Two-dimensional magnetism and spin-size effect in the S = 1 triangular antiferromagnet NiGa(2)S(4)

The triangular antiferromagnet is one of the most fundamental systems of geometrically frustrated magnets. NiGa(2)S(4) is a layered chalcogenide compound with an equilateral triangular lattice, and it is a prime candidate for an S = 1 triangular antiferromagnet. Here we focus on low temperature magnetism in NiGa(2)S(4), where quasi-static spins develop a spin-wave-like mode without forming any long-range ordering. We have studied low temperature magnetism of both polycrystalline samples and single crystals of Ni(1 - x)A(x)Ga(2)S(4) (A = Mn, Fe, Co, and Zn). A scaling law with a single energy scale of the Weiss temperature is found as an impurity effect and a hydrostatic pressure effect, providing evidence that it is in-plane interactions in the two-dimensional NiS(2) plane that drive the critical slowing down to the viscous spin liquid state at T(*) = 8.5 K and the spin-wave-like excitations of NiGa(2)S(4) that emerge below T ∼ 3 K. Furthermore, we find spin-size dependent impurity effects in the temperature dependence of the specific heat of Ni(1 - x)A(x)Ga(2)S(4). Even with a high impurity content, Zn(2+) (S = 0) and Fe(2+) (S = 2) substituted systems with weak XY anisotropy and integral spins retain the quadratic temperature dependence of the magnetic specific heat like pure NiGa(2)S(4). A spin glass-like phase, on the other hand, emerges at low temperatures with the substitution of magnetic impurities with half-odd integer spins: Ising Co(2+) (S = (3/2)) and weak XY Mn(2+) S = (5/2)) spins. This indicates that an integer size of spins is important for stabilizing the two-dimensional spin-wave-like behavior, and the unconventional spin state of NiGa(2)S(4) at low temperatures is distinct from a canonical spin glass.