Ab initio study of electrons and phonons in multiferroic BiFeO3

多铁性材料表现出铁磁性（或反铁磁性）、铁电性（或反铁电性）和铁弹性的两种或两种以上的特性。这种材料在受到外部磁场的作用时可以产生电极化，并且在受到外部电场的作用时可以产生磁化。已知钙钛矿BiFeO3是唯一在室温下表现出多铁性的材料。单晶形式是具有R3c空间群的菱面扭曲钙钛矿，具有铁电性，在643k时表现出反铁磁行为。这种“磁电”材料具有霍尔探头、超高频自旋波振荡器等技术应用，也可用于信息存储、自旋电子学和传感器等新兴领域的潜在应用。近年来，人们对BiFeO3的结构、电子和磁性进行了研究。在单晶菱面体相中，它的铁电极化值很小。然而，外延生长具有（001）取向伪四方结构的薄膜显示出高数量级的极化。用偏振拉曼散射方法研究了四方相的声子模式。然而，对其动力学特性，如模式分配和全声子色散关系，还没有系统的研究。由于固体的各种物理性质取决于其晶格动力学性质，因此对两种结构相（菱形和四边形）的声子进行系统的研究是很重要的。由于局域化Fe三维电子态的存在，进一步研究由于局域化态之间的自旋极化和强库仑排斥等电子效应而引起的声子模式的变化是可取的。本研究旨在开展多铁晶体bof3的电子结构和声子模式的ab-inito理论和计算研究。我们的工作有望帮助实验学家分析他们的拉曼散射测量结果。

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.