmap.com/Anti-ferroelectrichttp://en.goldenmap.com/Anti-ferroelectrichttp://en.goldenmap.com/Ferromagnetichttp://en.goldenmap.com/Antiferromagnetichttp://en.goldenmap.com/Antiferromagnetichttp://en.goldenmap.com/Ferromagnetichttp://en.goldenmap.com/Anti-ferroelectrichttp://en.goldenmap.com/Anti-ferroelectrichttp://en.goldenmap.com/Ferroelectrichttp://en.goldenmap.com/Multiferroichttp://en.goldenmap.com/Multiferroichttp://en.goldenmap.com/Piezoelectrichttp://en.goldenmap.com/Perovskitehttp://en.goldenmap.com/Chemical_compoundhttp://en.goldenmap.com/Inorganic


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Production of ferroelectricity in bismuth ferrite

Bi3+ and Fe3+cations displaced along the [1 1 1] threefold polar axis and off centered with

respect to the body centre of the oxygen octahedron, which in turn gives rise to

ferroelectricity.

Production of ferromagnetism in bismuth ferrite

It is produced due to rotations of adjacent oxygen FeO6 octahedral around the [1 1 1]

pseudo cubic direction (figure 3).

3.9.3. Applications of BiFeO31. Due to multiferroic nature of BiFeO3 it has broader applications in the field of

transducers, magnetic field sensors and information storage industry.

2. Due to its magneto electric coupling it has the advantage that data can be written

electrically and read magnetically. It exploits the best aspects of ferroelectricrandom access memory (Fe - RAM) and magnetic data storage. Multiferroism

leads to fast, low-power consumption, multifunctional memory devices exploiting

the best attributes of conventional ferroelectric and magnetic random-access

memories.

3. Recently, ferroelectric random access memories (Fe RAMs) have achieved fast

access speeds (5 ns), high densities (64 Mb)[39].

3.10. SummaryOngoing research has shown that synthesis of magnetic nano-particles is achievable. The

applications for magnetic nano-particles are very diverse. In order to make use of the

potential of magnetic nano-particles, work in synthesis refinement is necessary, to control

geometric and magnetic properties. From an application aspect, research is being geared

to find functional surfactants which serve to meet application requirements.

Biocompatibility in the case of drug deliver and detoxification applications and

conductivity in the case of electronic applications are examples of such. As researchevolves and solutions to current limitations are resolved, the integration of magnetic

nano-particles into everyday use becomes a clearer reality.


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