The Domain Theory of Magnetism

How can we explain these intriguing properties? The domain theory states that inside a magnet there are small regions in which the magnetic alignment of all the atoms are aligned in the same directions.

Within a domain the aligment of the magnetic direction is the same. In the next domain it may be in a completely different direction. On average over the many domains in the magnet there there is no preferential direction for the magnetic force. However, using a magnet the direction of the magnetic direction in each domain can be made to point in the same direction. In this way the magnetic field can be increased.

Why do Magnetic Domains Form?

Consider a bar magnet which has been been magnetised such that the entire magnet forms a single magnetic domain. Surface charges will appear at either end of the crystal. Associated with the surface charges is a secondary magnetic field called the demagnetising field which acts to reduce the magnetic field. The energy of the surface charges is called the magnetostatic energy.

Domain Formation in a Magnet

The magnetostatic energy can be reduced if the crystal forms a second domain, magnetised in the opposite direction. In this way, the separation of positive and negative surface charges are reduced decreasing the spatial extent of the demagnetising field.

Naturally, one might ask, if the magnetostatic energy is reduced by the formation of domains, can they carry on forming indefinitely? To which the answer is no. The reason being that energy is required to produce and maintain the region of transition from one domain to another, the domain wall. Equilibrium will be reached when the magnetostatic energy is equal to the energy required to maintain the domain walls. However, domains are much larger than the individual molecules within the magnet.

There are only 4 ferromagnetic elements at room temperature. Of these, iron (Fe), nickel (Ni), and cobalt (Co) are shown above. The fourth is gadolinium (Gd).

The pictures below show the formation made visible with the use of magnetic colloidal suspensions which concentrate along the domain boundaries. The domain boundaries can be imaged by polarized light, and also with the use of electron diffraction. Observation of domain boundary movement under the influence of applied magnetic fields has aided in the development of theoretical treatments. It has been demonstrated that the formation of domains minimizes the magnetic contribution to the free energy.

If a magnetic field is applied to the crystal, the domains that align with the magentic field will grow as the expense of the domains that are pointing in other directions.