One-Dimensional Magnetic Fluctuations in the Spin-2 Triangular Lattice α-NaMnO₂

The S=2 anisotropic triangular lattice α-NaMnO₂ is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T≤45  K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Δ≃7.5  meV) has been analyzed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k_B≃73  K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|/k_B≃3  K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. α-NaMnO₂ therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.