The domain theory of magnetism is a concept in physics that explains how ferromagnetic materials become magnetized. According to this theory, a ferromagnetic material is made up of many small regions called ‘domains’. Each domain is a microscopic area where the magnetic moments of atoms are aligned in the same direction, resulting in a net magnetic field within that domain.
When a ferromagnetic material is not magnetized, these domains are oriented randomly, and their magnetic fields cancel each other out. However, when the material is exposed to an external magnetic field, these domains begin to align with the field. As a result, more domains become oriented in the same direction, leading to an overall increase in the material’s magnetization.
This alignment occurs because the energy of the system decreases when more domains align with the external magnetic field. Ultimately, if the external field is strong enough, it can cause nearly all the domains within the material to align, resulting in a strong net magnetic field. When the external magnetic field is removed, some domains remain aligned, which is why many ferromagnetic materials can retain a degree of magnetization even after the external influence is gone.