First-principles study of electric polarization in piezoelectric and magnetoelectric materials

Abstract

First-principles calculations based on the density-functional theory (DFT) have proven to be extremely useful in the study of properties of matter. Not only do they provide a sufficient accuracy to reproduce experimental results, but also they make it possible to predict materials with enhanced or even new properties. Often first-principles calculations become a cheap alternative to real experiments or even allow one to investigate regimes not accessible experimentally either in principle or because of limitations of experimental techniques. But the real power of methods based on computer simulations is that they can help one to understand the microscopic mechanisms of physical processes inside the materials. In my thesis work I will analyze electric polarization properties of several materials and their dependence on some physical parameters such as strain, chemical doping, and magnetic order. In the first part of my thesis I will present an ab-initio study of wurtzite ZnO doped with Mg. Several ordered structures modeling the Zn1−xMgxO alloy are analyzed with different Mg concentrations. The electric polarization is studied as a function of Mg concentration x under different strain conditions. We find that to a good approximation the polarization depends linearly on x. We show that a simple model based on the piezoelectric response of pure ZnO can reproduce the results fairly well. In the second part, we study the magnetoelectric coupling in a spiral magnet TbMnO3. It is known from experiment that at low temperatures a ferroelectric phase appears simultaneously with the onset of a cycloidal magnetic order. Using first-principles methods, we demonstrate that ferroelectricity in this material is indeed driven by magnetic order. We show that spin-orbit coupling is essential for the electric polarization to appear. We also demonstrate that the ionic displacements induced by a cycloidal magnetic order, though tiny, play a crucial role in producing polarization. We do a detailed analysis of the forces on ions and ionic displacements from the mode-decomposition viewpoint, and find that simple models based only on nearest-neighbor interactions between Mn ions through oxygen are not able to account fully for the results.%%%%