A ferromagnetic material has a very high value of susceptibility, χ, and hence of relative permeability, µr. The value of µr can be several thousands. Like a paramagnetic material, the magnetization is in the direction of the applied field and a rod of a ferromagnetic material will align itself along the field leading to a phenomenon known as ferromagnetism.
In a paramagnetic substance which is not subjected to a magnetic field, the magnetic moments are oriented purely at random due to the thermal vibrations. In a ferromagnetic material exhibiting ferromagnetism, however, strong ‘interactions’ are present between the moments, of the nature of which requires quantum theory to understand it and is outside the scope of this work. These cause neighboring moments to align, even in the absence of an applied field, with the result that tiny regions of very strong magnetism are obtained inside the unmagnetized material called magnetic domains. Above a critical temperature called the Curie point, ferromagnetic become paramagnetic.
If all the domains were aligned completely, the material would behave like one enormous domain and the energy in the magnetic field outside is then considerable. Now all physical systems settle in equilibrium when their energy is a minimum because the external magnetic field energy is less. Thus the domains grow in the material. The region between two domains, where the magnetization changes direction, is called a domain wall and also contains energy. When the formation of a new domain wall requires more energy than is gained by the reduction in the external magnetic field, no more domains are formed. Thus, there is a limit to the number of domains formed. This occurs when the volume of the domains is of the order 10-4cm-3 or less.
Domains And Magnetization
Some of the phenomena in magnetization of ferromagnetic material(ferromagnetism) can now be explained. In an unmagnetized specimen, the domains are oriented in different directions. The net magnetization is then zero. If a small magnetic field H is applied, there is some small rotation of the magnetization within the domains, which produces an overall component of magnetization in the direction of H. If the field H is removed, the domain magnetization returns its original direction. Thus the magnetization returns to zero. The changes in the part AB of the curve are hence reversible. If the field H is increased beyond B in the region BC, the magnetization becomes greater. On removal of the field the magnetization does not return to zero, and so remanence occurs. Along BC, then, irreversible changes take place; the domains grow in the direction of the field, by movement of domain walls, at the expense of those whose magnetization is in the opposite direction. At very high applied fields H there is complete alignment of the domains and so the magnetization M approaches ‘saturation’.