James C. McGroddy Prize for New Materials Talk: What is new in multiferroicity?: Mott ferroelectrics!

Multiferroicity is an old topic. For example, linear magnetoelectric effect in materials such as Cr$_{2}$O$_{3}$ with broken time reversal and space inversion symmetry has been known since 1960's. However, giant cross-coupling effects such as flipping polarization or enormous change of dielectric constant by applied magnetic fields have been recently observed in systems such as Tb(Dy)MnO$_{3}$ and Tb(Dy)Mn$_{2}$O$_{5}$ [1-3]. The important ingredient for these giant magnetoelectric effects turns out to be associated with the presence of non-zero d electrons and their mutual interactions, leading to the Mott-insulator-type charge gap, magnetism, and collective phase transitions. Particularly, the collective nature of simultaneous magnetic-ferroelectric phase transitions results in the giant magnetoelectric effects. In addition, fascinating charge transport properties such as a switchable photovoltaic effect and characteristic conduction properties at domain walls stem from the (carrier-doped) Mott insulating nature of compounds such as BiFeO$_{3}$ and hexagonal YMnO$_{3}$ [4,5]. \\[4pt] [1] Kimura, T. \textit{et al}. Magnetic control of ferroelectric polarization. \textit{Nature} \textbf{426}, 55--58 (2003).\\[0pt] [2] Hur, N. \textit{et al}. Electric polarization reversal and memory in a multiferroic material induced by magnetic fields. \textit{Nature} \textbf{429}, 392--395 (2004).\\[0pt] [3] Cheong, S.-W. {\&} Mostovoy, M. Multiferroics: a magnetic twist for ferroelectricity. \textit{Nature Mater.} \textbf{6}, 13--20 (2007).\\[0pt] [4] Seidel, J. \textit{et al}. Conduction at domain walls in oxide multiferroics. \textit{Nature Mater.} \textbf{8}, 229--234 (2009).\\[0pt] [5] Choi, T., Lee, S., Choi, Y.J., Kiryukhin, V. {\&} Cheong, S.-W. Switchable ferroelectric diode and photovoltaic effect in BiFeO$_{3}$. \textit{Science} \textbf{324}, 63--66 (2009)