Thermal evolution of quasi-1D spin correlations within the anisotropic triangular lattice of $\alpha$-NaMnO${}_2$

Magnetic order on the spatially anisotropic triangular lattice of $\alpha$-NaMnO${}_2$ is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with $\mathbf{k}=(0.5,0.5,0)$ was found to occur below $T_{\mathrm{N}}=22$ K. Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state below $T_{\mathrm{I}\mathrm{C}}=45$ K whose two dimensional propagation vector evolves toward $\mathbf{k}=(0.5,0.5)$ as the temperature approaches $T_{\mathrm{N}}$. At high temperatures ($T>T_{\mathrm{I}\mathrm{C}}$), quasielastic scattering reveals one-dimensional spin correlations along the nearest neighbor Mn-Mn “chain direction” of the MnO${}_6$ planes. Our data are consistent with the predictions of a mean field model of Ising-like spins on an anisotropic triangular lattice, as well as the predominantly one-dimensional Heisenberg spin Hamiltonian reported for this material.