Two-dimensional magnetism and spin-size effect in theS = 1 triangularantiferromagnet NiGa2S4

Abstract

The triangular antiferromagnet is one of the most fundamental systems of geometrically frustrated magnets.NiGa2S4 is alayered chalcogenide compound with an equilateral triangular lattice, and it is a prime candidate foran S = 1 triangular antiferromagnet. Here we focus on low temperature magnetism inNiGa2S4,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 ofNi1 − xAxGa2S4 (A = Mn, Fe, Co, and Zn). A scaling law with a single energy scale of the Weisstemperature is found as an impurity effect and a hydrostatic pressure effect,providing evidence that it is in-plane interactions in the two-dimensionalNiS2 plane that drive the critical slowing down to the viscous spin liquid state atT* = 8.5 K and the spin-wave-likeexcitations of NiGa2S4 that emerge below T ∼ 3 K. Furthermore, we find spin-size dependent impurity effects in the temperature dependence of the specific heat ofNi1 − xAxGa2S4. Even with a highimpurity content, Zn2 + (S = 0) andFe2 + (S = 2) substitutedsystems with weak XY anisotropy and integral spins retain the quadratic temperature dependence of the magnetic specific heatlike pure NiGa2S4. A spin glass-like phase, on the other hand, emerges at low temperatureswith the substitution of magnetic impurities with half-odd integer spins: IsingCo2 + (S = 3⁄2) and weak XY Mn2 + S = 5⁄2) spins. This indicates that an integer size of spins is important for stabilizing thetwo-dimensional spin-wave-like behavior, and the unconventional spin state ofNiGa2S4 at low temperatures is distinct from a canonical spin glass.