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.
@article{arxiv.cond-mat/0410457,
title = {Three-Dimensional Spin Fluctuations in Na0.75CoO2},
author = {L. M. Helme and A. T. Boothroyd and R. Coldea and D. Prabhakaran and D. A. Tennant and A. Hiess and J. Kulda},
journal= {arXiv preprint arXiv:cond-mat/0410457},
year = {2007}
}