Switching from pyroelectric to ferroelectric order in Ni-doped CaBaCo4O7

We report ferroelectric ordering in Ni-substituted CaBaCo4O7. Magnetization showed ferrimagnetic transition at 60 K and an additional magnetic anomaly is found at ∼82K. The enhanced antiferromagnetic interactions and decrease in saturation magnetization are noticed with Ni substitution. The dielectric and pyroelectric measurements illustrate strong coupling between spin and charge degrees of freedom. Ferroelectric behavior is confirmed with enhanced ordering temperature (∼82K) and saturation polarization (250μC/m2). Neutron diffraction has revealed an increase in c-lattice parameter in the Ni sample. All the Co/Ni moments are reoriented in the a direction. A transition from a noncollinear ferrimagnetic to a collinear ferrimagnetic spin order, with ↑↑↓↓ antiferromagnetic ordering in the kagome layer, is observed with substitution. This results in the appearance of unequal bond distances in a two-dimensional collinear layer; most likely this is responsible for the transition from pyroelectric to ferroelectric, with the possibility of switching of electric polarization.