Spin Order by Frustration in Triangular Lattice NaCrO$_2$ : A Neutron Scattering Study

We report high resolution neutron scattering measurements on the triangular lattice antiferromagnet NaCrO$_2$. In contrast to the host compound of the 2D cobaltate superconductor Na$_x$CoO$_2$, no magnetic long-range order is observed down to 1.5K. For $T >$ 40K purely 2D quasi-static 120$^{\circ}$ spin correlations extend to a maximum of 20 lattice constants. For $T <$ 30K, a small in-plane incommensuration develops together with short-range $c$-axis correlations. Our observation of strong spin fluctuations over an extended temperature range and a long wave-length incommensurate modulation strongly suggest an out-of-plane frustration as the mechanism for stabilizing this rare 2D correlated phase in NaCrO$_2$.