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 all 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 120 degrees correlation with 2a period. Interestingly, however, a 2D linearly dispersive magnetic mode and a quasi-static spin component exist in the low temperature limit. Possibly relevant scenarios including topological phase transition associated with Z(2) vortex due to vector spin chirality, Spill quadrupolar nematic correlation, and C-3 bond-ordering are reviewed.