Magnetic materials may be classified into three categories :
(i) Diamagnetic substances : These are the substances in which feeble magnetism is produced in a direction opposite to the applied magnetic field. These substances have small negative values of magnetism and susceptibility and positive low value of relative permeability
The examples of diamagnetic substances are
bismuth, antimony, copper, lead, water, nitrogen (at STP) and sodium chloride.
(ii) Paramagnetic substances : These are the substances in which feeble magnetism is induced in the same direction as the applied magnetic field. These substances have small positive values of M and and relative permeability greater than 1, i.e.,
is a small positive number. The examples of paramagnetic substances are platinum, aluminium, calcium, manganese, oxygen (at S.T.P.) and copper chloride.
(iii) Ferromagnetic substances : These are the substances in which a strong magnetism is produced in the same direction as the applied magnetic field. These substances are characterised by large positive values of M and much greater than 1, i.e.,
It states that the magnetic susceptibility of paramagnetic substances is inversely proportional to absolute temperature, i.e.,
When temperature is increased continuously, the magnetic susceptibility of ferromagnetic substances decreases and at a stage the substance changes to paramagnetic. The temperature of transition at which a ferromagnetic substance changes to paramagnetic is called Curie temperature. It is denoted by It is different for different materials. In paramagnetic phase the susceptibility is given by
Diamagnetism is universal properties of all substances but it is weak in para and ferromagnetic substances and hence difficult to detect.
Electromagnets are made of soft iron which is characterised by high retentivity and low coercivity.
Permanent magnets are made of steel which is characterised by high retentivity and high coercivity.
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