According to the Landau theory applied to the mean field picture for magnetism, the free energy of a ferromagnetic material close to a phase transition can be written as:
where , the magnetization, is the order parameter,  is the applied magnetic field,  is the critical temperature, and  are material constants.
Close to the phase transition, this gives a relation for the magnetization order parameter:
where  is a dimensionless measure of the temperature.
Thus in a graph plotting  vs.  for various temperatures, the line without an intercept corresponds to the dependence at the critical temperature. Thus along with providing evidence for the existence of a ferromagnetic phase, the Arrott plot can also be used to determine the critical temperature for the phase transition.