Ordering in quasi-two-dimensional planar ferromagnets: A neutron scattering study of graphite intercalation compounds

The magnetic ordering of stage-2 ${\mathrm{CoCl}}_2$- and ${\mathrm{NiCl}}_2$-graphite intercalation compounds (GIC’s) has been investigated by neutron diffraction. Both of these compounds are observed to undergo a two-step ordering process. For temperatures within the range $T_l$$T_u$ purely two-dimensional (2D) spin correlations are observed, while at temperatures below $T_l$ ferromagnetically aligned planes couple weakly with an average antiferromagnetic interplanar correlation. No three-dimensional long-range magnetic order is observed even at the lowest temperatures attained. The absence of fine structure in scans along (h,k,l) with fixed h and k indicates that successive intercalate layers are translationally uncorrelated from each other. The in-plane peaks of ${\mathrm{CoCl}}_2$-GIC appear to have a shape characteristic of long-range spin order below $T_l$. However, the structure factor expected for a finite-sized bound vortex phase cannot at present be discounted for ${\mathrm{CoCl}}_2$-GIC. A similar scan at $T_l$$T_u$ is likewise inconclusive concerning the existence of the bound vortex phase of coplanar spins in the intermediate temperature regime.