Phase diagram and thermodynamic properties of the frustrated ferro-antiferromagnetic spin system on the octahedral lattice

The influence of anisotropy generated by the competition between the ferromagnetic and antiferromagnetic nearest-neighbor interactions on the magnetic and thermodynamic properties of the classical spin system on the octahedral lattice is investigated in the framework of the exactly solvable Ising-like model on the recursive octahedral lattice. The phase diagram of the model is found and the nature of all phase transitions is determined. It is shown that the model exhibits the existence of the intermediate phase that separates the ferromagnetic phase from the antiferromagnetic one. Moreover, it is also shown that while the intermediate phase and the antiferromagnetic one are separated by the critical curve of the second-order phase transitions the transitions between the ferromagnetic phase and the intermediate one have the first-order nature on the coexistence line. The magnetization and entropy properties of all phases of the model are studied. Depending on the value of the frustration parameter, four different ground states are identified with formation of a strict residual entropy hierarchy. Due to the frustration nature of the model the specific heat capacity exhibits anomalous (Schottky) behavior at low temperatures in the vicinity of the value of the frustration parameter, for which two highly macroscopically degenerated single-point-like ground states are formed. The presence of the intermediate phase in the model also leads to the existence of the series of even three successive second-order phase transitions in the temperature dependence of the magnetization as well as of the specific heat capacity.