The magnetic and thermodynamic properties of a spin-2 Heisenberg ferromagnetic system

The magnetic and thermodynamic properties such as the magnetization, internal energy, specific heat and susceptibility of spin-2 Heisenberg ferromagnetic system on a square lattice are studied by using Green׳s function technique. Without including the next nearest neighbor interaction, one doesn׳t observe the second-order phase transitions. We found that only when the next nearest neighbor interaction is greater than the nearest neighbor interaction, the second-order phase transitions exist for the small single-ion anisotropy values. Indeed, in the case of negative anisotropy which corresponds to first-order phase transitions, the energies have discontinuities. At the same time, the specific heat shows two peaks.