Magnetic field induced ferroelectricity and half magnetization plateau in polycrystalline R2V2O7(R=Ni,Co)

Low-dimensional frustrated antiferromagnet is a good model system to study exotic quantum physics. Here we report the observation of half magnetization plateau and ferroelectricity which emerge simultaneously in the bond-alternating skew-chain compounds ${R}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}\phantom{\rule{4pt}{0ex}}(R=\mathrm{Ni},\text{Co})$ induced by high magnetic fields. The half plateau is stabilized in fields of 8--30 T (7--12 T) for $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}\phantom{\rule{4pt}{0ex}}(\mathrm{C}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7})$, whereas two magnetic field induced ferroelectricities are located below and above this plateau. The remarkable high-field ferroelectricity for Ni or Co compound is about $50\text{\ensuremath{-}}60\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ for the polycrystalline sample. The resulting magnetic field-temperature $(H\text{\ensuremath{-}}T)$ phase diagrams are very complex and several distinct phase transitions are observed. The forced electric polarization by sweeping magnetic fields evidences a clear magnetoelectric coupling in $\mathrm{C}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ associated with the low-field ferroelectricity. Our magnetization data also reveal that $\mathrm{C}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ produces an effective spin-1/2 behavior at the magnetic ground state. These experimental findings improve the knowledge to multiferroics and pave the way for exploring the quantum state of frustrated antiferromagnets.