The thermodynamic properties of a spin-1/2 Heisenberg ferromagnetic system using Oguchi's method

The thermodynamic properties of spin-1/2 Heisenberg ferromagnetic system on the simple cubic lattice are investigated by using of Oguchi's approximation. The effects of the second-nearest-neighbor exchange interactions on the magnetization, internal energy, heat capacity, entropy and free energy of the Heisenberg ferromagnet are considered. One obtains typical ferromagnetic magnetization curves and obtains that the critical temperature increases when the second-nearest-neighbor exchange parameter increases. In the heat capacity curve, Schottky-like peaks are observed for a certain values of the second-nearest-neighbor interactions. The behaviors of magnetic entropy and free energy are presented. When the only nearest neighbor interaction is included, magnetic entropy does not reach maximum value, but the system with the second-nearest-neighbor exchange interaction reaches it. Free energy has a discontinuity point at the critical temperature.