Thermal evolution of quasi-one-dimensional spin correlations within the anisotropic triangular lattice of α−NaMnO2

Magnetic order on the spatially anisotropic triangular lattice of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{NaMnO}}₂$ 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 ${T}_{\mathrm{IC}}=45$ K whose two-dimensional propagation vector evolves toward $\mathbf{k}=(0.5,0.5)$ as the temperature approaches ${T}_{\mathrm{N}}$. At high temperatures ($T>{T}_{\mathrm{IC}}$), quasielastic scattering reveals one-dimensional spin correlations along the nearest-neighbor Mn-Mn ``chain direction'' of the ${\mathrm{MnO}}₆$ 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.