  Triangular-layered NiGa2S4, contrary to intuitive expectation, does not form
a noncollinear antiferromagnetic structure, as do isoelectronic NaCrO2 and
LiCrO2. Instead, the local magnetic moments remain disordered down to the
lowest measured temperature. To get more insight into this phenomenon, we have
performed first principles calculations of the first, second end third
neighbors exchange interactions, and found that the second neighbor exchange is
negligible, while the first and the third neighbor exchanges are comparable and
antiferromagnetic. Both are rapidly suppressed by the on-site Hubbard
repulsion.
