Successive electric-polarization switches in the S=1/2 skew chain Co2V2O7 induced by a high magnetic field

We report successive electric-polarization $(P)$ switches in the $S=1/2$ quantum magnet ${\mathrm{Co}}₂{\mathrm{V}}₂{\mathrm{O}}₇$ by application of a magnetic field along the $b$ axis, where a 1/2-magnetization plateau is seen at 5.4--11.6 T. Polarization reversal appears at $\ensuremath{\sim}5$ T from $\ensuremath{-}P\ensuremath{\parallel}b$ to $+P\ensuremath{\parallel}b$, leading to an irreversible magnetoelectric history involved with a memory effect, while the polarization flop in fields of 12--17 T is identified by a transition from the $P\ensuremath{\parallel}b$ to $P\ensuremath{\parallel}ac$ plane, different from those reported previously. These intriguing magnetoelectric phenomena are owing to the unique nature of the skew-chain-like magnetic structure of ${\mathrm{Co}}₂{\mathrm{V}}₂{\mathrm{O}}₇$ and can be understood by a change in symmetry of the magnetic order in applied fields. The emergent ferroelectricities deviating from the half-plateau state may arise from magnon Bose-Einstein condensation in this quantum magnet.