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I know the phase transition of a ferromagnet from order phase to disordered phase ( or ferromagnetic phase to paramagnetic phase). I understand this particular phase transition. However, in the first chapter of Baxter's "Exactly Solved Models in Statistical Mechanics", it said that the discountinuity for M(H) from +M to -M is a phase transition. Directly from the text, "The iron bar can be regarded as undergoing a phase transition at H=0, changing suddenly from negative to positive magnetization." and later on it also says that in the (T,H) half plane, the cut is a line of phase transitionenter image description here This cause a lot of confusion to me. Is from +Mo to -Mo a phase transition? And if so, why nobody study it? How does this relate to free energy? What order is it in?

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    $\begingroup$ Of course, it is a phase transition. It is of first-order (the magnetization, a first derivative of the free energy) being discontinuous. Note that the free energy is a function of two variables: the temperature and the magnetic field. It has certainly been much studied. The reason you might see the order/disorder transition discuss more often is that it is continuous and thus displays universality. $\endgroup$ May 1, 2017 at 18:47

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