Domain Theory of Magnetism

The domain theory states that inside a magnet there are small regions in which the magnetic direction of all the atoms are aligned in the same directions. These regions are known as domains. Within each atom, the orbiting electrons are also spinning as they revolve around the nucleus. The atom, due to its spinning electrons, has a magnetic field associated with it. In nonmagnetic materials, the net magnetic field is effectively zero since the magnetic fields due to the atoms of the material oppose each other. In magnetic materials such as iron and steel, however, the magnetic fields of groups of atoms numbering in the order of $10^{12}$ are aligned, forming very small bar magnets. This group of magnetically aligned atoms is called a domain. Each domain is a separate entity; that is, each domain is independent of the surrounding domains. For an unmagnetized sample of magnetic material, these domains appear in a random manner, such as shown in Fig. 1(a). The net magnetic field in any one direction is zero. When an external magnetizing force is applied, the domains that are nearly aligned with the applied field will grow at the expense of the less favorably oriented domains, such as shown in Fig. 2(b). Eventually, if a sufficiently strong field is applied, all of the domains will have the orientation of the applied magnetizing force, and any further increase in external field will not increase the strength of the magnetic flux through the core-a condition referred to as saturation. The elasticity of the above is evidenced by the fact that when the magnetizing force is removed, the alignment will be lost to some measure, and the flux density will drop to $B_R$. In other words, the removal of the magnetizing force will result in the return of a number of misaligned domains within the core. The continued alignment of a number of the domains, however, accounts for our ability to create permanent magnets. At a point just before saturation, the opposing unaligned domains are reduced to small cylinders of various shapes referred to as bubbles. These bubbles can be moved within the magnetic sample through the application of a controlling magnetic field. These magnetic bubbles form the basis of the recently designed bubble memory system for computers.