  We present macroscopic and neutron diffraction data on multiferroic lightly
Co-doped Ni$_3$V$_2$O$_8$. Doping Co into the parent compound suppresses the
sequence of four magnetic phase transitions and only two magnetically ordered
phases, the paraelectric high temperature incommensurate (HTI) and
ferroelectric low temperature incommensurate (LTI), can be observed.
Interestingly, the LTI multiferroic phase with a spiral (cycloidal) magnetic
structure is stabilized down to at least 1.8 K, which could be revealed by
measurements of the electric polarization and confirmed by neutron diffraction
on single crystal samples. The extracted magnetic moments of the LTI phase
contain besides the main exchange also fine components of the cycloid allowed
by symmetry which result in a small amplitude variation of the magnetic moments
along the cycloid propagation due to the site-dependent symmetry properties of
the mixed representations. In the HTI phase a finite imaginary part of the
spine magnetic moment could be deduced yielding a spin cycloid instead of a
purely sinusoidal structure with an opposite spin chirality for different spine
spin chains. The magnetic ordering of the cross-tie sites in both phases is
different in comparison to the respective ones in the pure Ni compound. A wider
temperature stability range of the HTI phase has been observed in comparison to
Ni$_3$V$_2$O$_8$ which can be explained by an additional single-ion easy-axis
anisotropy due to Co-doping. The larger incommensurability of the Co-doped
compounds yields a larger ratio between the competing next-nearest neighbour
and nearest neighbour interaction, which is $J_2/J_1$=0.43 (0.47) for a doping
level of 7% (10%) Co compared to 0.39 in the parent compound.
