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 R2V2O7 (R = Ni, Co) induced by high magnetic fields. The half plateau is stabilized in fields of 8-30 T (7-12 T) for Ni2V2O7 (Co2V2O7), 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 50-60 mu C/m(2) for the polycrystalline sample. The resulting magnetic field-temperature (H-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 Co2V2O7 associated with the low-field ferroelectricity. Our magnetization data also reveal that Co2V2O7 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.