Ab initio study of electrons and phonons in multiferroic BiFeO3

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Multiferroic materials exhibit two or more properties of ferromagnetism (or antiferromagnetism), ferroelectricity (or antiferroelectricity) and ferroelasticity. Such materials can develop electric polarisation when subjected to an external magnetic field, and can develop magnetisation when subjected to an external electric field. The perovskite BiFeO3 is known to be the only material that exhibits multiferroism at room temperature. In single crystal form itis rhombohedrally distorted perovskite with space group R3c, and is ferroelectric and exhibits antiferromagnetic behaviour up to 643 K. Such 'magneto-electric' materials have technological applications as Hall probes, UHF spin-wave oscillators, and can also be employed in potential applications in information storage, in the emerging field of spintronics, and sensors.Several works have recently been done on the study of the structural, electronic and magnetic properties of BiFeO3. In its single crystal rhombohedral phase it shows only small values of ferroelectric polarisation. However, epitaxially grown films with (001)-oriented pseudotetragonal structure shows an order of magnitude higher polarisation. Polarlized Raman scattering method has been employed to examine some phonon modes for the tetragonal phase. However, no systematic studied have emerged on dynamical properties, e.g. mode assignment and full phonon dispersion relations. As a wide range ofphysical properties of solids depend on their lattice dynamical properties, it is important to undertake a systematic study of phonons for the two structural phases (rhombohedral and tetragonal). Due to the presence of localised Fe 3d electronic states, it is further desirable to investigate changes in phonon modes due to electronic effects such as spin polarisation and strong Coulomb repulsion between the localised states.The proposed research is aimed at carrying out ab-inito theoretical and computational studies of the electronic structure and phonon modes in the multiferroic crystal BiFO3. Our work is expected to help experimentalists analyse their Raman scattering measurements.