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

The temperature dependence of the magnetic order on the spatially anisotropic triangular lattice of α-NaMnO₂ will be presented. Single crystals were studied via time-of-flight and triple-axis neutron diffraction measurements, which revealed the transition into a commensurate, collinear antiferromagnetic ground state with k = (0.5, 0.5, 0) occurs below T_N = 22 K. Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state below T_IC = 45 K whose two dimensional propagation vector evolves toward k = (0.5, 0.5) as the temperature approaches T_N. At high temperatures (T > T_IC), 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.