Magnetic ordering in a quasi-two-dimensional planar ferromagnet

The magnetic ordering of stage-2 CoCl₂-Graphite Intercalation Compound (GIC) has been investigated by neutron diffraction. The static spin correlations are found to exhibit pronounced two-dimensional (2D) character at all temperatures down to 4.4 K, with a weak antiferromagnetic c axis coupling below T_ℓ ≈ 8.8 K. A longitudinal scan through the Co(100) reflection at 4.4 K does not exhibit the peak shape characteristic of a bound vortex phase from finite-sized islands. This assessment is corroborated by scanning across the magnetic ridge along the c^* axis. The measured 2D structure factor has been succesfully fit to a sum of two terms: a short-range order term, which is of maximum intensity and minimum width near Tu ≈ 9.5 K, and a long-range order term of constant width which vanishes above T_u. The scattering near T_u does not reflect the form expected for a bound vortex phase.