Spin correlations in Ca3 Co2 O6: Polarized-neutron diffraction and Monte Carlo study

We present polarized-neutron diffraction measurements of the Ising-type spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature TN. Below TN, a clear evolution from a pure 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 analyze the magnetic diffuse scattering data. We show that both intrachain and interchain correlations persist above TN, but are essentially decoupled. Intrachain correlations resemble the one-dimensional ferromagnetic Ising model, while interchain 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.