  The polar magnet CaBaCo$_{4}$O$_{7}$ is known to exhibit the largest
magnetic-field-driven electric polarization change ($\Delta P$) associated with
an antiferromagnetic (AFM)-ferrimagnetic (FIM) transition in a narrow
temperature range between 62 and 69 K. In this work, we investigate the effect
of Ni doping on its multiferroic properties, by means of magnetization,
electric polarization, dielectric constant, and magnetostriction measurements
on single crystals of CaBaCo$_{3.9}$Ni$_{0.1}$O$_{7}$ up to 50 T. In the doped
material, two kinds of AFM phases appear below 78 K, accompanying negative
$\Delta P$. Upon the application of a magnetic field along any crystallographic
axis, giant positive $\Delta P$ of up to $11 \sim 12$ mC/m$^{2}$ is observed
along with an AFM-FIM transition in the whole temperature range below 78 K. The
giant magnetoelectric effect inherent in CaBaCo$_{4}$O$_{7}$ can be further
enhanced just by a small amount of chemical substitution, in terms of (i)
increasing the magnitude of $\Delta P$ and (ii) expanding the temperature range
in which giant $\Delta P$ appears.
