Thermal evolution of quasi-one-dimensional spin correlations within the anisotropic triangular lattice of α - NaMnO 2

Magnetic order on the spatially anisotropic triangular lattice of $\alpha -{\text{NaMnO}}_2$ is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with $\text{k}=(0.5,0.5,0)$ was found to occur below $T_{\text{N}}=22\text{K}$ . Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state below $T_{\text{IC}}=45\text{K}$ whose two-dimensional propagation vector evolves toward $\text{k}=(0.5,0.5)$ as the temperature approaches $T_{\text{N}}$ . At high temperatures $\left(T>T_{\text{IC}}\right)$ , quasielastic scattering reveals one-dimensional spin correlations along the nearest-neighbor Mn-Mn "chain direction" of the MnO₆ 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.