Para-, Ferro-, Antiferro-, and Ferrimagnetism

The assumption that fiionB/(kT) -C 1 is for the most part an excellent one. See Prob. 15.3. 

It should be pointed out that Eq. (15.30) is slightly incorrect because the electrons were assumed to be aligned either with or opposite to the applied field. In reality, the electron momenta may have any direction in between, and an angular dependence should be accounted for in deriving Eq. (15.30). This problem was tackled by Langevin, with the final result being 

In a certain class of magnetic materials, namely, ferro- and ferrimagnets, the temperature dependence of Xmag obeys not Curie s law, but rather the modified version 

The situation is quite analogous to melting. The stronger the bond between the atoms the higher the melting points. The Curie temperature can be considered to be the temperature at which the magnetic ordering melts.  

Given that ferromagnetism exists up to a finite temperature above absolute zero and then disappears, one is forced to postulate that in these materials  

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