CHAPTER - I INTRODUCTION TO MAGNETISM AND MAGNETIC MATERIALS. Magnetic phenomena have been known and exploited for many centuries.

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1 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 1

2 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 2

3 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 (Fe 3 O 4 ) 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 (µ B ), 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. 3

4 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 4

5 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 (1s 2 2s 2 2p 6 3s 2 3p 6 )3d 6 4s 2 with a net moment of 4 m b, In minerals, the transition elements are in a variety of oxidation states. Fe commonly occurs as Fe 2+ and Fe 3+. When losing electrons to form ions, transition metals lose the 4s electrons first, so we have for example, Fe 3+ with a structure of (1s 2 2s 2 2p 6 3s 2 3p 6 )3d 5, or 5 m b. Similarly Fe 2+ has 4 m b and Ti 4+ has no unpaired spins. Iron is the main magnetic species in geological materials, but Mn 2+ (5 m b ) and Cr 3+ (3 m b ) occur in trace 5

6 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). 6

7 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]. 7

8 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 Fe 2 O 3 ), 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 (Fe 3 O 4 ), Manganese Bismuth (Mn-Bi), and Cobalt Ferrite (CoFe 2 O 4 ) 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) 8

9 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 6 th century B.C., is that of a ferrite, the mineral magnetite, a compound containing negative oxygen ions O 2- and positive iron ions in two states, iron (II) ions, Fe 2+, and iron (III) ions, Fe 3+. The oxygen ions are not magnetic, but both iron ions are. In magnetite crystals, chemically formulated as Fe 3 O 4, for every four oxygen ions, there are two 9

10 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 10

11 Table 1: Magnetic behavior versus values of magnetic susceptibility Magnetic Behavior Value of χ Example Diamagnetic Paramagnetic Ferromagnetic small and negative small and positive large and positive Au Cu Mn Pt 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 11

12 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 (M r )), 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 (M s ). 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 (H c ). Both ferromagnetic and ferromagnetic materials are classified as either soft or hard on the basis of their hysteresis characteristic. 12

13 Soft Magnetic Materials- Soft 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 Mn x Zn (1-x) Fe 2 O 4 ) and nickel-zinc (Ni-Zn, with the formula Ni x Zn (1-x) Fe 2 O 4 ). 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]. 13

14 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 materials- Magnetic 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 14

15 (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 15

16 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 10 5 to 10 7 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 16

17 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 Me 2+ -Fe 2 O 4, where Me 2+ is a divalent metallic ion such as Zn 2+, Ni 2+, Cu 2+, Mg 2+ 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. 17

18 (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 (M 2+ ) A [Me 3+ ] B O 4. Where M represents divalent ions and Me for trivalent ions. A typical example of normal spinel ferrite is bulk ZnFe 2 O 4. 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 (Me 3+ ) A [M 2+ Me 3+ ] B O 4. A typical example of inverse spinel ferrite is Fe 3 O 4 in which divalent cations of Fe occupy the octahedral [B] sites. Fig.1.6: Inverse spinel 18

19 (C) Random spinel Spinel with ionic distribution, intermediate between normal and inverse are known as mixed spinel e.g. 2+ δ 2+ δ 2+ δ 3+ δ ( M Me 1 ) A[ M 1 Me 1+ ] BO 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 MgFe 2 O 4 and MnFe 2 O 4. Fig.1.7: Random spinel Garnet: Garnet ferrites have the structure of the silicate mineral garnet and the chemical formula M 3 (Fe 5 O 12 ), 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 Mn 3 Al 2 Si 3 O 12 [18]. They produced the first silicon free garnet Y 3 Al 5 O 12 by substituting Y III +Al III for Mn II +Si IV. Bertaut and Forrat 19

20 prepared Y 3 Fe 5 O 12 in 1956 and measured their magnetic properties [19]. In 1957 Geller and Gilleo prepared and investigated Gd 3 Fe 5 O 12 which is also a ferromagnetic compound [20]. The general formulas for the unit cell of a pure iron garnet have eight formula units of M 3 Fe 5 O 12, 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 (Fe 12 O 19 ), 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 Hexa ferrites are hexagonal or rhombohedral ferromagnetic oxides with formula MFe 12 O 19, 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]. 20

21 Ortho ferrite: Ortho ferrites have the general formula MeFeO 3 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 HoFeO 3 and ErFeO 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 21

22 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. 22

23 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). 23

24 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

25 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 CoFe 2 O 4 phase [41]. 25

26 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-Fe 2 O 4 (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

27 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

28 References: [1]. J.M.D.Coey, Magnetism and Magnetic Materials Cambridge University Press, (2009). [2]. Carmen-Gabriela Stefanita, Magnetism: Basics and Applications, Springer (2012). [3]. B.D.Cullity, C.D.Graham, Introduction to Magnetic Materials (2 nd Ed.). Wiley- IEEE Press. (2008). [4]. Kei Yosida, Theory of Magnetism Springer (India) Pvt. Ltd. (2006). [5]. S.O.Pillai, Solid State Physics. [6]. Charles Kittel, Introduction to Solid State Physics, 8th Edition. [7]. V.Raghvan, Material science and engineering: A First course, Fifth Edition. [8]. Leszek Malkinski, Advanced Magnetic Materials Published by InTech (2012). [9]. K.H.J.Buschow, Handbook of Magnetic Materials Elsevier B.V. (2014). [10]. S.Muralidharan, V.Saraswathy, L.J. Berchmans, K. Thangavel, K.Y. Ann, Sensors Actuators, B: Chemical, 145: [11]. H.G.Liu, X. Wang, J. Zhang and Y. Chen et al., Acta Biomaterialia, 7: (2011) [12]. I.R.Harris, A.J.Williams, Mater. Sci. & Eng. Vol. II Magnetic Materials [13]. Kurikka V.P.M. Shafi, Yuri Koltypin, Ahron Gedanken, Ruslan Prozorov, Judit Balogh, Janos Lendvai, Israel Felner, J. Phys. Chem. B 101(1997)6409. [14]. H.Lgarashi, K.Okozaki, J. Am. Ceram. Soc. 60 (1977) 51. [15]. R. Sato Turtello, Giap V. Duong, W Nunes, R Grpssinger, M. Knobel, J Magn. Magn. Mater 320 (2008)

29 [16]. A.Hannour, D.Vincent, F.Kahlouche, A.Tchangoulian, S.Neveu, V.Dupuis, J. Magn. Magn. Mater. 353 (2014) [17]. N.H.Hong, A.T.Raghavender, O.Ciftja, M.H.Phan, K.Stojak, H.Srikanth, Yin Hua Zhang, Appl. Phys. A (2013) 112 : [18]. K.J.Standley, Oxide Magnetic Materials, 2 nd edition (1972). [19]. F.Bertaut, F. Forrat, Compt. Rend, 242, (1956) [20]. D.S.Geller, M. Gilleo, J. Phys. Chem. Solids 3, 30 (1957). [21]. Robert C. Pullar, Progress in Materials Science, Vol.57, 7, (2012) [22]. Gopal Reddy C V, Manorama S V, Rao V J (1999) Sensors and Actuators B55 90 [23]. Luo H Y, Yue Z X, Zhou J (2000) J. Magn. Magn. Mater [24]. P.Acharya, R.Desai, V.K.Aswal, R.V. Upadhyay, J. Physics Pramana, 71 (2008) [25]. I.Safarik, M.Safarikova, Magn. Nanopart. & Biosciences, in: H. Hofmann, Z. Rahman, U. Schubert (Eds.), Nano-structured Materials, Springer, Vienna (2002) 1. [26]. Kasapoglu N, Birsoz B, Baykal A, et al. Central European J. Chemistry, 2007, 5(2): [27]. Cao S. W., Zhu Y. J., Cheng G.F., et al. J. Hazardous Materials, 2009, 171 (1-3): [28]. D.Hoegmann, L.Josephson, R.Weissleder, J.P.Basilion, Bioconjugate Chemistry 11 (2000) 941. [29]. Elina Manova, Boris Kunev, Daniela Paneva, Ivan Mitov, Lachezar Petrov, Chem. Mater. 16 (2004)

30 [30]. P.L.Andrade, V.A. J.Silva, J.C.Maciel, M.M.Santillan, N.O.Moreno, L.De Los Santos Valladares, Angel Bustamante, S. M. B. Pereira, M. P. C. Silva, J. Albino Aguiar, Hyper. Interact. DOI /s [31]. M.Bradiceanu, P.Vlazan, S.Novaconi, I.Grozescu, P.Barvinschi, Chem. Bull. Politehnica" Univ. (Timi oara) 52 (66) (2007) 1-2. [32]. A.M.Abdeen, O.M.Hemeda, E.E.Assem, M.M. El-Sehly, J. Magn. Magn. Mater. 238 (2002) [33]. Ping Hu, De-an Pan, Xin-feng Wang, Jian-jun Tian, Jian Wang, Shen-gen Zhanga, Alex A. Volinsky, J. Magn. Magn. Mater. 323 (2011) [34]. Noppakun Sanpo, James Wang and Christopher C. Berndt, J. Austr. Cer. Soc. 49 (1) (2013) [35]. A.A.Sutka, G.Mezinskis, A.Lusis, M.Stingaciu, Sensors and Actuators B Chemical : [36]. A.Mahmoud, Goodarz Naseri, E.B.Saion, Hossein Abbastabar Ahangar, Abdul Halim Shaari, Hashim Mansor, J. Nanomaterials [37]. C.V.Gopal reddy, S.V.Manorama, V.J.Rao, J. Mater. Sci. Letters 19 (2000) [38]. H.M.Joshi, J. Nanopart. Res. (2013) 15:1235. [39]. N.H.Hong, A.T.Raghavender, O.Ciftja, M.H.Phan, K.Stojak, H.Srikanth, Y.H. Zhang, Appl Phys. A (2013) 112: [40]. F.B.Peeples, V.Goornavar, C.Peeples, D.Spence, V.Parker, C.Bell, D.Biswal, G.T.Ramesh, A.K.Pradhan, J. Nanopart Res (2014) 16:2290. [41]. L.X.Phua, F.Xu, Y.G.Ma, C.K.Ong, Thin Solid Films 517 (2009) [42]. S.R.Nalage et al, Thin Solid Films 520 (2012)

31 [43]. S.M.Chavan, M.K.Babrekar, S.S.Maore, K.M.Jadhav, J. Alloys & Comp.507 (2010) [44]. K.Kamala Bharathi, R.S.Vemuri, C.V.Ramana, Chemical Physics Letters 504 (2011) [45]. Ke Sun, Zhongwen Lan, Zhong Yu, Xiaoliang Nie, Lezhong Li, Chengyong liu,, J. Magn. Magn. Mater. 320 (2008) [46]. Gleiter H: Nanocrystalline materials. Prog. Mater. Sci. 1989, 33: [47]. I.C.Nlebedim, K.W.Dennis, R.W.McCallum, D.C.Jiles, J. Appl. Phys. 115, 17A519 (2014). [48]. N.Somaiah, T.V.Jayaraman, P.A.Joy, D.Das, J. Magn. Magn. Mater. 353, 324:14 (2012),