Neutron-scattering measurement of incommensurate short-range order in single crystals of the S=1 triangular antiferromagnet NiGa(2)S(4).

Neutron scattering is used to investigate spin correlations in ultrapure single crystals of the S=1 triangular lattice NiGa(2)S(4). Despite a Curie-Weiss temperature of Θ(CW)=-80(2)  K, static (τ>1  ns) short-range (ξ(ab)=26(3)  Å) incommensurate order prevails for T>1.5  K. The incommensurate modulation Q(0)=(0.155(3),0.155(3),0), Θ(CW), and the spin-wave velocity (c=4400  m/s) can be accounted for by antiferromagnetic third-nearest-neighbor interactions J(3)=2.8(6)  meV and ferromagnetic nearest-neighbor coupling J(1)=-0.35(9) J(3). Interplane correlations are limited to nearest neighbors and weakened by an in-plane field. These observations show that the short-range ordered glassy phase that has been observed in a number of highly degenerate systems can persist near the clean limit.