A First Principles Study Of Magnetoelectric Effects In Some Perovskite Oxides

With the rapid development of information industry, people usually pursuit devices of a better performance with large capacity, high speed and low energy cost. Multiferroic materials hold both the charge and spin orders and their coupling effe ct, possessing more freedoms to design high quality devices to meet the needs of modern information industry. The single phase multiferroic materials have been studied widely, as their magnetism and ferroelectricity can be strongly coupled. However, coexistence of ferromagnetic and ferroelectricity in single-phase magnetoelectric materials is relatively rare, while most of magnetoelectric materials show antiferromagnetic coupling, not conducive for practical application. This dissertation is aimed to design new types of magnetoelectric materials and analysis their magnetic and ferroelectric coupling mechanism. The main results were listed as following:The effects of Co doping on the structure, magnetic and the dielectric properties of Bi Fe O3 are predicted through ab initio calculation based on density function theory(DFT). The calculations revealed that the B-site Co-doping has significant effect on the electronic and magnetic properties of Bi Fe O3, which are determined by Fe 3d-O 2p and Fe 3d-Fe 3d interactions. The induced magnetism should be ascribed to the superexchange interaction of Fe3+–O2-–Co3+. Magnetic measurements confirmed that Co substitution can effectively induce the a ppearance of the spontaneous magnetization. Both the Fe-O bond anisotropy in the Fe O6-octahedron cage and the off-centering ferroelectric polarization along the hexagonal [001]h are predicted to be substantially increased by the Co-doping. These results may shed some light on the controlling and tuning of the multiferroic properties of Bi Fe O3.The structural, electronic, magnetic, and ferroelectric properties of Ni Ti O3 are predicted through ab initio calculations based on the density functional theory(DFT). The theoretical structure parameters matched well with those obtained experimentally. The electronic structure results show that the antiferromagnetic(AFM) phase of Li Nb O3(LN)-type Ni Ti O3 has a direct band gap of 2.16 e V. The calculated local magnetic moment of Ni ion is 1.61μB. The calculated Born effective charges(BECs, denoted by tensor Z*) show that the Z* of Ti and O atoms are significantly and anomalously large. Interestingly, ferroelectric spontaneous polarization is predicted to be along [111] direction with a large magnitude of 94μC/cm2. B-site Ti ions in 3d0 state dominate ferroelectric polarization of multiferroic Ni Ti O3, whereas A-site Ni ions having partially filled eg orbitals are considered to contribute to the antiferromagnetic properties of Ni Ti O3. Furthermore, the current study also found that the polar lattice distortion can induce weak ferromagnetism.The ground state structural, electronic, magnetic and ferroelectric properties of Tl Ni O3 are calculated by using density functional theory within the generalized gradient approximation. The calculations reveal that Tl Ni O3 has an antiferromagnetic ground state with a direct band gap of 0.53 e V. The local magnetic moment of Ni(1) and Ni(2) are 1.735μB and 0.809μB, respectively. Plots of the density of states(DOS) exhibit hybridization of Ni(1), Ni(2)-3d, and O-2p states. However, the calculated charge density and electron localization function(ELF) show a largely ionic character of the Ni-O bonds which is also supported by the anomaly in the calculated Born effective charges(BECs) with respect to the corresponding nominal ionic charges. We also find a spontaneous polarization of 2.13μC/cm2 along the b-axis which is due to charge order induced by magnetic ordering.The structure-property relation is a key outstanding problem in the study of magnetoelectric multiferroic materials. First-principles calculations were used to explore the influences of Hubbard-U on the ground state of double-perovskite Lu2 Co Mn O6 determined by a series of complex electronic and structural parameters. The origin of ferroelectricity in the multiferroic Lu2 Co Mn O6 is elucidated by the calculations of collinear and non-collinear magnetic structure. It is confirmed that the up-up-down-down spin arrangement plays a significant role in the ferroelectric(FE) polarization by the contrast of the lattice geometries and electronic structures in different magnetic orders. The current study shows that both the mechanisms of exchange strictive effect and spin-charge ordering are simultaneously active and lead to the ferroelectric polarization in Lu2 Co Mn O6. Similar to the early discoveries in Ho Mn O3 and Dy Fe O3, we also found and explained the mechanism of switching the ferroelectric polarization via a 180° coherent rotation of Co and Mn spins in double-perovskite Lu2 Co Mn O6.