Unconventional spin order in the triangular lattice system NaCrO_2: A neutron scattering study

We report high resolution neutron scattering measurements on the rhombohedrally stacked triangular antiferromagnet NaCrO_2 which has recently been shown to exhibit an unusually broad fluctuating cross-over regime extending far below the onset of spin freezing at T_c. Our results show that at T_c purely two-dimensional quasi-static spin correlations of the 120° type exist. Below some cross-over temperature (T∼0.75T_c) a small incommensuration develops which helps resolve the inter-layer spin frustration and drives short-range three-dimensional magnetic order. This incommensuration assisted dimensional cross-over suggests that inter-layer frustration is responsible for stabilizing the rare 2D correlated phase above 0.75T_c.