Curie-Weiss Theory Classical Approach

We suspect that spontaneous magnetism must have something to do with the d-electrons in the transition metals and the/-electrons in the rare earth metals since they have unpaired spins. We can postulate that there must be some self-generated ordering force that keeps 

Putting this result into Equation 25.29, we can solve for the magnetization as a function of temperature  

Solutions exist for T Tq only if the applied field H 0. Equation 25.29 may be approximated by 

Universal magnetization versus reduced temperature curve. The model gives very good agreement with measured data. 

It should also be mentioned that peff obtained from the saturation magnetism was only 2.22, whereas the value for Fe with four impaired spins estimated from 2y/S (S + 1) = 4.90. There are several possible reasons for this lower value of peff in the pure metal spin-orbit interactions, interference with the conduction electrons, and the Fermi level cutting off some of these higher energy states. Also some fraction of the d-electrons participate in forming covalent bonds. It is not a perfect theory. 

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