Three-Dimensional Spin Fluctuations in Na_0.75CoO₂

We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na_0.75CoO₂. The data confirm ferromagnetic correlations within the cobalt layers and reveal antiferromagnetic correlations perpendicular to the layers, consistent with an A-type antiferromagnetic ordering. The magnetic modes propagating perpendicular to the layers are sharp, and reach a maximum energy of ∼12  meV. From a minimal spin-wave model, containing only nearest-neighbor Heisenberg exchange interactions, we estimate the interlayer and intralayer exchange constants to be 12.2±0.5  meV and -6±2  meV, respectively. We conclude that the magnetic fluctuations in Na_0.75CoO₂ are highly three dimensional.