Three-Dimensional Spin Fluctuations in Na0.75CoO2
  We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na0.75CoO2. 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-neighbour Heisenberg exchange interactions, we estimate the inter- and intra-layer exchange constants to be 12.2 +/- 0.5 meV and -6 +/- 2 meV, respectively. We conclude that the magnetic fluctuations in Na0.75CoO2 are highly three-dimensional. 
L. M. Helme, A. T. Boothroyd, R. Coldea, D. Prabhakaran, D. A.   Tennant, A. Hiess and J. Kulda