Unravel the mystery of magnetic reference state

Magnetic order/disorder transition significantly influences thermochemical and thermophysical properties of materials and can remarkably impact phase equilibria and phase transformations. The Inden-Hillert-Jarl model and its various extensions have often been used to describe the magnetic contribution to the Gibbs energy. They have been implemented and are available in many CALPHAD tools, including Thermo-Calc. It is well known that the choice of the reference state for describing the magnetic contribution sometimes leads to dramatically different phase diagrams, and the reason behind it is still unknown. This work aims to unravel this mystery.