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

The S = 2 anisotropic triangular lattice alpha-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 (Delta similar or equal to 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) similar or equal to 73 K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|= k(B) similar or equal to 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. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.