Magnetic Susceptibility and Ferromagnetic Substances:
Magnetic Susceptibility: The magnetic susceptibility (c) of a substance is defined as the ratio of the intensity of magnetisation (I) and the intensity of the applied magnetic field (H). It has no units
Magnetic retentivity: Retentivity of a magnetic material is a property by virtue of which it retains certain amount of intensity of magnetisation even after the removal of applied magnetic field.
Diamagnetic Substances: Diamagnetic Substances are those in which the resultant magnetic moment of individual atoms is zero. Ex: Copper, Bismuth, Air, Water, Gold, Alcohol, Mercury, Hydrogen.
Para magnetic Substances (Mar 99): Paramagnetic Substances are those in which the resultant magnetic moment of individual atoms is not zero.
Ferromagnetic Substances: Ferromagnetic substances are those in which the resultant magnetic moments of individual atoms align themselves in parallel because of a special effect present in them giving rise to spontaneous magnetisation. Ex: Iron, Cobalt, Nickel, Gadolinium, Dysprosium
Domain: Domain in a ferro magnetic specimen is a small local region in which all the magnetic dipoles align parallel to each other giving to certain magnetisation with in the domain.