Abstract
This paper presents results of a recent study of multiferroic \CCO\
by means of single crystal neutron diffraction. This system has two
close magnetic phase transitions at $T\nsub{N1}=24.2$~K and
$T\nsub{N2}=23.6$~K. The low temperature magnetic structure below
$T\nsub{N2}$ is unambiguously determined to be a fully 3-dimensional
proper screw. Between $T\nsub{N1}$ and $T\nsub{N2}$
antiferromagnetic order is found that is essentially 2-dimensional.
In this narrow temperature range, magnetic near neighbor
correlations are still long range in the ($H,K$) plane, whereas
nearest neighbors along the $L$-direction are uncorrelated. Thus,
the multiferroic state is realized only in the low-temperature
3-dimensional state and not in the 2-dimensional state.
