Magnetic properties of the spin-1 two-dimensional $J_1-J_3$ Heisenberg   model on a triangular lattice
  Motivated by the recent experiment in NiGa$_2$S$_4$, the spin-1 Heisenberg model on a triangular lattice with the ferromagnetic nearest- and antiferromagnetic third-nearest-neighbor exchange interactions, $J_1 = -(1-p)J$ and $J_3 = pJ, J > 0$, is studied in the range of the parameter $0 \leq p \leq 1$. Mori's projection operator technique is used as a method, which retains the rotation symmetry of spin components and does not anticipate any magnetic ordering. For zero temperature several phase transitions are observed. At $ p \approx 0.2$ the ground state is transformed from the ferromagnetic order into a disordered state, which in its turn is changed to an antiferromagnetic long-range ordered state with the incommensurate ordering vector at $p \approx 0.31$. With growing $p$ the ordering vector moves along the line to the commensurate point $Q_c = (2 \pi /3, 0)$, which is reached at $p = 1$. The final state with the antiferromagnetic long-range order can be conceived as four interpenetrating sublattices with the $120\deg$ spin structure on each of them. Obtained results offer a satisfactory explanation for the experimental data in NiGa$_2$S$_4$. 
P. Rubin, A. Sherman and M. Schreiber
cond-mat.str-el