\sect{Baruch's C03.}
\begin{itemize}
\item [(a)]
Antiferromagnetism is a phenomenon akin to ferromagnetism. The
simplest kind of an antiferromagnet consists of two equivalent
antiparallel sublattices ${A}$ and ${B}$ such that members of
${A}$ have only nearest neighbors in ${B}$ and vice versa. Show
that the mean field theory of this type of (Ising)
antiferromagnetism yields a formula like the Curie-Weiss law for
the susceptibility ${\chi \sim \left(T- T_{c}\right)^{-1} }$,
except that ${T- T_{c}}$ is replaced by ${T+T_{c};\, T_{c}}$ is
the transition temperature into antiferromagnetism (Neel's
temperature).
\item [(b)]
Below ${T_{c}}$ the susceptibility ${\chi}$ of an
antiferromagnet drops again. Show that in the mean field theory of
(a) the rate of increase of immediately below ${T_{c}}$ is twice
the rate of decrease immediately above. (Assume that the applied
field is parallel to the antiferromagnetic orientation.)\\
\end{itemize}
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