Magnetic behaviour near the critical point

Let us consider a gas of molecules with a constant magnetic moment /T, which is placed into a homogeneous magnetic field of strength H (Rumer and Ryvkin, 1977). 

The first approximation implies that there is no interaction between the molecules. The potenticil energy of a molecule is 

The ratio of the m2tgnetic energy to the mean heat motion energy x = pH/kT has the meaning of the orientation (reorder) parameter. 

The magnetization of ferromagnetics reaches its saturation in relatively weak fields, and there is spontaneous magnetization even without external field. This indicates that the orientation of magnetic moments is basically caused by the internal orienting field (related to the interactions among magnetic moments) rather than by the external field. 

Under the Weiss hypothesis, the internal field is proportional to the existing magnetization 

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