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

TL;DR

This study investigates the thermal evolution of spin correlations in $ ext{NaMnO}_2$, revealing a transition from short-range incommensurate order to a commensurate antiferromagnetic state, with one-dimensional spin correlations dominating at high temperatures.

Contribution

It provides detailed neutron diffraction data on the temperature-dependent magnetic ordering and spin correlations in $ ext{NaMnO}_2$, confirming theoretical models of anisotropic triangular lattice magnetism.

Findings

Transition to commensurate antiferromagnetic order below 22 K

Presence of incommensurate short-range order below 45 K

Dominant one-dimensional spin correlations at high temperatures

Abstract

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 $T_\mathrm{{IC}}=45$ 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 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.