Um estudo de primeiros princípios sobre a origem e os mecanismos da ferroeletricidade nos compostos multiferróicos RMnO3 (R=Y, Lu)

Abstract

The RMnO3 (R=Y,Lu) compounds, members of the family of hexagonal manganites, are materials which exhibit strong magneto-electric coupling characteristic for multiferroic compounds. Despite the numerous studies with objective to reveal the origin of this phenomenon, the mechanism that is causing it is still not fully understood. The question that attracts special attention of scientific community is about the cause of ferroelectric distortion that occurs at certain temperatures in these materials. Although this issue is discussed in many papers, there is still no consensus what mechanism is responsible for it: (1) hybridization between the Mn dz2 orbital and O pz orbital (Mn d0 –ness model), (2) hybridization between the R dz2 orbital and O pz orbital (R d0 –ness model), (3) geometric effects or (4) charge transfer from Mn-O bonds to R-O bonds. Understanding of the mechanism that leads to ferroelectric polarization is necessary to speed application of these materials in ferroelectric memories or spintronics devices. Objective of the present work was to evaluate the first two possible mechanisms that might cause the ferroelectric distortion in RMnO3. Adopted strategy was to investigate what happens with the Mn-O and R-O chemical bonds after the structural phase transition from paraelectric to ferroelectric phase that occurs at high temperature, at which both phases are characterized by paramagnetic order of the Mn magnetic moments. For that purpose, the first-principles calculations based on density functional theory were carried on, with usage of the most modern exchange-correlation potentials. The chemical bonds were evaluated (1) qualitatively, by analysis of density of electronic states (DOS) and maps of electronic density along the bonds, and (2) quantitatively, in terms of topological analysis of Bader. The results revealed that no significant change occurred with Mn-O bonds, while the R-O bonds were affected by phase transition in the sense that R dz2 and O pz orbital exhibited enhanced hybridization in the ferroelectric phase. Therefore, the present study substantiates the R d0 –ness mechanism as the probable cause of ferroelectric distortions in RMnO3 compounds.