Download I INTRODUCTION TO MAGNETISM AND MAGNETIC MATERIALS

CHAPTER - I
INTRODUCTION TO MAGNETISM AND MAGNETIC MATERIALS
1.1 Magnetism
Magnetic phenomena have been known and exploited for many centuries. The
earliest experiences with the magnetism involved Magnetite, the only material that
occurs naturally in a magnetic state. This mineral was also known as Lodestone,
after its property of aligning itself in certain directions if allowed to rotate freely,
thus being able to indicate the positions of North and South, and to some extent
also latitude. The other well known property of Lodestone is that two pieces of it
can attract or even repel each other.
After the production of iron from its ores had become possible, it was realized that
magnetite could also attract iron. There are many magnetic materials known today,
and it is therefore useful first of all to give a very important rule for what is called
magnetic material.
If two objects attract each other and also repel each other (depending on their
relative operations) then those objects might be called magnets. There are also
other objects that are attracted to, but not repelled by magnets, and are not attracted
or repelled by each other. Such objects are said to consist of magnetic materials [1,
2].
Origin of Magnetism:
The macroscopic magnetic properties of materials are consequence of magnetic
moments associated with individual electrons. Each electron in an atom has
magnetic moments that originate from two sources. One is related to its orbital
motion around the nucleus; being a moving charge and electron may be considered
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to be a small current loop, generating a very small magnetic field, and having a
magnetic moment along its axis of rotation. Each electron may also be thought of
as spinning around an axis the other magnetic moment originates from this electron
spin, which is directed around the spin axis. Spin magnetic moments may be only
in an “up” direction or in antiparallel “down” direction. Thus each electron in an
atom can be thought of as being a small permanent magnet having orbital spin
magnetic moments in each individual atom, orbital moments of some electron pairs
cancel each other; this also holds for spin moments. For example, the spin moment
of one electron with spin up will cancel the one with spin down. The net magnetic
moment, then, for an atom is the sum of magnetic moments of each constituent
electron, including both orbital and spin contributions, and taking into account
moment calculations for an atom having completely filled electron shells or
subshells, when all electrons are considered, there is total cancellation of both total
and spin moments. Thus materials are composed of atoms having completely filled
electron shells are not capable of being permanently magnetized. This category
includes the inert gases (Ar, Ne, He etc) as well as some ionic materials [3].
Fig.1.0: Origin of Magnetism
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1.2 Theory of Magnetism:
Magnetism, the power of attracting iron by a material, is known to mankind for
centuries before Christ. The oldest magnetic material or simply magnet, so called
magnetite (Fe3O4) is a mineral was initially found in the district of Magnesia of the
modern Turkey. The word magnet is a Greek word and known from the name of
district. Almost everyone is familiar with what a magnetic material can do but very
few know how a magnet works. The magnetic properties of materials are entirely
due to the motion of electrons of the atoms. To understand this phenomenon one
must first grasp the inextricable connections that exist between magnetism and
electricity. A simple electromagnet can be produced by wrapping copper wire into
the form of a coil and connecting the wire to a battery. A magnetic field is created
in the coil but it remains there only while electricity flows through the wire. The
field created by the magnet is associated with the motions and interactions of its
electrons, the minute charged particles which orbit the nucleus of each atom.
Electricity is the movement of electrons, whether in a wire or in an atom, so each
atom represents a tiny permanent magnet in its own right. The circulating electron
produces its own orbital magnetic moment, measured in Bohr magnetrons (µB),
and there is also a spin magnetic moment associated with it due to the electron
itself spinning, like the earth, on its own axis (illustrated in fig.1) [4]. In most
materials there are resultant magnetic moments, due to the electrons being grouped
in pairs causing the magnetic moment to be cancelled by its neighbour. In a certain
magnetic material the magnetic moments of a large proportion of the electrons
align, producing an unfilled magnetic field. The field produced in the material (or
by an electromagnet) has a direction of flow and any magnet will experience a
force trying to align it with an externally applied field, just like a compass needle.
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Fig 1.1 (a, b): Origin of magnetism-(a) orbital magnetic moment
(b) Spin magnetic moment
These forces are used to drive electric motors, produce sounds in a speaker system,
control the voice coil in a CD player, etc. The interactions between magnetism and
electricity are therefore an essential aspect of many devices we use every day. The
magnetic moments of the electrons are so oriented that they cancel one another out
and the atom as a whole has no net magnetic moment. This leads to diamagnetism
and the cancellation of magnetic moment is only partial and the atom is left with a
net magnetic moment and the atom is called a magnetic atom. This leads to
Paramagnetism, Ferromagnetism, Ferrimagnetism and Antiferromagnetism [5, 6].
Weber's Theory
A popular theory of magnetism considers the molecular alignment of the material.
This is known as Weber's theory. This theory assumes that all magnetic substances
are composed of tiny molecular magnets. Any unmagnetized material has the
magnetic forces of its molecular magnets neutralized by adjacent molecular
magnets, thereby eliminating any magnetic effect. A magnetized material will have
most of its molecular magnets lined up so that the north pole of each molecule
points in one direction and the south pole faces the opposite direction. A material
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with its molecules thus aligned will then have one effective north pole, and one
effective south pole. When a steel bar is stroked several times in the same direction
by a magnet, the magnetic force from the north pole of the magnet causes the
molecules to align themselves.
Domain Theory
A more modern theory of magnetism is based on the electron spin principle. From
the study of atomic structure it is known that all matter is composed of vast
quantities of atoms, each atom containing one or more orbital electrons. The
electrons are considered to orbit in various shells and sub shells depending upon
their distance from the nucleus. The structure of the atom has previously been
compared to the solar system, wherein the electrons orbiting the nucleus
correspond to the planets orbiting the sun. Along with its orbital motion about the
sun, each planet also revolves on its axis. It is believed that the electron also
revolves on its axis as it orbits the nucleus of an atom.
An atom with an atomic number of 26, such as iron (Fe), has 26 protons in the
nucleus and 26 revolving electrons orbiting its nucleus. If 13 electrons are spinning
in a clockwise direction and 13 electrons are spinning in a counterclockwise
direction, the opposing magnetic fields will be neutralized. When more than 13
electrons spin in either direction, the atom is magnetized. Fe has a structure of
(1s22s22p63s23p6)3d64s2 with a net moment of 4 mb, In minerals, the transition
elements are in a variety of oxidation states. Fe commonly occurs as Fe2+ and Fe3+.
When losing electrons to form ions, transition metals lose the 4s electrons first, so
we have for example, Fe3+ with a structure of (1s22s22p63s23p6)3d5, or 5 mb.
Similarly Fe2+ has 4 mb and Ti4+ has no unpaired spins. Iron is the main magnetic
species in geological materials, but Mn2+ (5 mb) and Cr3+ (3 mb) occur in trace
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amounts. The elements with the most unpaired spins are the transition elements
which are responsible for most of the paramagnetic behavior observed in rocks.
1.3 Kinds of Magnetism
When a material is placed within a magnetic field, the magnetic forces of the
material’s electrons will be affected. This effect is known as Faraday’s law of
magnetic induction. However, materials can react quite differently to the presence
of an external magnetic field. This reaction is dependent on a number of factors,
such as the atomic and molecular structure of the material and the net magnetic
field associated with the atoms.
The magnetic moments associated with the atoms have three origins. These are the
electron orbital motion, the change in orbital motion caused by an external
magnetic field and the spin of the electrons. In most atoms, electrons occur in
pairs. Electrons in a pair spin in opposite directions. So, when electrons are paired
together, their opposite spins cause their magnetic fields to cancel each other.
Therefore, no net magnetic fields exist. Alternately, materials with some unpaired
electrons will have a net magnetic field and will react more to an external field.
Most materials can be classified as diamagnetic, paramagnetic, ferromagnetic,
antiferromagnetic and ferrimagnetic.
All materials can be classified in terms of their magnetic behavior falling into one
of five categories depending on their bulk magnetic susceptibility. The two most
common types of magnetism are diamagnetism and paramagnetism, which account
for the magnetic properties of most of the periodic table of elements at room
temperature (Fig 1.2).
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Fig 1.2: A periodic table showing the type of magnetic behavior of each
element at room temperature.
These elements are usually referred to as nonmagnetic, whereas those which are
referred to as magnetic are actually classified as ferromagnetic. The other type of
magnetism observed in pure elements at room temperature is antiferromagnetism.
Finally, magnetic materials can also be classified as ferrimagnetic although this is
not observed in any pure element but can only be found in compounds, such as the
mixed oxides, known as ferrites, from which ferrimagnetism derives its name. The
value of magnetic susceptibility falls into a particular range for each type of
material.
The classification of magnetic materials is based on how they respond to magnetic
fields. Although as surprising as it may sound, all matter is magnetic to varying
degrees. The main delineating factor is that in some materials there is no collective
long range interaction between atomic magnetic moments, whereas in other
materials there is a very strong interaction [7, 8].
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Diamagnetism: Materials such as quartz, water, acetone, copper, lead and carbon
dioxide are diamagnetic. These materials are very weakly affected by magnetic
fields. To the extent that they are affected, they become magnetically polarized in
the opposite direction from the magnetic field. If the magnetic field is not uniform,
they feel a force away from the higher field region. Diamagnetism results from the
effects of magnetic fields on all of the electrons in the material. Thus, all materials
are diamagnetic. However, the other forms of magnetism are stronger than
diamagnetism, so the diamagnetism can usually be ignored unless it is the only
magnetism present.
Paramagnetism: Materials such as sodium (Na), oxygen (O), iron oxide (FeO or
Fe2O3), and platinum (Pt) are paramagnetic. They are affected somewhat more
strongly than diamagnetic materials; they become polarized parallel to a magnetic
field. Thus, in a non-uniform magnetic field, they feel a force towards the higher
field region.
Paramagnetism results from the magnetic forces on unpaired electrons. Electrons
move around atoms in orbitals and maximum of two electrons can go into each
orbital. Electrons that are alone in an orbital are said to be unpaired.
Ferromagnetism: Materials such as iron (Fe), nickel (Ni), gadolinium (Gd), iron
oxide (Fe3O4), Manganese Bismuth (Mn-Bi), and Cobalt Ferrite (CoFe2O4) are
ferromagnetic. These materials are very strongly affected by magnetic fields. They
become strongly polarized in the direction of the magnetic field, thus, they are
strongly attracted to the high field region when the field isn't uniform.
Furthermore, they retain their polarization after the magnetic field is removed.
Once polarized ferromagnetic materials produces magnetic fields of their own.
Since these fields are usually not uniform (particularly near the ends of the piece)
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ferromagnetic materials are capable of attracting each other. All of the materials
that we are used to calling "magnets" are ferromagnetic materials.
Ferromagnetism results from the interactions among the electrons in the material.
This is why a ferromagnet can remain magnetically polarized even if there is no
magnetic field applied to it from the outside. It should be no surprise that most
applications of magnetic materials call for ferromagnetic materials. These are the
ones that interact most strongly with magnetic fields. Within this category there are
several important subcategories. These have to do with how easily the magnetic
polarization (magnetization) of the material can be changed.
Ferrimagnetism: Ferrimagnetism, type of permanent magnetism that occurs in
solids in which the magnetic fields associated with individual atoms spontaneously
align themselves, some parallel, or in the same direction (as in ferromagnetism),
and others generally antiparallel, or paired off in opposite directions (as in
antiferromagnetism). The magnetic behaviour of single crystals of ferrimagnetic
materials may be attributed to the parallel alignment; the diluting effect of those
atoms in the antiparallel arrangement keeps the magnetic strength of these
materials generally less than that of purely ferromagnetic solids such as metallic
iron.
Ferrimagnetism occurs chiefly in magnetic oxides known as ferrites. The natural
magnetism exhibited by lodestones, recorded as early as the 6th century B.C., is
that of a ferrite, the mineral magnetite, a compound containing negative oxygen
ions O2- and positive iron ions in two states, iron (II) ions, Fe2+, and iron (III) ions,
Fe3+. The oxygen ions are not magnetic, but both iron ions are. In magnetite
crystals, chemically formulated as Fe3O4, for every four oxygen ions, there are two
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iron (III) ions and one iron (II) ion. The iron (III) ions are paired off in opposite
directions, producing no external magnetic field, but the iron (II) ions are all
aligned in the same direction, accounting for the external magnetism.
The spontaneous alignment that produces ferrimagnetism is entirely disrupted
above a temperature called the Curie point, characteristic of each ferrimagnetic
material. When the temperature of the material is brought below the Curie point,
ferrimagnetism revives.
Fig 1.3: Types of magnetism:
(A) Paramagnetism (B) Ferromagnetism
(C) Antiferromagnetism (D) Ferrimagnetism
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Table 1: Magnetic behavior versus values of magnetic susceptibility
Magnetic Behavior
Value of χ
Example
Diamagnetic
small and negative
Au
Cu
Paramagnetic
small and positive
Mn
Pt
Ferromagnetic
large and positive
Fe
Antiferromagnetic
small and positive
Cr
Ferrimagnetic
large and positive, function
of applied field,
microstructure dependent
Ba-ferrite
1.4 Magnetic Materials:
There are two basic types of magnetic materials: Metallic and Metallic Oxide or
ceramics, etc. The most common metallic material is the familiar laminated steel
that we see in mains power transformers. This material works well at mains
frequencies, but rapidly becomes ineffective at frequencies above, say, the audio
spectrum. The other type of metallic magnetic material can basically be described
as iron powder. The iron dust is acid treated to produce an oxide layer on the outer
surface. This oxide layer effectively insulates each iron particle from the next. The
powder is mixed with a (non-magnetic) bonding material and pressed or formed
into useful shapes, the most common being the toroid or ring core. The use of
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individual particles of iron each insulated from each other gives many of the
benefits of steel (e.g. good low frequency performance) but without the
disadvantages (e.g. high eddy-current losses). Metallic Oxide materials are called
ferrites. Ferrites are essentially ceramics; the ingredients are mixed, pre-fired,
crushed, dried, shaped and finally pressed or extruded and fired into their final
hard, brittle state. Newer ferrite materials are called rare earth types. They are
primarily used as permanent magnets. Like all ceramics they are very stable, with
the excellent characteristic of fairly high resistivity [9].
Magnetic materials are grouped into two types, soft and hard, depending on the
nature of magnetic behavior. The classification is based on their ability to be
magnetized and demagnetized, not their ability to withstand penetration and
abrasion. Soft magnetic materials are easy to magnetize and demagnetize. They
have low coercive fields. Hard magnetic materials retain their magnetization once
they are magnetized and possess large coercive fields. The characterization of soft
and hard ferrites is based upon some important parameters like:
1) The residual magnetism (remanence (Mr)), that though materials retains
when the external field is removed.
2) The saturation flux or maximum magnetic field that can be induced in
the material that is saturation magnetization (Ms).
3) The demagnetization field or the value of the external field applied in
the negative direction that residual magnetic field i.e. coercive force /
coercivity (Hc).
Both ferromagnetic and ferromagnetic materials are classified as either soft or hard
on the basis of their hysteresis characteristic.
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Soft Magnetic MaterialsSoft ferrites are class of magnetic material which easily magnetize and
demagnetize, they possess low coercive field. The low coercivity means the
material's magnetization can easily reverse direction without dissipating much
energy (hysteresis losses), while the material's with high resistivity prevents eddy
currents in the core, another source of energy loss. In addition to low coercivity,
the permeability and saturation magnetization are low for soft ferrites. The electric
and magnetic field of soft ferrite is arises from the interactions between ions
situated at different sites relative to the oxygen ions in the spinel crystalline
structure. Soft ferrites have certain advantages over other electromagnetic
materials includes their inherent high electrical resistivity which result in low eddy
current losses over wide frequency range. Because of their comparatively low
losses at high frequencies, they are extensively used in the cores of RF
transformers and inductors in applications such as switched-mode power supplies
(SMPS).
The most common soft ferrites are manganese-zinc (Mn-Zn, with the formula
MnxZn(1-x)Fe2O4) and nickel-zinc (Ni-Zn, with the formula NixZn(1-x)Fe2O4). Ni-Zn
ferrites exhibit higher resistivity than Mn-Zn and are therefore more suitable for
frequencies above 1 MHz. Mn-Zn have in comparison higher permeability and
saturation induction. Ferrites that are used in transformer or electromagnetic cores
contain nickel, zinc, and / or manganese compounds. Some of the low frequency
applications of soft ferrites include magnetic recording heads, inductor and
transformer core, filter cores magnetostrictive vibrator etc [10, 11].
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These are used in devices that are subjected to alternating magnetic field and in
which energy losses must be low. For this reason the relative area within the
hysteresis loop must be small; it is characteristically thin and narrow, as
represented in Fig.1.4 Consequently, a soft magnetic material must have a high
initial permeability and a low coercivity. A material possessing these properties
may reach its saturation magnetization with a relatively low applied magnetic field
and has low hysteresis energy losses.
Using an appropriate heat treatment, a square hysteresis loop may be produced,
which is desirable in some magnetic amplifier and pulse transformer application.
In addition soft magnetic materials are used in generator, motors dynamos and
switching circuits.
- Easy to magnetize and demagnetize.
- Remanence is minimum.
- Low coercivity
Applications:
Electromagnet, motors, transformers, relays and switching circuits etc.
Hard magnetic materialsMagnetic hardness is due to fine particles having shape and crystalline anisotropy.
A large crystalline anisotropy is characteristics of hard ferrites. Hence a large
coercivity is almost an inherent property of hard ferrite. Barium and strontium
ferrites are widely studied hard ferrites. The coercivity of these materials is more
than 3000 Oe which is far in excess compared to other materials. The hard ferrites
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(Hexagonal ferrite) are used for constructing permanent magnet. These materials
are ferrimagnetic and considering the proportion of iron within the material have
quite a low remanence (∼ 400 mT). The low remanence means that the maximum
energy product is only ∼ 40 kJm-3, which is lower than the alnicos, but due to the
high coercivity these magnets can be made into thinner sections. The hard ferrite
(Hexaferrite) finds applications in motors, generator, loud speaker, telephones,
meter switches, magnetic separators, toy, flexible and rubber magnet, magnetic
latch, magnetic levitation.
These are used in permanent magnets, which must have a high resistance to
demagnetization. In terms of hysteresis behavior, a hard magnetic material has a
high remanence, coercivity and saturation flux density, as well as a low initial
permeability and high hysteresis energy losses [12].
-Hard to magnetize and demagnetize
-Can be made into permanent magnet
-High coercivity
Applications:
Recording media, Micro-sized motors, Mini-pumps etc.
Fig.1.4 Hysteresis of Ferrites
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1.5 Introduction to Ferrite
The term “ferrites’’ derived from the Latin word for iron has different meanings
for different scientists. To metallurgists, ferrite means pure iron. To geologists,
ferrites are a group of minerals based on iron oxide. To an electrical engineer,
ferrites are also a group of materials based on iron oxide, which have particular
useful properties: magnetic properties and dielectric properties.
Ferrite is a general term used for any ferrimagnetic ceramic material. Ferrites are a
very well-established group of magnetic materials. Various types of ferrites are
commercially important.
Ferrite
is categorized as electroceramics with
ferrimagnetic properties. Each one has a unique crystal structure, magnetic, electric
and dielectric properties.
Ferrite exhibits ferrimagnetism due to the super-exchange interaction between
electrons of metal and oxygen ions. The opposite spins in ferrite results in the
lowering of magnetization compared to ferromagnetic metals where the spins are
parallel. Due to the intrinsic atomic level interaction between oxygen and metal
ions, ferrite has higher resistivity of the order 105 to 107 ohm-cm compared to
ferromagnetic metals. This enables the ferrite to find applications at higher
frequencies and makes it technologically very valuable [13, 14].
In general ferrites are composed of iron oxide as their main constituent and metal
oxides. Among the different spinel type ferrite material, cobalt ferrites are of great
importance because of their excellent chemical stability, good mechanical
hardness, high electrical resistivity, low eddy current and dielectric losses, high
coercivity, moderate saturation magnetization, positive anisotropy constant and
high magnetostriction [15, 16]. Owing to their important properties cobalt ferrite
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are widely used magnetic materials in high frequency applications. They belong to
inverse spinel structure category. The crystal structure allows incorporating
different metallic ions which can considerable influence the magnetic and
electrical properties. The important magnetic properties originate mainly from the
magnetic interaction between cations that are present in the tetrahedral A and
octahedral B site. Cobalt and substituted cobalt ferrite has been studied intensively
due to their versatile properties and numerous applications [17].
1.6 Classification of Ferrites on the basis of Structure:
Ferrites are ceramic ferromagnetic materials with the iron oxides as their main
constituent. On the basis of crystal structure ferrites are grouped into three main
classes namely spinel, garnet, hexagonal and ortho ferrites [18]. Each class of
ferrites has its own importance and applications in several fields. Among the
various types of ferrite, spinel ferrites are the most important and widely studied
magnetic material.
Spinel ferrite:
The spinel ferrites are unique materials exhibiting ferrimagnetic and semiconductor
properties and can be considered as magnetic semiconductors. Spinel ferrites have
general chemical formula Me2+-Fe2O4, where Me2+ is a divalent metallic ion such
as Zn2+, Ni2+, Cu2+, Mg2+ etc. These materials has been extensively used in several
applications including magnetic recording media, antenna rods, loading coils,
microwave devices, medical applications, core material for power transformers in
electronics and telecommunication applications.
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(A) Normal Spinel
If there is only one kind of cations on octahedral [B] site, the spinel is normal. In
these ferrites the divalent cations occupy tetrahedral (A) sites while the trivalent
cations are on octahedral [B] site. Square brackets are used to indicate the ionic
distribution of the octahedral [B] sites. Normal spinel have been represented by the
formula (M2+)A[Me3+]BO4. Where M represents divalent ions and Me for trivalent
ions. A typical example of normal spinel ferrite is bulk ZnFe2O4.
Fig.1.5: Normal spinel
(B) Inverse spinel
In this structure half of the trivalent ions occupy tetrahedral (A) sites and half
octahedral [B] sites, the remaining cations being randomly distributed among the
octahedral [B] sites. These ferrites are represented by the formula (Me3+)A
[M2+Me3+]BO4. A typical example of inverse spinel ferrite is Fe3O4 in which
divalent cations of Fe occupy the octahedral [B] sites.
Fig.1.6: Inverse spinel
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(C) Random spinel
Spinel with ionic distribution, intermediate between normal and inverse are known
as mixed spinel e.g.
(M δ2 + Me 12−+δ ) A [M 12−+δ Me 13++δ ] B O 4 ) , where, δ is inversion
parameter. Quantity δ depends on the method of preparation and nature of the
constituents of the ferrites. For complete normal spinel ferrite δ = 1, for complete
inverse spinel ferrite δ =0, for mixed spinel ferrite, δ ranges between these two
extreme values. For completely mixed ferrite δ = 1/3. If there is unequal number of
each kind of cations on octahedral sites, the spinel is called mixed. Typical
example of mixed spinel ferrites are MgFe2O4 and MnFe2O4.
Fig.1.7: Random spinel
Garnet:
Garnet ferrites have the structure of the silicate mineral garnet and the chemical
formula M3 (Fe5O12), where M is yttrium or a rare-earth ion. In addition to
tetrahedral and octahedral sites, such as those seen in spinels, garnets have
dodecahedral (12-coordinated) sites. The net ferrimagnetism is thus a complex
result of antiparallel spin alignment among the three types of sites. Garnets are also
magnetically hard. Yoder and Keith reported in 1951 that substitutions can be
made in ideal mineral garnet Mn3Al2Si3O12 [18]. They produced the first silicon
free garnet Y3Al5O12 by substituting YIII+AlIII for MnII+SiIV. Bertaut and Forrat
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prepared Y3Fe5O12 in 1956 and measured their magnetic properties [19]. In 1957
Geller and Gilleo prepared and investigated Gd3Fe5O12 which is also a
ferromagnetic compound [20]. The general formulas for the unit cell of a pure iron
garnet have eight formula units of M3Fe5O12, where M is the trivalent rare earth
ions (Y, Gd, Dy). Their cell shape is cubic and the edge length is about 12.5 Å.
They have complex crystal structure. They are important due to their applications
in memory structure.
Hexagonal Ferrite:
The hexagonal ferrites have the formula M (Fe12O19), where M is usually barium
(Ba), strontium (Sr) or lead (Pb). The crystal structure is complex, but it can be
described as hexagonal with a unique c axis, or vertical axis. This is the easy axis
of magnetization in the basic structure. Because the direction of magnetization
cannot be changed easily to another axis, hexagonal ferrites are referred to as hard.
This was first identified by Went, Rathenau, Gorter and Van Oostershout 1952 and
Jonker, Wijn and Braun 1956. Hexa ferrites are hexagonal or rhombohedral
ferromagnetic oxides with formula MFe12O19, where M is an element like Barium
(Ba), Lead (Pb) or Strontium (Sr). In these ferrites, oxygen ions have closed
packed hexagonal crystal structure. They are widely used as permanent magnets
and have high coercivity. They are used at very high frequency. Their hexagonal
ferrite lattice is similar to the spinel structure with closely packed oxygen ions, but
there are also metal ions at some layers with the same ionic radii as that of oxygen
ions. Hexagonal ferrites have larger ions than that of garnet ferrite and are formed
by the replacement of oxygen ions. Most of these larger ions are barium, strontium
or lead [21].
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Ortho ferrite:
Ortho ferrites have the general formula MeFeO3 where Me is a large trivalent metal
ions such as rare earth or Yttrium. They crystallize in a distorted perovskite
structure with an orthorhombic unit cell. These types of ferrites show a weak
ferromagnetism. The examples of these types of ferrites are HoFeO3 and ErFeO3.
1.7 Applications of Ferrite
Ferrite has been recognized as one of the most important electro-ceramics in
modern industries and its processing and application technology has been
improved incessantly in the last two decades. From the 1950 as radio and
television spreads ferrites established a significant position in industries and now
ferrite are most essential material in electronic industries.
Ferrites are used at both radio and microwave frequencies. Ferrite applications at
below microwave frequencies are numerous. When ferrite rod is inserted into a coil
of wire acting as an antenna, it concentrates the electromagnetic energy in the core
because of its high permeability. The high resistivity of ferrite combined with high
permeability also makes them a suitable filter in inductor applications. The ferrites
are also used in cores for magnetic memories and switches. These applications
involved the use of microsecond pulses for transmitting signals and reading
information expressed in binary code. Other non-microwave applications are IF
transformer and tune inductors [22, 23].
Ferrites are used at microwave frequencies for somewhat different reason. At these
frequencies they exhibit non reciprocal properties i.e. the attenuation and phase
shift of microwave propagating through them have different values for the two
opposite directions of propagation in a wave guide. A rather renowned Faraday
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effect is observed at microwave frequencies i.e. the plane of polarization of the
wave is rotated as it travels through an axially magnetized ferrite pencil in a
circular wave-guide. This effect can be utilized to build a whole class of nonreciprocal devices such as unilines, gyrators and differential phase shifter etc.
Recently, ferrites were considered as one of the most versatile magnetic materials
for multiplayer chip inductor (MLCI) applications and surface mount devices
(SMDs) due to their high electrical resistivity and permeability. The ferrite
material system exhibits super-paramagnetic behavior, display little or no
remanence and coercivity while keeping a very high saturation magnetization have
potential applications in biomedicine, magnetic drug delivery and cell sorting
systems . Now ferrites are most essential material in electronic industries. Ferrites
are widely used magnetic materials due to their high electrical resistivity, low eddy
current and dielectric losses. Nanosized ferrites may have extraordinary electric
and magnetic properties that are comparatively different from microstructured
materials, tailoring them to modern technologies, as well as providing novel
applications such as ferrofluids [24], magnetic drug delivery [25], high density
information storage [26], photocatalysis [27], gas sensors [28], etc.
The few applications of ferrites are described below:
Magnetic shielding
A radar absorbing paint containing ferrite has been developed to render an aircraft
or submarine invisible to radar.
Magnetic sensors:
These are used for temperature control and these can be made using ferrite with
sharp and definite Curie temperature. Position and rotational angle sensors
(proximity switches) have also been designed using ferrites.
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Electrical uses
Ferrite cores are used in electronic inductors, transformer and electromagnet where
the high electrical resistance of the ferrite leads to very low eddy current losses.
They are commonly seen as a lump in a computer cable called ferrite bead, which
helps to prevent high frequency electrical noise for entering the equipment. The
deflection yoke core in a television picture tube is an example of the use of ferrite
of the nickel-zinc-iron. The electron beam in television picture tube is deflected
vertically and horizontally thus projecting a picture. Because of their high
resistivity and the consequent low eddy current loss, use of ferrite cores here
greatly increases the efficiency of the operation for the same reason cores of fly
back transformer used in television scanning are made of ferrite. Ferrites are also
used in core of magnetic memories and switches.
Pollution control
There are several Japanese installations, which use precipitation of ferrite
precursors to savage pollutant materials such as mercury from waste stream. The
ferrite produced subsequently can be separated magnetically along with pollutant.
Ferrite electrodes
Because of their high corrosion resistance, ferrites having the appropriate
conductivities have been used as electrodes in application such as chromium
plating.
Coating
a) Ferrite powders are used in the coatings of magnetic recording tapes (e.g.
Iron oxide).
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b) Ferrite particles are used as components of radar-absorbing materials or
coatings used in stealth aircraft and in the expensive absorbing tiles lining
the room used for electromagnetic compatibility measurements.
Ferrite magnet
a) Most common radio magnets, including those used in loudspeaker are
ferrite magnets. Due to their low cost, ferrite magnet enjoys a very wide
range of applications.
b) Motors and loudspeakers to toys and crafts, and is the most widely used
permanent magnet today.
Technological applications
Ferrites are inexpensive, more stable and have range of technological applications
in transformer core, high quality filters and radio wave circuit devices etc.
Application in computer devices
Single crystals Mn-Zn Ferrite are found quite stable used as a core material in
computer, microprocessor and VCR system. It is also used as memory chips,
storage devices, recording media etc.
1.8 Literature Survey
A large number of reports are available in the literature on the synthesis,
characterization, electrical and magnetic properties of cobalt and other spinel ferrites
[29-31].
A. M. Abdeen et al have reported the structural, electrical and transport phenomena
of cadmium substituted cobalt ferrite [32].
The effect of fuel additives and heat treatment effects on nanocrystalline zinc ferrite
phase composition prepared by the auto combustion method using citric acid, acetic
24
acid, carbamide and acrylic acid as fuel additives have been studied by Ping Hu et al
[33].
Influence of cheating agents such as polyvinyl alcohol, citric acid synthesized by sol
gel auto combustion on the microstructure and antibacterial property of cobalt ferrite
nano powders has been reported by Noppakun Sanpo et al [34].
Gas sensing properties of zinc doped p-type nickel ferrite synthesized by sol-gel auto
combustion technique have been reported by A. Sukta et al [35].
Mahmoud Goodarz Naseri et al have reported simple synthesis and characterization
of cobalt ferrite nanoparticles by thermal treatment method. They have used PVP as
an agglomeration capping agent in the synthesis [36].
C. V. Gopal Reddy et al have reported the preparation and characterization of ferrites
as gas sensor materials. They studied various ferrites such as copper ferrite, cobalt
ferrite, zinc ferrite and nickel ferrite prepared by citrate process [37].
H. M. Joshi reported MR imaging applications of multifunctional metal ferrite
nanoparticles [38].
N. H. Hong et al studied ferrite nanoparticles for future heart diagnostics [39].
B. Peeples et al investigated structural, stability, magnetic and toxicity study of
nanocrystalline iron oxide and cobalt ferrite for biomedical applications [40].
L. X. Phua et al reported that cobalt ferrite films were prepared by spray pyrolysis
with post annealing. For the as-deposited film, the differential scanning calorimetry
measurement shows a crystallization peak at around 375 °C during the isochronal
heating at 20 °C/min, and the X-ray diffraction pattern shows its amorphous-like
characteristic. The magnetic hysteresis loops of as-deposited and annealed films show
that both the saturation magnetization and coercivity increase with the annealing
temperature, due to the crystallization of CoFe2O4 phase [41].
25
S. R. Nalage et al studied structural, optical morphological properties. Optical
absorption studies show low absorbance in IR and visible region with wide band gap
also depicted that a uniform surface morphology and the particles are fine [42].
S. M. Chavan et al have studied the structural and optical properties of nano
crystalline Ni-Zn ferrite thin film obtained by using chemical bath deposition
technique. They concluded that the band gap increases with increase in zinc
substitution and leads to structural changes [43]
K. Kamala Bharathi et al have studied the substitutional effect of rare earth ion
dysprosium in nickel ferrite thin film. With dysprosium substitution magnetization
increases, coercivity decreases, lattice constant increases [44].
Ke Sun et al studied the magnetic properties of Sn substituted Ni-Zn ferrite thin films
and obtained some interesting results. They have studied structural, micro-structural
and magnetic properties. The lattice parameter increases with Sn substitution.
Hysteresis loop demonstrate that substituted thin films get easily magnetized than that
of the thin film without substitution [45].
1.9 Aim of the Present Work
The spinel ferrites with chemical formula M-Fe2O4 (M is divalent metal ions) are
of great interest to the scientist and technologist as they exhibits combined
electrical and magnetic properties and have many applications. The important
electrical and magnetic properties of spinel ferrites are greatly influenced by
synthesis techniques and synthesis parameters. The ceramic technique is used to
prepare the spinel ferrite in bulk form whereas wet chemical methods are used for
the synthesis of nanosized spinel ferrites. The advantages of wet chemical methods
26
are: 1. Requires low temperature, 2. Easy and low cost 3. Produces nanosize
particles 4. Better homogeneity etc. Therefore, in the recent years many spinel
ferrites have been synthesized using wet chemical method like sol gel, chemical
co-precipitation, microemulsion etc. The properties of nanosized spinel ferrites are
found to be superior to that of their bulk counterpart [46]. Among the spinel
ferrites, cobalt ferrite is of much importance because of its unique properties such
as hard magnetic material with high coercivity and moderate magnetization.
In the literature, cobalt ferrite has been extensively studied in nanosize nature by
different wet chemical methods [47, 48]. The cations like Zn, Al etc have been
incorporated in the lattice of cobalt ferrite and modification in the electrical and
magnetic properties are achieved. The substitution of Mg ions in cobalt ferrite has
not been reported in the literature.
Keeping in mind the above facts, the aim of the present work is to synthesize Mg
substituted cobalt ferrite samples in nanosize form using sol-gel auto combustion
technique and to investigate the structural, morphological, electrical, dielectrical
and magnetic properties.
27
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