CLASSIFICATION OF MAGNETIC MATERIALS

CLASSIFICATION OF MAGNETIC MATERIALS

Although all materials have magnetic properties of some kind being either diamagnetic, paramagnetic or ferromagnetic, the term“magnetic material"is customarily applied only to substances which exhibit ferromagnetism.

1.Paramagnetic Materials:

The materials,which are not strongly attracted by a magnet,such as aluminium,tin,platinum, manganese etc.are known as paramagnetic materials. Their relative permeability is small but positive.For example,the relative permeability for aluminium,air and platinum are 1.00000065, 1.0000031 and 1.00036 respectively.Such materials are slightly magnetized when placed in a strong magnetic field and act in the direction of the magnetic field.

The paramagnetic materials the individual atomic dipoles are oriented in a random fashion. The resultant magnetic field is, therefore,negligible. When an external magnetic field is applied,the permanent magnetic dipoles orient themselves parallel to the applied magnetic field and give rise to a positive magnetization.Since the orientation

of the dipoles parallel to the applied magnetic field is not complete,the magnetization is small.These materials have little application in the field of electrical engineering.

2.Diamagnetic Materials:

The materials which are repelled by a magnet such as zinc,mercury,lead,sulphur, copper, silver, bismuth, wood etc. are known as diamagnetic materials.Their permeability is slightly less than unity.For example the relative permeabilities of bismuth,copper and

wood are 0.99983,0.999995 and 0.9999995 respectively.They are slightly magnetized when placed in a strong magnetic field

and act in a direction opposite to the applied magnetic field.

In diamagnetic materials,the two relatively weak magnetic fields(one caused due to orbital revolution and other due to axial rotation) are in opposite directions and cancel each other.Permanent magnetic dipoles are absent in them. Diamagnetic materials are unimportant from the point of views of application in the field of electrical engineering.

3.Ferromagnetic Materials:

The materials,which are strongly attracted by a magnet such as iron,steel,nickel,cobalt

and some of their alloys,are known as ferromagnetic materials.

Their permeability is very high(varying from several hundreds to many thousands).

The opposing magnetic effects of electron orbital motion and electron spin do not eliminate each other in an atom of Such a material.There is a relatively large contribution from each atom which aids in the establishment of an internal magnetic field,so.that when the material is placed in a magnetic  field,its value is increased many times the value that was present in the free space before the material was placed there.

For the purpose of electrical engineering it will suffice to classify the materials simply as non-ferromagnetic and ferromagnetic materials. The former includes materials of relative permeabilities practically equal to unity while the latter have relative permeabilities many times greater than unity. Paramagnetic and diamagnetic materials fall in the former(i.e. non-ferromagnetic)class.

Ferromagnetic materials are of two types:(a)those easily magnetized,called the soft magnetic materials and

(b)those retaining their magnetism with great tenacity,designated as hard magnetic materials.

(a)Soft Ferromagnetic Materials:

The  high relative permeability,low coercive force,easily magnetized and demagnetized and have extremely small hysteresis. Soft ferromagnetic materials are iron and its alloys with nickel, cobalt,tungsten and aluminium. Ease of magnetization and demagnetization  makes them highly suitable for applications involving changing magnetic flux as in electromagnets,electric motors, generators, transformers,inductors,telephone receivers, relays etc.They are also useful for magnetic screening.Their properties may be greatly enhanced through careful manufacture, and by heating and slow annealing so as to achieve a high degree of crystal purity.Large magnetic moment at room temperature makes soft ferromagnetic materials extremely useful for magnetic circuits but ferromagnetics are very good conductors and suffer energy loss from eddy currents produced within them.There is additional energy loss due to the fact that magnetization does not proceed smoothly but in minute jumps.This loss is called magnetic  residual loss and it depends purely on the frequency of the changing flux density and not on its magnitude.

(b)Hard Ferromagnetic Materials:

They have relatively low permeability,and very high coercive force.These are difficult to magnetize and demagnetize. Typical hard

ferromagnetic materials include cobalt steel and various ferromagnetic alloys of nickel, aluminium and cobalt.They retain high percentage of their magnetization and have

relatively high hysteresis loss.They are highly suited for use as permanent magnets in loud speakers etc.Sometimes they are also called the permanent magnetic materials.During

manufacturing, a  high degree of dislocation is introduced.Hard ferromagnetic materials are frequently heated to high temperature and then quenched in suitable liquid to introduce

strains.

4.Ferrites:

This is a special group of ferromagnetic materials that occupy an intermediate position between ferromagnetic and nonmagnetic materials.They consist of extremely fine particles of a ferromagnetic material possessing high permeability,and are held together with a binding resin.

The magnetization produced in the ferrites is.large enough to be of commercial value but their magnetic saturations are not as high as those of ferromagnetic materials.As in the case of terromagnetics,ferrites may be soft or hard ferrites.

(a)Soft Ferrites:

Ceramic Magnets,also called ferromagnetic ceramics and ferrites, are made of an iron oxide, Fe2O3, with one or more divalent 

Oxides such as NiO, MnO or ZnO. This magnets have a square histeresis loop and high Resistance to demagnetization.and are valued for magnets For Computing machines where a high resistance is desired. The great advantage of ferrites is their high resistivity. Commercial ferrites can have resistivities . Eddy currents resulting from alternating fields are therefore, reduced to a minimum, range of application of these magnetic materials is extended to high frequencies,even to microwaves. Ferrites are carefully made by mixing powdered oxides,

compacting and sintering at hightemperatures. High-frequency transformers in television and frequency modulated receivers are almost always made with ferrite cores.Ferrites,with large magnetostrictive effects,are sometimes used in electro- mechanical transducers.In high-frequency applications, magnetostriction in ferrites can lead to undesirable noise.

(b)Hard Ferrites:

These are ceramic permanent magnetic. materials.The most important family of hard ferrites has the basic composition MO.Fe2O3, where M is a barium (Ba) ion or a strontium (Sr)ion. These materials have a hexagonal structure and low in cost and density. A Hard ferrites are used in generators, relays and motors. Electronic applications include. the  magnets for loud speakers,telephone ringers and receivers. The also used in holding devices for door closers,seals,latches and in several toy designs. 

The separate class of magnetic material, known as super-paramagnetic, is made from powdered iron or other magnetic particles. This material is used in electronics and transformers for cores. The Permalloy (molybdenum- nickel-iron powder) is the best known example of this important category of magnetic materials.

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