Magnetic properties of the spin-1 J1 - J3 Heisenberg model on a triangular lattice

We study the spin-1 J(1) - J(3) Heisenberg model with antiferromagnetic nearest neighbors J(1) > 0 and the ferromagnetic third nearest neighbor J(3) < 0 for a triangular lattice. The ground-state and thermodynamic properties are evaluated under the Popov-Fedotov functional integral method and the Luttinger-Tisza procedure. Perturbation expansion of the auxiliary field around mean-field theory was carried out to investigate the effect of thermal fluctuations on ground state energy and sublattice magnetization. The results are compared with those of the spin wave approximation and other methods.