title: Three-dimensional spin fluctuations in Na0.75CoO2.
abstract: 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 approximately 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 Na0.75CoO2 are highly three dimensional.
authors.affiliation: Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom.
authors.forename: L M
authors.initials: LM
authors.lastname: Helme
authors.forename: A T
authors.initials: AT
authors.lastname: Boothroyd
authors.forename: R
authors.initials: R
authors.lastname: Coldea
authors.forename: D
authors.initials: D
authors.lastname: Prabhakaran
authors.forename: D A
authors.initials: DA
authors.lastname: Tennant
authors.forename: A
authors.initials: A
authors.lastname: Hiess
authors.forename: J
authors.initials: J
authors.lastname: Kulda