| The perovskite oxides have been attracted much attention in the fields of modern electronic information due to the abundant physical and chemical properties, such as ferroelectricity, piezoelectric, acousto-optic, photovoltaic and electromagnetic effect, metal-insulator transition and colossal magnetoresistance. They have great application prospect in engineering such as optical, induction microwave and high density non-volatile memory devices. Recently, with the demands of the information technology, the microelectronic devices are developing towards the trends of miniaturization, integration, and multi-functions. This trend not only provides new chance for the research of the perovskite oxides, but also brings a serial of new challenges. First of all, the miniaturizations of electronic devices requires that the size of fabricated materials sustain within the nanoscale, which results in the properties of materials affected by the quantum size and surface effect. A typical example is the critical thickness of ferroelectricity effect that the ferroelectricity will vanish when the thickness of ferroelectric film is smaller than a critical size. Thus, it is an important subject how to lower the critical thickness of ferroelectric materials. On the other hand, the multi-functions trend of electronic devices expected to integrate various properties in the same material, such as the coupling of magnetic, electricity, mechanics and optics. Meanwhile, the multi-electric properties in the fabricated materials are expected to have a rapid response to the external fields. Owing to the conflict of mechanism between ferroelectricity with the formal d0configuration and ferromagnetism with partial filled d states in perovskite oxides, there are thus few single phase multiferroic materials in the natural surrounding. Therefore, it becomes the hot spots of functional materials and devices design for the research and exploration of the perovoskite materials with the strong magnetoelectric coupling as well as the controlling of electronic and magnetic properties of these materials by the external fields.In this thesis, the first-principles calculations within the density-functional theory have been used to explore the effective methods to obtain the magnetism in perovskite-oxide ferroelectrics, and three feasible ways have been proposed, including of the introduction of vacancy, magnetic impurity doping, and the composite materials of ferroelectricity/ferromagnetism. The method has been proposed and developed to lower the critical thickness of ferroelectrics and increase magneto-electric coupling effect in the multi-junctions by using the asymmetrical metal electrodes. The electronic and magnetic properties of manganite superlattice controlled by the strains have been analyzed and discussed, and the mechanism of phase transitions of magnetic ordering and electric conductivity in the perovskite-oxide superlattices have been explained, and the following results have been obtained.1. A comparative study of the stability and various types of vacancies-induced magnetism on different terminations of BaTiO3(001) surfaces. It is found that different vacancies can be obtained by adjusting the growth conditions. Under a metal-rich condition, O vacancies are abundant on both BaO-terminated and TiO2-terminated surfaces, while the Ti vacancy is energetically more favorable than O vacancy under the O-rich condition. Furthermore, the neutral O vacancies are stable for most of the allowed range of the Fermi level under metal-rich conditions. In contrast, the charged Ti vacancy is dominated for the O-rich condition. The analysis of electronic and magnetic properties of BaTiO3(001) with the vacancies has indicated that the local magnetic moments of O vacancy on the BaO-terminated surface and Ti vacancy on the TiO2-terminated surface induce are1.12μB and3.98μB, respectively. It is found that the magnetism induced by O vacancy on the BaO-terminated surface mainly arises from the spin-polarized electrons trapped in the vacancy basin, while the magnetism induced by Ti vacancy on the TiO2-terminated surface originates from the polarized2p electrons of O atoms localized around the Ti vacancy. However, O vacancy on the TiO2-terminated surface does not induce magnetism. The mechanism is that the lone electrons of Ti atoms near O vacancy prefer to pair together, which results in the deduction of the system energy.2. We have studied the electronic and magnetic properties of Co-doped perovskite BaTiO3using the GGA+U method of the first-principle calculations, and analyzed the O-vacancy effect on the electronic and magnetic properties of doped system. It is found that the Co-doped BaTiO3have presented the following magnetic trends:for the doping concentration x=3.7%, the total magnetic moment of system is4.78μB. When the doping concentration of Co impurities is increasing to7.4%, the total magnetic moments are5.86μB and6.60μB for the neighboring Co-Co interactions and far-distance Co-Co interactions, respectively. The results indicated that the origin of magnetism in all considered systems is arising from the Co dopant. If the O vacancy is introduced into the Co-doped BaTiO3, the magnetism of system will be decreased rapidly. For instance, the magnetism of Co-doped BaTiO3decreases from4.78μB to1.00μB when the3.7%O vacancies are introduced into the system. On the other hand, we have found the electrical properties of Co-doped BaTiO3system can be affected strongly by the O vacancy. Moreover, the increasing band-gap states lead to the enhanced electric conductivity of system with the increasing concentration of O vacancy. The present results can be applied to explain the experimental observations.3. A comparative study of critical thickness of ferroelectric barrier and magnetoelectric coupling effect in multijunction with symmetric and asymmetric metal electrodes. It is found the critical size of the ferroelectric barrier layer in Co/BaTiO3/Co is the thickness of four unit cells, about1.8nm. When the thickness of BaTiO3barrier layer is lower than the critical size, the ferroelectricity will be disappear due to the depolarization field caused by the unscreened polarized charge that is from the incomplete charge counteract between the Co electrode and BaTiO3layer. In contrast, the ferroelectricity of Co/BaTiO3/Fe can be maintained when the thickness of barrier layer is lower than four unit cells. The decreased critical size of ferroelectricity mainly originates from the work function difference of two asymmetrical electrodes. On the other hand, the use of asymmetry electrodes leads to the increase of the magnetoelectric coupling effect. With a four-unit thickness BaTiO3layer, the magnetoelectric coupling coefficient of multijunction with symmetry electrodes and asymmetry electrodes are2.46x10-10Gcm2/V and14.31×10-10Gcm2/V, respectively.4. Uniaxial strain effect on the magnetic ordering and the electric conductivity in (LaMnO3)2/(SrMnO3)2superlattices. The calculated results have shown that the transition of magnetic ordering from antiferromagnetism to ferromagnetism occurs at the uniaxial tensile strain of～1.4%. With the increase of the uniaxial compressive strain, the out-of-plane electrical conductivity (along the grown direction of superlattice) decreases. Moreover, the out-of-plane electrical conductivity will decrease to zero when the compressive strain is up to～12%. In this case, the electrical transport is transformed from3-dimension to2-dimension. The analysis of band structures and density of states have shown that the in-plane electronic states, such as Mn-dx2-y2, O-px, and O-py are responsible for the2-D conductivity. Based on these results, the out-of-plane conductivity should be controlled by adjusting the external strains. 5. The shear strain effect on the electronic, magnetic and transport properties of (LaMnO3)2/(SrMnO3)2superlattices. The results have found:without the external strain, the superlattices possesses a strong spin-polarized conductivity and half-metal characteristic, and show the ferromagnetic ordering in three dimensions. When a4%-tensile pyramidal symmetry (TPS) is loaded in the superlattice, all in-plane (the plane perpendicular to the grown direction of superlattice) electronic states including of Mn3dx2-y2orbital, O2px, and2py orbital are removed from the Fermi level, thus the in-plane electronic transport will be suppressed and the electrical conductivity will transit from three-dimension to one-dimension out-of-plane. Meanwhile, the loaded strain leads to the suppression of the double-exchange interaction which results in the transition of magnetic ground state from ferromagnetism to the C-type antiferromagnetism. On the other hand, when a4%-compression pyramidal symmetry (CPS) is loaded in the superlattice, it is found that all the out-of-plane electronic states around the Fermi level, such as the Mn3d3z2-1orbital and O2pz orbital are disappear. Here the out-of-plane electronic transport is suppressed and the conductivity of superlattice changes from3-dimension to2-dimension. The loaded strain also induces the magnetic ground state transition of superlattice from ferromagnetism to A-type antiferromagnetism. |
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