  We report ferroelectric ordering in Ni substituted CaBaCo4O7. Magnetization
showed ferrimagnetic transition at 60 K and an additional transition is found ~
82 K, further, enhanced antiferromagnetic interactions and decrease in
saturation magnetization are noticed with Ni substitution. The dielectric and
pyroelectric measurements illustrate a strong coupling between spin and charge
degrees of freedom; ferroelectric behavior is confirmed with enhanced ordering
temperature (~82 K) and saturation polarization (250 muC/m2. Neutron
diffraction has revealed an increase in c-lattice parameter in Ni sample and
all the Co/Ni moments are reoriented in a- direction; evidently a non-collinear
ferrimagnetic to collinear ferrimagnetic spin order is observed. The coupling
between the triangular and Kagome layers weakens and leads to up-up-down-down
AFM ordering in the Kagoma layer. This can be viewed as a 2D-collinear layer
with unequal bond distances and most likely responsible for the switching of
electric polarization.
