Neutron-Scattering Measurement of Incommensurate Short-Range Order in Single Crystals of the S = 1 Triangular Antiferromagnet NiGa_2S_4

摘要

Neutron scattering is used to investigate spin correlations in ultrapure single crystals of the S = 1 triangular lattice NiGa_2S_4. Despite a Curie-Weiss temperature of O_(CW) = -80(2) K, static (τ > 1 ns) short-range (ζ_(ab) = 26(3) A) incommensurate order prevails for T > 1.5 K. The incommensurate modulation Q_o = (0.155(3), 0.155(3), 0), OCw> and the spin-wave velocity (c = 4400 m/s) can be accounted for by antiferromagnetic third-nearest-neighbor interactions 7_3 = 2.8(6) meV and ferromagnetic nearest-neighbor coupling J_1 = -0.35(9) 7_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.