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Home , Antiferromagnetism, Diamagnetism, Ferromagnetism

Diamagnetism: Materials such as quartz, , acetone, , and 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 . 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 in the material. Thus, all materials are diamagnetic. However, the other forms of 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), oxide (FeO or Fe2O3), and (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 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), (Ni), (Gd), iron oxide (Fe3O4), (Mn-Bi), and (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) 9 ferromagnetic materials are capable of attracting each other. All of the materials that we are used to calling "" are ferromagnetic materials. 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 () of the material can be changed.

Ferrimagnetism: , type of permanent magnetism that occurs in 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 ).

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.

In crystals, chemically formulated as Fe3O4, for every four oxygen ions, there are two 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 11 Table 1: Magnetic behavior versus values of 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 . 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 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 ( (Mr)), that though materials retains when the external field is removed. 2) The 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 / (Hc). Both ferromagnetic and ferromagnetic materials are classified as either soft or hard on the basis of their characteristic. 13 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 in applications such as switched-mode power supplies (SMPS). The most common soft ferrites are manganese- (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 or electromagnetic cores contain nickel, zinc, and / or manganese .

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: , 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 15 (Hexagonal ferrite) are used for constructing permanent . 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 . 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:

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 , magnetic, electric and properties. Ferrite exhibits ferrimagnetism due to the super- between electrons of and oxygen ions. The opposite spins in ferrite results in the lowering of magnetization compared to ferromagnetic 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 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 17 are widely used magnetic materials in high frequency applications.

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 properties and can be considered as magnetic . 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.

(12-coordinated) sites. The net ferrimagnetism is thus a complex result of antiparallel alignment among the three types of sites. Garnets are also magnetically hard.