Classification of Magnetic Materials

Classification of Magnetic Materials

There is a very wide range of magnetic materials from which permanent magnets may be made. The materials differ in the nature of their elements and their composition. Each of them has its own values, its own characteristics and consequently its own uses.

The most commonly used alloy in the manufacture of permanent magnets is Alnico, This is composed of Aluminium, Nickel, Iron and Cobalt. The last of these four metals, namely cobalt, is costlier than the others and Iron is the cheapest of all. As researches have all along tried to make the magnetic material more stable and economical, Alnico alloy has been divided into several standards. The approximate chemical composition of some standards of Alnico is given below :

 
Name of Metal General Alnico II Alnico III Alnico V
 Al (Aluminium), percent  18  10  12  8
 Ni (Nickel), percent  —  20  24  14
 Co (Cobalt), percent  12  12  —  24
 Cu (Cuprum-copper),percent  6  6  3  3
 Fe (Ferrum-iron), percent  64  52  61  51

The different metallic alloys from which magnets are made are called magnetic materials or magnetic substances. The magnetic substances are broadly classified in three main categories, namely:

(i) Ferromagnetic.Classification of Magnetic Materials
(ii) Paramagnetic.
(iii) Diamagnetic.

The general characteristics of these substances are as follows:

(i) Ferromagnetic. These substances have got very large values of magnetic permeability and are, therefore, capable of high degree of magnetisation. They include the metals which are found to be attracted by magnets or magnetic fields. Such substances are : Iron, Steel, Nickel and Cobalt. The difference between the properties of Iron and Steel is that soft iron has far greater retentivity than steel, but has far less coercivity ; in other words, steel retains magnetism for a long time, whereas soft iron loses it earlier. As such soft iron is used in electromagnets and steel is used for permanent magnets.

(ii) Paramagnetic. These substances represent the materials which are feebly attracted when placed in a magnetic field. In a non-uniform field, paramagnetic substances will experience an attractive force towards the strongest part of the field. Such substances include Aluminium, Chromium, Copper sulphate, Manganese, Palladium, Platinum, Potassium and Tungsten, etc.

(iii) Diamagnetic. These substances are the materials which are not attracted by magnets. They have a tendency to move from stronger to weaker parts of magnetic field and are characterised by negative susceptibility. To this class belong Antimony, Bismuth, Copper, Diamond, Gold, Mercury, Silver, Sulphur, Tin and Zinc.

Gases and liquids are also found to belong to the classes of paramagnetic or diamagnetic substances. Air and oxygen are found to be paramagnetic while Alcohol, Hydrogen, Nitrogen and water are diamagnetic in their properties.
Permanent alloy magnets are generally used for door latches, fans, filter coils, gramophones, loud speakers, magnetoes, magnetic generators, magnetic separators, meters, radios, scooters, sugar mills, telephones, television receivers, toy-motors, and other novelties.