Two-dimensional ferromagnetic correlations in CoCl2-intercalated graphite

We have investigated, by quasielastic neutron scattering, the spin correlation of the planar ferromagnet Co${\mathrm{Cl}}_2$ intercalated in graphite. At all temperatures down to 4.4 K the magnetic scattering exhibits pronounced two-dimensional character. The structure factor has been successfully fitted to a sum of two parts: a diffuse term with Lorentzian line shape from either short-range order or critical fluctuations, and a narrow component with Gaussian line shape from long-range order. The width of the Gaussian component, corresponding to a correlation length of 900±150 Å, stays roughly constant up to the ordering temperature $T_u\approx 9.4$ K. For an ideal two-dimensional $\mathrm{xy}$ ferromagnet a cusp-shaped structure factor is expected below the ordering transition from the power-law decay of the pair correlation function. The present data, however, do not support this picture: Conventional short-range-order correlation predominates for all temperatures, blurring the differences between a Gaussian and cusp-shaped structure factor within the narrow component.