Novel Geometrical Frustration Effects in the Two-Dimensional Triangular-Lattice Antiferromagnet NiGa₂S₄ and Related Compounds

Abstract

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 NiGa 2 S 4 and related compounds. NiGa 2 S 4 provides the unique example of a S = 1 2D antiferromagnet on a regular exact triangular lattice. Extensive studies using high-purity samples of NiGa 2 S 4 have revealed that Ni 2+ 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 120° correlation with 2 a 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, spin quadrupolar nematic correlation, and C 3 bond-ordering are reviewed.