We report ferroelectric ordering in Ni substituted CaBaCo 4 O 7 . 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 μC/m 2 ). 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 noncollinear ferrimagnetic to collinear ferrimagnetic spin order is observed. The coupling between the triangular and Kagome layers weakens and leads to ↑↑↓↓ 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.

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We present a study of terahertz frequency magnetoelectric effect in ferrimagnetic pyroelectric CaBaCo 4 O 7 and its Ni-doped variants. The terahertz absorption spectrum of these materials consists of spin excitations and low-frequency infrared-active phonons. We studied the magnetic-fieldinduced changes in the terahertz refractive index and absorption in magnetic fields up to 17 T.We find that the magnetic field modulates the strength of infrared-active optical phonons near 1.2 and 1.6 THz. We use the Lorentz model of the dielectric function to analyze the measured magnetic-field dependence of the refractive index and absorption. We propose that most of the magnetoelectric effect is contributed by the optical phonons near 1.6 THz and higher-frequency resonances. Our experimental results can be used to construct and validate more detailed theoretical descriptions of magnetoelectricity in CaBaCo 4−x Ni x O 7 .

We report dielectric and pyroelectric responses of solid solutions of Ga2–xFexO3 (x = 0.75, 1.0, and 1.25), exhibiting combined relaxations (Maxwell-Wagner type relaxation and Debye relaxation); however, the dominating contribution is found to be from extrinsic Maxwell-Wagner type relaxation. Furthermore, the emergence of polarization in these systems is understood in terms of the thermally stimulated depolarization current effect caused by freezing of defect dipoles associated with charged oxygen vacancies rather than the intrinsic ferroelectric behavior.

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