In this article we will discuss about:- 1. Introduction to Magnetic Materials 2. Magnetic Parameters 3. Classification of Magnetic Materials 4. Paramagnetism 5. Diamagnetism 6. Feebly Magnetic Materials 7. Cast and Cermet Permanent Magnets 8. Metal Ceramic Technique of making Magnets 9. Ageing of a Permanent Magnet.
Introduction to Magnetic Materials:
Magnetic materials are those materials in which a state of magnetisation can be induced. Such materials when magnetised create a magnetic field in the surrounding space.
In a material all the molecules contain electrons which orbit around the nucleus; these orbits are therefore, equivalent to circulating currents and so develop an m.m.f. Each m.m.f. due to an individual orbit, in most molecules, is neutralized by an opposite one. But in magnetic materials such as iron and steel there are a number of unneutralized orbits, such that a resultant axis of m.m.f. exists which produces a magnetic dipole.
In unmagnetised specimens, because of mutual attraction and repulsion among the dipoles, the molecular m.m.f. axes lie along continuous closed paths, and no external magnetic effect can be detected.
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In magnetised specimens, the dipoles line up parallel with the exciting m.m.f. and when the exciting m.m.f. is removed, a number of diples may remain aligned in the direction of the external field and thus exhibit permanent magnetism.
The readiness of a material to accept magnetism is expressed by its permeability. For all materials except a few magnetic ones, the permeability is that of free space and is constant. It is denoted by μ0 (= 4 x 10-7).
For magnetic materials the permeability equals μ0 times the relative permeability which is denoted by μr (i.e., μ = μ0 μr).
The relative permeability varies with the degree of magnetisation of the material and may have a value as high as 2500.
Magnetic Parameters:
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The various magnetic parameters are described below:
It has been observed that a coil carrying a current I, when located in a region of uniform magnetic flux density, will experience, a torque whose magnitude depends on the area of the coil, the current and the component of flux density in the plane of the coil.
The magnetic dipole moment is defined by the following relation:
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Pm = I x A
Where, pm = Magnetic dipole moment-Ampere metre2 (A-m2) and is a vector quantity,
I = Current, and
A = Area of the coil.
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A permanent bar magnet also experiences a torque when placed in a uniform field, which tends to align it with the field. If the magnet is supposed to have free poles of opposite kind, we can define the dipoles moment, as the product of poles strength and distance between them.
It has been found that all materials are affected by the presence of a magnetic field, in that they are found experimentally to acquire magnetic moments. The magnitude of this moment per unit volume, is called the magnetisation of the medium and is described by the vector M.
The magnetisation M of a material may be expressed in terms of its elementary magnetic dipole moment, pm by-
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M = Npm … [8.1 (a)]
where, N is the number of magnetic dipoles per unit volume.
To prove for a magnetic material: B = μ0 (M + H):
Consider a solenoid.
Let, I = Length of the solenoid,
A = Area of cross-section of solenoid,
N = Number of turns, and
I = Current through the solenoid.
If the solenoid is placed in vacuum the flux density will be:
The magnetic susceptibility depends on the nature of the magnetic material and on its state, i.e., temperature etc. The susceptibility of a sample may change on cold working. If copper is cold worked, the susceptibility may change from a -ve to a +ve value and on annealing after cold work, it may again become -ve.
The susceptibility may be determined by measuring the force exerted on a magnetic material when it is placed in a magnetic field. The susceptibility of a ferromagnetic substance is very strongly dependent on the field strength. Therefore, the magnetic properties of ferromagnetic materials are best described by giving the magnetisation or the flux density as a function of the field strength.
The M-H and B-H curves of the ferromagnetic materials are very similar in nature. The slope of the former when divided by the magnetic space constant, μ0 gives the magnetic susceptibility and the slope of the latter gives the magnetic permeability, μ (= μ0 μr).
For ferromagnetic materials the susceptibility (or, the permeability) is not constant. It has a low value at weak fields. As the field intensity is increased, the susceptibility increases, reaches a maximum value and then begins to drop, ultimately reaching a constant value on saturation.
Classification of Magnetic Materials:
All materials possess magnetic properties to a greater or lesser degree and these are determined by the facts that-
(1) A magnetic field exerts forces and torques on the bodies,
(2) A body placed in a magnetic field distorts the field.
The magnetic properties of the materials are characterised by their relative permeabilities.
In accordance with the value of relative permeability the materials may be classified in the following three ways:
1. Ferromagnetic Materials:
The relative permeabilities of these materials are much greater than unity and are dependent on the field strengths. They attract the lines of force strongly. The principal ferromagnetic elements are iron, cobalt, nickel. Gadolinium however, also comes under this classification. These have high susceptibility.
2. Paramagnetic Materials:
These have relative permeability slightly greater than unity and are magnetised slightly. They attract the lines of forces weakly. Aluminium, platinum and oxygen belong to this category.
The relative permeability of these materials is slightly less than unity. They repel the lines of force slightly. The examples are bismuth, silver, copper and hydrogen.
Paramagnetism:
a. Let us take the case of a material which is composed of large number of identical atoms. Each atom can be visualised as a positive point charge at the centre of a negative charge cloud. Now if an electric field is applied the nucleus will displace slightly in one direction and centres of the negative electron cloud will move slightly in the opposite direction.
The application of electric field produces a dipole (A dipole is a pair of equal but opposite electric charges which are very close). This process is known as induced polarisation. However, in certain materials, formation of dipoles takes place even in the absence of an electric field. These are permanently polarised.
In a material which is permanently magnetised, the dipoles interact weakly. They are oriented at random and have a low net magnetisation. When these materials are placed in a magnetic field, the dipoles orient themselves up in the direction of magnetic field and the substance shows magnetisation. This phenomenon is known as paramagnetism and the materials exhibiting this property are known as paramagnetic substances.
b. Since paramagnetism requires existence of permanent dipole moments paramagnetic susceptibility may be considered to be analogous to orientational susceptibility associated with dielectrics. In both cases the susceptibility is positive and temperature dependent. Thus the paramagnetic susceptibility varies inversely with the absolutely temperature for ordinary fields and temperatures.
Or, x = C/T … (8.13)
This law is known as the Curie law of paramagnetism and the constant C is called the Curie constant. Fig. 8.15 shows Curie law for a paramagnetic material.
Paramagnetic materials have a small positive susceptibility of the order of 10-3 at room temperature, which is much larger than the negative contribution due to the diamagnetic effect.
In general, paramagnetism is a relatively small effect that has found a few technical applications-
(i) Paramagnetic salts have been used in obtaining very low temperatures of the order of 10-3 K by adiabatic demagnetisation.
(ii) They are also the essential materials used in the solid state MASER.
Diamagnetism:
Diamagnetism is the property of material due to which it, when placed in a magnetic field, becomes weakly magnetised in a direction opposite to the magnetisation of the external field. Consequently purely diamagnetic substances are repelled by a magnetic field.
Diamagnetism refers to the magnetic moment induced in the individual atoms and molecules by an applied field, the process being similar to that by which currents are induced in a conducting loop of wire when the flux linking the loop is changed. The induced moment is directly opposite to the applied field.
The diamagnetic susceptibility is very small and negative. This is seen in the repulsion experienced by diamagnetic materials when placed in a magnetic field. Further since the susceptibility is determined by the electron structure of the system, it does not depend on external conditions, such as temperature. It is additive just like molecular refraction.
The diamagnetic effect becomes observable only when the net paramagnetic atomic moment is zero in zero field. That is, when the paramagnetic susceptibility is not zero it is generally much larger than the diamagnetic susceptibility. The paramagnetic susceptibility arises because of a favourable orientation of atomic moments in the directions of the external field.
The diamagnetic susceptibility, on the other hand, arises due to a change in magnitude of the electronic orbital moments. Since diamagnetism is associated with the individual electrons in an atom, diamagnetic susceptibility has always a small value even if the atom has zero initial moment.
Practically all organic substances are diamagnetic. Diamagnetism, however, is of little practical importance because of the small value of diamagnetic susceptibility.
Feebly Magnetic Materials:
Feebly magnetic materials are not in themselves useful as electromagnet cores, they may be important in such designs to provide structural members which are “nonmagnetic”. They are often employed to reduce eddy-current heating and to reduce energy losses of such parts as rotor-coil binding wire, shafts, bolts, filters, and pole-supports castings.
Austenitic 18-8 stainless steel, steels having 14% manganese with 1.25% carbon, 10% manganese with 3% nickel and 0.7% carbon, and 18% manganese with 1.5% nickel and 0.3% carbon are typical feebly magnetic materials supplied in wrought form. All have poor corrosion resistance except the stainless steels. The high carbon manganese steels are also hard to machine.
As castings, Nomag with 11% Ni, 5.2% Mn, 2.7% C, is the only cast iron in use; 14% Mn with 1.2% C is recommended for steel castings where grinding is required; 10% Mn, 6% Ni and 0.25% C and 18% Mn, 1.5% Ni and 0.3% C are steel alloys recommended when machining is involved.
Cast and Cermet Permanent Magnets:
Several alloys of materials employed for making permanent magnets are difficult to machine due to the following reasons:
(i) Extreme hardness.
(ii) Large crystal structure.
The fabrication of magnets and components is done by either casting or ceramic techniques.
Cast permanent magnets show good resistance to ageing.
Natural ageing depends on the following factors:
(i) For a magnet of given cross-sectional area, the shorter the length the more intensive is the ageing of the magnet.
(ii) Partial demagnetisation by an alternating magnetic field accelerates ageing.
(Ageing is a spontaneous change in properties of metal after a heat treatment or cold- working operation. Ageing tends to restore the material to an equilibrium condition and to remove the unstable condition induced by the prior operation and usually results in increased strength of the metal with corresponding loss of ductility).
Below are given the properties of cast permanent magnets:
The above magnetic characteristics are obtained by a specific heat treatment which varies according to the alloy to be treated. Magnets which are made from cast alloys and do not contain cobalt can be rough machined by cemented-carbide tipped cutting tools.
They can be ground with electrical corundum wheels in two stages, i.e., rough and finish. Rough finish machining can be done by electric spark machining methods. Before machining, the work pieces should be annealed to remove hardness and brittleness.
Metal Ceramic Technique of making Magnets:
Two methods of making magnets by metal ceramic technique are described below:
First Method:
(i) Mixture of pure metal powders or requisite powdered alloy is moulded at reduced pressure and temperature in first stage.
(ii) It is further moulded at full pressure in the second stage.
(iii) It is then sintered at the requisite temperature.
(iv) The components are finally heat treated.
Second Method:
(i) The material is made up into blank plates from which strips and sheets can be rolled.
(ii) The components are then die punched from the strip or sheets.
(iii) Finally the components are sintered.
This method entails the following advantages:
b. Greater density is available.
Metal ceramic methods of making magnets claim the following advantages:
1. No casting defects.
2. Better grindability.
3. Higher mechanical strength.
4. Homogeneous material.
Ageing of a Permanent Magnet:
Ageing of a permanent magnet is the process of normal or accelerated change, under continued normal or specified artificial conditions, in the strength of the magnetic field maintained. The change in field strength due to ageing is usually expressed in percent.
Ageing may be of the following two types:
1. Metallurgical Ageing.
2. Magnetic Ageing.
Each of the above type of ageing may occur either singly or in combination with the other.
It is a result of a change in the metallurgical condition of the magnet, which changes its ability to maintain itself in a magnetized condition. This change may begin immediately after the magnet is hardened and, at room temperature, may continue at diminishing rate for a long period of time. High temperature usually accelerates these metallurgical changes.
Hence most permanent magnet materials are stabilized by baking for about 24 hours at temperature between 100 and 200°C. The change may also be accelerated in some materials by cooling below room temperature, by cyclic temperature changes, and possibly by mechanical vibration.
Magnets that have been metallurgically aged cannot be restored to their original strength by remagnetisation.
It is the result of some external influence which causes a change in the strength of the magnetic field being maintained by the magnet, but does not alter the metallurgical condition of the magnet as far as its magnetic properties are concerned.
Magnets that have suffered only magnetic ageing may be restored to their original strength by remagnetisation.
The magnetic ageing may be caused by the following:
(i) Presence of strong external fields.
(ii) Changes in the external magnetic circuit, such as increasing the air gap length or removing the “keeper”.
(iii) Mechanical vibration.
(iv) Temperature condition.