| Multiferroic material is a kind of material simultaneously exhibiting two or more of the primary ferroic properties (ferroelectricity, ferromagnetism, ferroelasticity and ferrotoroidicity), more interestingly, couplings between these ferroic orderings have potential applications. As a pioneer example in researching multiferroic material, BiFeO3(BFO) is the most attractive candidate among single-phase multiferroics at room temperature, and it shows both ferroelectricity and antiferromagnetism. Its magnetoelectric effect and electric-field control of magnetism have attracted intense interest. As one of the most important field in spintronics, electric-field control of magnetism has became the focus in information science studying, and played an important role in the field of fundamental research and information storage.It is a common belief that the ferroelectric of BFO originates from Bi-6s lone pair. However, it can not explain the ferroelectric polarization magnititude in tetragonal BFO as well as the strong couplig between the direction of ferroelectric polarization and the easy axis of magnetization. Here we use the first-principles calculation with orbital selective external potential (OSEP) method to clarify the ferroelectric origin of BFO, and reveal the physical mechanism of electric-field control of magnetism. The main works and innovations of this dissertation are listed as follows:1Cooperating with Prof. Wan in Nanjing university, we have developed OSEP method which can shift the energy level of a specific atomic orbital and illustrate the effects of atomic orbital for physical or chemical properties. In the OSEP method, multiple orbital-dependent potentials can be applied to the system simultaneously, providing great flexibility to study various effects on the problem of origin and hybridization.2In indirect magnetic exchange systems, interatomic magnetic exchange interaction is favourable to ferroelectric phase, which is a new microscopic mechanism for ferroelectric origin. If magnetic energy can overcome elastic energy, the system shows ferroelectric phase. In the case of classicical antiferromagnetic material MnO, compressive epitaxial strain can increase magnetic energy and decrease elastic energy to induce ferroelectric phase.3The ferroelectric origin of BFO is investigated. Besides the influence of Bi-6s lone pair, Fe-3d states also contribute to ferroelectricity, and the influence of Fe-3d states include Fe-O-Fe superexchange interaction and Fe-O bonding. Because Fe-O-Fe superexchange interaction is enhanced in tetragonal phase, the effect of Fe-3d states play a more important role in ferroelectricity. The ferroelectric polarization can be also increased correspondingly.4Electric-field control of the magnetic ordering and metal-insulator transition are investigated in tetragonal-like BFO. A transition from Cl to G-type antiferromagnetic phase exists at the [001] polarized state with the in-plane constant3.91A, and such magnetic phase transition can be explained by the competition between the heisenberg exchange constants J1c and J2c under a biaxial strain. At the same time [111] polarized state remains G-type antiferromagnetic phase. Therefore, under appropriate epitaxial strains, electric-field control of the polarization direction from [001] to [111] can influence the magnetic ordering. Researching the linear and nonlinear optical properties of tetragonal-like BFO, we find that at low frequencies second-harmonic generation susceptibility X(1) of C1-type antiferromagnetic are larger than that of G-type antiferromagnetic. Therefore, we can take advantage of X(2) to detect C1and G-type antiferromagnetic ordering. The strain and polarization direction can not only influence the magnetic ordering, but also induce metal-insulator transition. Increasing the lattice constant, the [001] polarized state changes from insulator to metal, but the [110] polarized state changes from metal to insulator. Therefore, electric-field control of the polarization direction can induce metal-insulator transition under appropriate epitaxial strains. 5The magnetocrystalline anisotropy energy and net magnetic moment are investigated in BFO. When the polarization direction is changed from [111] to [001] direction, magnetocrystalline anisotropy energy can be increased by an order of magnitude, which can increase Dzyaloshinskii-Moriya interaction. Therefore, the role of noncollinear magnetic ordering should be considered. The calculations show that both canting angle and net magnetic moment increase, representing the effect of electric-field control of net magnetic moment. |
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