Magnetic Properties of Materials - Diamagnetism | Paramagnetism | Ferromagnetism

The magnetic properties of materials are classified into three parts that are diamagnetism, paramagnetism, ferromagnetism. In terms of susceptibility X, a material is diamagnetic if X is negative, paramagnetic if X is positive and small, and ferromagnetic if X is positive and large.

Diamagnetism

Diamagnetic substances are those in which the resultant magnetic moment is zero. When the magnetic field is applied, those electrons having orbital magnetic moment in the same direction slow down and those in opposite direction speed up. Thus, substance develops net magnetic moment in opposite direction to that of the applied field and hence repulsion occurs.

Properties of Diamagnetic Substances 

• They have a tendency to move from the stronger to the weaker part of the magnetic field.

• The field lines are repelled or expelled and the field inside the material is reduced.

• When placed in non-uniform magnetic, the bar will tend to move from high to the low field.

Some Important Points

• Some diamagnetic materials are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride.

• The field lines are completely expelled by the superconductors. Superconductors repel a magnet and are repelled by the magnet.

• The phenomenon of perfect diamagnetism in superconductors is called Meissner Effect.

Paramagnetism 

The individual atoms of a paramagnetic material possess a permanent magnetic moment of their own. Due to the random thermal motion of the atoms, no net magnetisation is seen. In an external magnetic field, the individual atomic dipole moment can be made to align and point in the same direction as an external magnetic field. The field lines get concentrated inside the material, and the field inside is enhanced (10⁵).

Properties of Paramagnetic Substances

·         They have a tendency to move from weak magnetic field to strong magnetic field.

·         They get weakly magnetized when placed in an external magnetic field.

·         In a non-uniform magnetic field, the bar will tend to move from a weak to a strong magnetic field.

Some Important Points

·         Paramagnetic materials are aluminum, calcium, copper chloride, sodium, and oxygen (at STP),  etc

·         Magnetisation is inversely proportional to absolute temperature T.

·         Curie’s law

Ferromagnetism

The individual atoms possess a dipole moment. They interact in such a way that they align themselves in the same direction over a macroscopic volume called domain. Magnetisation varies randomly from domain to domain and there is no bulk magnetisation. In an external magnetic field, the domains orient themselves in the direction of B₀ and its size grows.

Properties of Ferromagnetic Substances

·         They have a strong tendency to move from weak to the strong magnetic field.

·         They get strongly magnetised when placed in an external magnetic field.

·         In a non-uniform magnetic field, it tends to move towards the region of the high field.

Some Important Points

·         Ferromagnetic materials are iron, cobalt, nickel, gadolinium, etc. The relative ferromagnetic permeability is 1000!

·         Hard magnetic materials (or hard ferromagnets) – In some ferromagnetic materials the magnetisation persists even when the external field is removed like alnico, aluminium, nickel, cobalt, and copper.

·         Soft ferromagnetic materials – Magnetisation disappears on the removal of the external field. Soft iron is one such example.

·         The naturally occurring materials like lodestone form permanent magnets used in a compass needle.

·         The ferromagnetic property depends on temperature. At high enough temperature a ferromagnetic becomes paramagnetic.