What is Ferromagnetism? Explain it on the basis of domain theory.

Answer:

Ferromagnetism: the property of certain materials (iron, cobalt, nickel, gadolinium and their alloys) by which they are strongly magnetised in the direction of an applied magnetic field and can retain magnetism after the field is removed. Such materials have a large positive susceptibility and relative permeability much greater than 1. Above a temperature called the Curie temperature they become paramagnetic.

Domain theory:

1. In a ferromagnetic material the atoms have permanent magnetic moments. Due to a strong quantum mechanical interaction (exchange coupling), the moments of neighbouring atoms align parallel to each other over small regions called domains (size about 1 mm or less, containing about \(10^{17}\) to \(10^{21}\) atoms).
2. Each domain is magnetised to saturation, but in an unmagnetised specimen the domains are oriented randomly, so the net magnetic moment of the specimen is zero.
3. When an external field is applied, (a) domains whose moments are along the field grow in size by movement of domain boundaries (domain walls), at the expense of others, and (b) at stronger fields the domains rotate so that their moments align with the field.
4. As a result the specimen becomes strongly magnetised. When nearly all domains are aligned, the material reaches magnetic saturation.
5. On removing the field, the domain boundaries do not fully return to their original positions, so some magnetism remains (retentivity). This explains hysteresis and the use of ferromagnetic materials for permanent magnets.
6. On heating above the Curie temperature, thermal agitation destroys the domain alignment and the material becomes paramagnetic.

[Figure: (a) randomly oriented domains in an unmagnetised specimen; (b) domains aligned with the applied field B in a magnetised specimen.]