Electronic Ferroelectric Properties in Mixed Valence Iron Oxide RFe₂O₄

Abstract

Mixed-valence ferrites RFe₂O₄（R＝trivalent rare earth ions）consist of an equal number of Fe^2＋ and Fe^3＋ in a triangular lattice. The Fe^2＋ and Fe^3＋ exhibit long-range charge ordering at room temperature. It has long been pointed out that the charge ordering breaks the inversion symmetry of the crystal, and RFe₂O₄ has been a candidate for electronic ferroelectric material at room temperature. We report here our recent success in synthesizing RFe₂O₄ single crystals with high chemical stoichiometry by the floating zone method and high-quality thin films by the magnetron sputtering technique. We also describe the details of the synthesis of RFe₂O₄ crystals mentioning the history of RFe₂O₄ research so far. We will also report on the observation of electronic ferroelectric polarization in RFe₂O₄ through neutron scattering measurements and second harmonic generation measurements using the obtained single crystals, and our recent attempts at ultrafast optical control of the electric polarization in RFe₂O₄.