  We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy.
