To explain the phenomenon of ferromagnetism, Weiss proposed a hypothetical concept of ferromagnetic domains. He postulated that the neighboring atoms of the ferromagnetic materials, due to certain mutual exchange interactions, from several number of very small regions, called domains.

Weiss theory of ferromagnetism is also called domain theory of ferromagnetism. It has following points:

• The domains which are aligned approximately along the direction of the applied magnetic field grow in size at the cost of unfavorably oriented domains, that is, those align opposite to the field direction get reduced. In other words, the domain boundaries move so as to expand the favorable domains.
• Also domains rotate and orient themselves in the direction of the external magnetic field.

In the presence of the weak external field, the magnetisation in the material occur mostly by the process of domain growing, but in the strong magnetic field the material is magnetised mostly by the process of domain alignment. When the field is removed, the domain boundaries do not recover their original positions and thus the material is not completely demagnetised, but some residual magnetism remains in it.

Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0