Magnetization, spontaneous definitions

In a ferromagnetic system the order parameter is given by the spontaneous magnetization, M, which in an ideal second-order transition rises from zero at 7J-(see, e.g., Belov 1959). As mentioned in sect. 2, and are related by C, — TdM IQ T and therefore, if the temperature of the maximum of is taken as the thermodynamical definition of T., it corresponds to the maximum slope of M. Many authors define TJ- (T for us) as the temperature where the maximum slope of extrapolates to Ms = 0. Although this is not a thermodynamical definition, the difference ATc = 7 — Tc is a measure of the fluctuations above Tq as was mentioned in sect. 2. 

---