  We present a numerical treatment of a spin-orbital polaron model for
Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the
polarons account for the peculiar magnetic properties of this layered compound:
They explain the large susceptibility; their internal degrees of freedom lead
both to a negative Curie-Weiss temperature and yet to a ferromagnetic
intra-layer interaction, thereby resolving a puzzling contradiction between
these observations. We make specific predictions on the momentum and energy
location of excitations resulting from the internal degrees of freedom of the
polaron, and discuss their impact on spin-wave damping.
