Neutron scattering is used to investigate spin correlations in ultrapure single crystals of the S=1 triangular lattice NiGa2S4. Despite a Curie-Weiss temperature of ΘCW=-80(2)K, static (τ>1ns) short-range (ξab=26(3)Å) incommensurate order prevails for T>1.5K. The incommensurate modulation Q0=(0.155(3),0.155(3),0), ΘCW, and the spin-wave velocity (c=4400m/s) can be accounted for by antiferromagnetic third-nearest-neighbor interactions J3=2.8(6)meV and ferromagnetic nearest-neighbor coupling J1=-0.35(9) J3. 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.
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