First-principles study of magnetoelectric effects and ferroelectricity in complex oxides
Creators: name: Ye, Meng
Publisher: name: No Publisher Supplied
Year: 2016

description: This thesis contains several investigations of magnetoelectric effects and ferroelectricity  in complex oxides studied via   first-principles calculations. We start by reviewing the  mechanisms of ferroelectricity and magnetoelectric e ffects, and then we give a brief introduction  to the  first-principles computational methods that are involved. Next, our  investigations are divided into two parts. The first half focuses on the magnetoelectric  effects, while the second half is mainly on ferroelectricity. The first half aims to examine  the lattice contribution to the magnetoelectricity by investigating the dynamical  magnetic charge tensors induced by different mechanisms. Through the study of Cr2O3  and a  fictitious material KITPite, we  find that the dynamical magnetic charges driven  by exchange striction are more significant than the ones induced by spin-orbit coupling.  Since the lattice contribution to the magnetoelectric effect is proportional to the dynamical  magnetic charges, we also study the magnetic charges and the magnetoelectric  coupling in hexagonal manganite RMnO3 and ferrite RFeO3. Our results further confirm    the importance of the exchange-striction mechanism in inducing large magnetic   charges, but we also notice that the magnetoelectric contributions from various phonons  tend to cancel each other, leading to a great reduction of the total coupling. These investigations  not only provide a prediction of the magnetoelectric coupling constant in  RMnO3 and RFeO3, but also emphasize the importance of phonons in magnetoelectric  coupling. In the second half of the thesis, we focus on predicting new ferroelectrics  in the family of corundum derivatives. Many new corundum derivatives have been  synthesized recently; these are automatically polar, and many are magnetic as well.  However, a polar material is only called ferroelectric if the polarization is reversible by  an external field, and it is not yet clear whether or not this is the case for these new  materials. Motivated by this question, we use a structural constraint method to study  the ferroelectric reversal path and energy barrier of several corundum derivatives. As  a result, we predict several FE candidates with insulting reversal paths and low barrier  energies. Since the hysteresis behavior of ferroelectrics is attributed to the ferroelectric  domain wall motion, we further investigate the formation and motion of ferroelectric  domain walls in corundum derivatives. Our study predicts the atomic structure and orientation  of the ferroelectric domain wall, as well as the shape of ferroelectric domains.  In addition, we fi nd novel properties at domain walls, including a strong magnetoelectric  coupling and an interlocking between chirality and polarization. Moreover, we use  the structural constraint method to study the barrier energy of ferroelectric domain  wall reversal. Our results suggest that the barrier energy is linearly correlated with the  bond valence sum, which can be used as a guide to  find new ferroelectrics in the family  of corundum derivatives.