Properties of Multiferroic BiFeO3 from First Principles

In this dissertation, a first-principle-based approach is developed to study magnetoelectric effect in multiferoic materials. Such approach has a significant predictive power and might serve as a guide to new experimental works. As we will discuss in the course of this work, it also gives an important insight to the underlying physics behind the experimentally observed phenomena. We start by applying our method to investigate properties of a generic multiferroic material. We observe how magnetic susceptibility of such materials evolves with temperature and compare this evolution with the characteristic behavior of magnetic susceptibility for pure magnetic systems. Then we focus our attention to particular multiferroic - BiFeO3 - and reproduce its magnetic states with all of their essential features. Those magnetic states include (i) antiferromagnetic state, (ii) state with weak ferromagnetism resulting from canting of magnetic moments, and (iii) cycloidal magnetic structure. All of those magnetic states were also studied under external electric and magnetic fields. Under such electric fields magnetic order parameters of the systems undergo interesting transformations and sometimes take unexpected path. Finally, we study the material under strain and explore possibilities of favoring one magnetic state over another and even creating states that can be stable only under the strain