Multicritical phase diagram of the three-dimensional Ashkin-Teller model including metastable and unstable phases

The mean field approximation from the Bogoliubov inequality is applied to study the ferromagnetic Ashkin-Teller model in three dimensions. Phase transitions for the stable, metastable and unstable states is observed. From the critical values in the free energy of the model, those transitions are classified. Besides the free energy, magnetization, critical frontiers and tricritical points are obtained. Phase diagram is presented for a better illustration of the phase transitions and its order.