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Let *H*=0 (symmetric case). Entropy wants spins to be disoriented,
energy--to be parallel

- At large
*T*entropy wins:*M*=0 - At small
*T*energy wins: - In the
*Curie point**T*_{c}--phase transition. This is a*second order*phase transition*(why?)*.

If , magnetization *M* is parallel to *H*. There is a function
*M*(*H*,*T*). What happens at ?

- If
*T*>*T*_{c}, the system is disordered at*H*=0:

- If
*T*<*T*_{c}, the system is ordered at*H*=0:

- If
*T*=*T*_{c}, the points*M*and -_{0}*M*coincide--the tangent is vertical_{0}

In critical point susceptibility is infinity!

Since amplitude of fluctuations is

the fluctuations infinitely grow at

© 1997

Mon Oct 13 22:07:20 EDT 1997