Ferromagnetic in-plane spin fluctuations in NaxCoO2 observed by neutron inelastic scattering.

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies ( approximately 10 meV), which is localized in wave vector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.