The first question one might ask about ferromagnetic material is the source of the large magnetism. The saturation magnetization is so large virtually all the magnetic dipoles in the ferromagnetic material must be lined up with the field. To make our model fit the facts, it is necessary to add a term to the energy which will make the dipoles all line up. By adding a new energy term we are actually making the same assumptions as Weiss. He was the first to postulate the existence one direction such a field. It states that the field produces at any point by all neighbouring molecules, is propositional to magnetization and acting in the same direction as the intensity of magnetization.

                                     Χ_m = C/T-T_c

This is curie Weiss law. Here C is a material specific curie constant and critical temperature is the Curie temperature measured in Kelvin. The Curie Weiss law describes the magnetic susceptibility of a ferromagnetic in the paramagnetic region above the Curie point.