Neutron scattering investigation of a quasi-two-dimensional planar ferromagnet

Publisher Website
Google Scholar
Abstract
The magnetic ordering of stage‐2 CoCl2‐graphite intercalation compound has been investigated by neutron diffraction. This compound is a layered structure, with ferromagnetic coupling between xy‐type spins within the same layer and a much weaker antiferromagnetic coupling between successive layers. The interplanar coupling is sufficiently weak that the ordering occurs in two steps, leaving a temperature window between Tl≊8.8 K and Tu≊9.4 K where the ordering is purely two dimensional. Below Tl the spins become weakly correlated along the c axis, but this correlation never grows beyond two magnetic layers. We have measured the static spin correlation function by neutron scattering. At all relevant temperatures the scattering is markedly two dimensional and consists of two parts: a short‐range order term with maximum correlation length and intensity at Tu, and a sharp Bragg‐like ridge of constant width, which vanishes above Tu and relates to either long‐range order or a power‐law decay of the order parameter. Results will be analyzed in light of current theories, in particular, of the vortex binding mechanism proposed by Kosterlitz and Thouless.