Polar Nanodomains and Giant Converse Magnetoelectric Effect in Charge-Ordered Fe$_{2}$OBO$_{3}$

Charge ordering (CO) is considered to be an important issue that leads to metal-insulator transitions in numerous materials and also shows possible correlations to many notable physical phenomena, such as colossal magnetoresistance, superconductivity and multiferroics. In recent investigations, oxyborate Fe$_{2}$OBO$_{3}$ has been found to show certain structural and physical features in connection with a continuous CO transition [1, 2]. By using\textit{ In-situ }TEM technique, we revealed that the charge-ordering transition characterized by an incommensurate modulation could evidently result in remarkable polar nanodomains at low temperatures. This kind of nanodomain could play a critical role in triggering a high dielectric constant and notable dielectric dispersion as observed in Fe$_{2}$OBO$_{3}$. Moreover, measurements of the magnetoelectric coupling under electrical field demonstrate the existence of giant electrically induced changes in magnetization around the magnetic transition [1, 2]. 1.Y. J. Song et al., Phys. Rev. B 81, 020101(R) (2010). 2.H. X. Yang et al., Phys. Rev. Lett. 106, (2011) 016406.