On the Origin of Improper Ferroelectricity in Manganites and Normal Ferroelectricity in Ferrites

Abstract

Multiferroics exhibit simultaneous ferroic properties such as ferroelectricity, ferromagnetism, and ferroelasticity with coupled electric, magnetic, and structural order parameters. Multiferroic materials are particularly appealing owing to their potential for enabling entirely new device paradigms, especially in non-volatile random access memory and magnetic data storage. Over the past decade, a variety of theoretical models were proposed to account for the origins of improper ferroelectricity as well as proper ferroelectricity. However, little progress has been made in our understanding of the ferroelectricity origin. In view of this, it is of great scientific interest to identify the main driving force of the ferroelectric origin in multiferroics. In view of this, we have investigated the origin of the ferroelectricity in different types of multiferroic materials by exploiting first-principles density-functional theory (DFT) calculations. We have first carried out the magnetically-induced ferroelectric origin of orthorhombic manganites such as TbMnO3, DyMnO3, YMnO3, and TbMn2O5. These four manganites have been the most extensively studied because they exhibit a strong tendency of the magnetoelectric coupling which stems from a spin-ordering-induced improper ferroelectricity. Among these, the orthorhombic TbMnO3 and DyMnO3 are best known examples of the spin-orbit-coupling-driven ferroelectiric materials. Based on our DFT calculations, the spin-orbit-coupling-driven reverse Dzyaloshinskii-Moriya (DM) interaction, Si × Sj type mechanism, is primarily responsible for the improper ferroelectricity in TbMnO3 and DyMnO3. We have also shown that the electric polarization along the c-axis can be flopped to the electric polarization along the a-axis by changing from bc to ab cycloidal spin structure. On the contrary to this, the ferroelectricity in orthorhombic YMnO3 having a collinear spin structure and TbMn2O5 having a noncollinear spin structure is driven by an exchange-striction type mechanism of Si • Sj without invoking the spin-orbit coupling. In order to clarify the origin of orthorhombic ferroelectricity in YMnO3 and TbMn2O5, we have carried out DFT calculations by considering the spin-orbit interaction. Our DFT calculations predict that the present calculations clarify the ferroelectricity origin that the electric polarization in TbMn2O5 is mainly caused by an exchange-striction (Si ∙ Sj). We have further successfully reproduced the field-induced polarization change by adopting a spin configuration which is likely to occur under a bias magnetic field in YMnO3 and TbMn2O5. Second, we show that the asymmetric d-p orbital hybridization is the main cause of the ferroelectricity in Hexagonal P63cm ReFeO3 (Re = Y, Lu, Yb). In fact, all of the ReFeO3-type oxides (R = rare-earth) belong to orthorhombic ferrites (orthoferrites) in a centrosymmetric Pbnm (or Pnma) perovskite crystal. Accordingly, all of the rare-earth orthoferrites are known to be non-ferroelectric. More recently, however, we demonstrated that an epitaxial ReFeO3 thin-film heterostructure fabricated by adopting a hexagonal template is ferroelectric with a six-fold hexagonal symmetry. According to our DFT calculations, the asymmetric Re dz2-O pz hybridization along the c-axis of P63cm is the origin of this extraordinary ferroelectricity in Hexagonal P63cm ReFeO3. Finally, we have investigated the ferroelectric properties of BiFeO3-based multiferroics. The stereochemically active 6s lone-pair electrons of Bi are responsible for the ferroelectricity origin of BiFeO3. In addition, our DFT calculations show that the La-doping substantially reduces not only the ferroelectric polarization but also the displacive ferroelectric character.