Thermal Evolution Of Quasi-One-Dimensional Spin Correlations Within The Anisotropic Triangular Lattice Of Alpha-Namno2

Magnetic order on the spatially anisotropic triangular lattice of alpha-NaMnO2 is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with k = (0.5, 0.5, 0) was found to occur 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.