Novel geometrical frustration effects in the two-dimensional triangular-lattice antiferromagnet NiGa2S4 and related compounds

Geometrical frustration may suppress conventional magnetic long-range order and possibly promote a novel type of ordering associated with a higher order degree of freedom than spin dipolar-moment such as vector spin chirality and spin quadrupole moment. After a brief overview of such phenomena due to vector spin chirality, we focus on the two-dimensional (2D) frustrated magnetism in the layered chalcogenide Mott insulator NiGa2S4 and related compounds. NiGa2S4 provides the unique example of a S=1 2D antiferromagnet on a regular exact triangular lattice. Extensive studies using high-purity samples of NiGa2S4 have revealed that Ni2+ S=1 Heisenberg spins exhibit resonant critical slowing down at T*=8.5 K without forming a magnetic long-range order, signaling a viscous spin-liquid state. The critical spin-fluctuation regime extends over almost an order of magnitude in temperature both above and below T*. Even well below T*, the spin-spin correlation remains short-ranged at an incommensurate wave vector close to (1/6, 1/6, 0), corresponding to a 120deg correlation with 2a period. Interestingly, however, a 2D linearly dispersive magnetic mode and a quasi-static spin component exist in the low temperature limit. Possibility relevant scenarios including topological phase transition associated with Z2 vortex due to vector spin chirality, spin quadrupolar nematic correlation, and C3 bond-ordering are reviewed. (author)