Gigantic ferroelectric polarization and magnetoelectric coupling in a ferrimagnetic oxide CaBaCo$_{4}$O$_{7}$

From both fundamental and applications points of view, improper ferroelectrics that exhibiting a strong coupling between polarization and magnetic structure are challenging the scientific community. Several multiferroics belonging to that category have been reported; however, they exhibit rather small values of electric polarization combined with low magnetic ordering temperatures. Only the CuO (tenorite), the ordered perovskites LBaCuFeO$_{5}$ and the Z-type hexaferrites display magnetic ordering temperatures near room temperature, but they all suffer from polarization much smaller than that of proper ferroelectrics. Here, we report a ferrimagnetic cobaltite, CaBaCo$_{4}$O$_{7}$, crystallizing in a polar space group, which enters an improper ferroelectric phase below T$_{\mathrm{C}}=$ 64 K. Single crystals of CaBaCo$_{4}$O$_{7}$ demonstrate the highest polarization value reported among improper ferroelectrics to date, reaching 10 mC/m$^{2}$ at T$_{\mathrm{C}}$ and approaching 16 mC/m$^{2}$ at 8 K. Moreover a large magnetoelectric coupling coefficient is also evidenced near T$_{\mathrm{C}}$. This result points to routes for exploring new multiferroics among ferrimagnetic phases.