Magnetic Transformations and Phase Diagrams

When magnetic phases are included on phase diagrams some unfamiliar features appear. Most magnetic transformations are not first-order phase transformations and therefore their depictions on phase diagrams do not have to follow the well-known construction requirements such as those based on the Gibbs Equilibrium Phase Rule. In particular, the transformation from a paramagnetic to ferromagnetic phase should be designated differently than those of the more widely known first-order transformations. The transformation curve (Curie curve) does not display a two-phase equilibrium: rather it shows the limit of stability of the disordered (paramagnetic) phase. In this paper, various examples of phase diagrams which include such transformations will be presented and discussed. The role of externally applied magnetic fields will be presented based on fundamental thermodynamic principles and the role that applied magnetic fields play in changing the degrees of freedom of systems (alloys) will be discussed.