The RMn 2 O 5 manganite compounds represent one class of multiferroic family with magnetic origins, which has been receiving continuous attention in the past decade. So far, our understanding of the magnetic origins for ferroelectricity in RMn 2 O 5 is associated with the nearly collinear antiferromagnetic structure of Mn ions, while the exchange striction induced ionic displacements are the consequence of the spin frustration competitions. While this scenario may be applied to almost all RMn 2 O 5 members, its limitation is either clear: the temperature-dependent behaviors of electric polarization and its responses to external stimuli are seriously materials dependent. These inconsistences raise substantial concern with the state-of-the-art physics of ferroelectricity in RMn 2 O 5 . In this mini-review, we present our recent experimental results on the roles of the 4f moments from R ions which are intimately coupled with the 3d moments from Mn ions. DyMn 2 O 5 is a golden figure for illustrating these roles. It is demonstrated that the spin structure accommodates two nearly collinear sublattices which generate respectively two ferroelectric (FE) sublattices, enabling DyMn 2 O 5 an emergent ferrielectric (FIE) system rarely identified in magnetically induced FEs. The evidence is presented from several aspects, including FIE-like phenomena and magnetoelectric responses, proposed structural model, and experimental check by nonmagnetic substitutions of the 3d and 4f moments. Additional perspectives regarding possible challenges in understanding the multiferroicity of RMn 2 O 5 as a generalized scenario are discussed.

Through first-principles density functional calculations, the magnetic ground state and electronic structure of mixed-valent manganite SrMn7O12 in the rhombohedral symmetry are investigated in detail. Our calculations show clearly that the possible magnetic ground state accommodates a particular helical spin order responsible for the insulating state in SrMn7O12. The giant ferroelectric polarization is revealed, which is almost purely from the electronic contribution rather than the ionic displacements and is closely related to the magnetism. The influence of the on-site Coulomb interaction and spin-orbit coupling on the electronic structure and multiferroicity is also discussed. The present results suggest that SrMn7O12 is a promising multiferroic candidate with desirable properties.