Multiferroicity due to Charge Ordering

In this contribution I discuss multiferroicity that is driven by different forms of charge ordering, presenting first the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. In type-I multiferroics [1], ferroelectricity and magnetism have different origins and occur at different temperatures. There is a number of specific classes of materials for which this is relevant. Discussed will be in some detail (i) perovskite manganites of the type (PrCa)MnO3 [2,3], (ii) the complex and interesting situation in magnetite Fe3O4, (iii) strongly ferroelectric frustrated LuFe2O4 and (iv) an example of a quasi-one-dimensional organic system [4]. In type-II multiferroics [1], ferroelectricity is completely due to magnetism, but with charge ordering playing an important role [5], such as (v) multiferroic Ca3CoMnO6, (vi) possible ferroelectricity in rare earth perovskite nickelates of the type RNiO3 [6,7], (vii) multiferroic properties of manganites of the type RMn2O5 [8], (viii) perovskite manganites with magnetic E-type ordering. \\[4pt] [1] J. van den Brink and D. Khomskii, J. Phys.: Condens. Matter 20, 434217 (2008).\\[0pt] [2] D.V. Efremov, J. van den Brink and D.I. Khomskii, Nature Materials 3, 853 (2004).\\[0pt] [3] G. Giovannetti, S. Kumar, J. van den Brink, S. Picozzi, Phys. Rev. Lett. 103, 037601 (2009).\\[0pt] [4] G. Giovannetti, S. Kumar, A. Stroppa, J. van den Brink and S. Picozzi, Phys. Rev. Lett. 103, 266401 (2009). \\[0pt] [5] J. Betouras G. Giovannetti and J. van den Brink, Phys. Rev. Lett. 98, 257602 (2007).\\[0pt] [6] G. Giovannetti, S. Kumar, D. Khomskii, S. Picozzi and J. van den Brink, Phys. Rev. Lett. 103, 156401 (2009).\\[0pt] [7] S. Kumar, G. Giovannetti, J. van den Brink and S. Picozzi, Phys. Rev. B 82, 134429 (2010).\\[0pt] [8] G. Giovannetti and J. van den Brink, Phys. Rev. Lett. 100, 227603 (2008).