铋系多铁氧化物的铁电起源及相关电控磁性机制的第一性原理研究
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摘要
多铁性材料是一类同时具有两种或两种以上铁性特征(铁电性、铁磁性、铁弹性或铁涡性)的多功能材料,而其中各种铁性之间存在耦合关系使得多铁性材料具有潜在的应用前景。作为多铁性材料的研究先锋,BiFeO3(BFO)是少数在室温条件下同时具有反铁磁性和铁电性的单相多铁性材料,其磁电耦合效应和电控磁性受到研究者的广泛关注。而作为自旋电子学的一个重要分支,电控磁性成为目前信息科学的研究热点之一,并且在基础研究与信息存储领域有着重要的意义。
目前普遍的观点认为BFO的铁电性起源于Bi-6s孤对电子,但这很难解释四方相BFO的铁电极化强度以及铁电极化方向和磁化易轴存在的耦合关系。本论文采用第一性原理计算方法,结合我们发展的轨道选择性外场(Orbital Selective External Potential, OSEP)方法,阐明了BFO的铁电起源,并揭示了相关电控磁性的物理机制。主要内容和创新点如下：
1和南京大学万贤纲教授合作发展了OSEP方法。该方法可以精确地控制特定原子轨道的移动,有助于研究原子轨道对材料物理和化学性质的影响。OSEP方法可以同时在不同原子轨道上施加外势场,这给研究体系的某些现象或是起源问题提供了多种选择性。
2在间接磁交换体系中发现一种全新的铁电形成机制,磁性相互作用有利于形成铁电相,若磁性能具备克服弹性能的条件,体系会呈现铁电相。以经典的反铁磁材料MnO为例,计算得到双轴压应力可以增加磁性能,降低弹性能,从而导致铁电相的出现。
3研究了BFO的铁电起源。计算发现除了Bi-6s孤对电子对铁电性有影响之外,Fe-3d态对铁电性也有贡献,包括间接超交换作用和Fe-O成键。尤其是在四方相BFO中,由于Fe-O-Fe超交换作用的增强,使得Fe-3d态对铁电性的影响更加明显,铁电极化强度也随之增强。
4研究了类四方相BFO的电控磁有序和金属-绝缘相变。结果发现[001]极化态的BFO在晶格常数为3.91A附近其反铁磁序出现从C1型向G型的转变,而这种改变可以用海森堡交换积分常数Jc和J2c的竞争来解释。同时,由于[111]极化态一直保持在G型反铁磁序,因此在一定条件下外加电场使得铁电极化从[001]转到[111]方向,相应的反铁磁序从C1型转变为G型,从而实现了电控磁有序。而通过计算类四方相BFO的线性和非线性光学性质,发现在低频处二次谐波系数χzzz(2)对C1型反铁磁序的响应要比G型反铁磁序强很多,因此可以通过探测χzzz(2)来区别C1型和G型反铁磁序。应力和铁电极化方向不仅能改变磁有序,也能诱导金属-绝缘相变,当晶格常数逐渐增加时,[001]极化态从绝缘态变为金属态,而[110]极化态却从金属态变为绝缘态,因此在一定的晶格常数下外加电场可以改变铁电极化方向来引起金属-绝缘相变的出现。
5研究了BFO的磁晶各向异性能和净磁矩。计算发现当铁电极化从[111]转到[001]方向时,其磁晶各向异性能将会增加一个数量级,这增强了体系中Dzyaloshinskii-Moriya相互作用。通过考虑BFO的非共线磁序,计算得到磁矩偏角以及净磁矩都会出现明显的增加,从而体现出一种电控净磁矩的效应。
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.
目前普遍的观点认为BFO的铁电性起源于Bi-6s孤对电子,但这很难解释四方相BFO的铁电极化强度以及铁电极化方向和磁化易轴存在的耦合关系。本论文采用第一性原理计算方法,结合我们发展的轨道选择性外场(Orbital Selective External Potential, OSEP)方法,阐明了BFO的铁电起源,并揭示了相关电控磁性的物理机制。主要内容和创新点如下：
1和南京大学万贤纲教授合作发展了OSEP方法。该方法可以精确地控制特定原子轨道的移动,有助于研究原子轨道对材料物理和化学性质的影响。OSEP方法可以同时在不同原子轨道上施加外势场,这给研究体系的某些现象或是起源问题提供了多种选择性。
2在间接磁交换体系中发现一种全新的铁电形成机制,磁性相互作用有利于形成铁电相,若磁性能具备克服弹性能的条件,体系会呈现铁电相。以经典的反铁磁材料MnO为例,计算得到双轴压应力可以增加磁性能,降低弹性能,从而导致铁电相的出现。
3研究了BFO的铁电起源。计算发现除了Bi-6s孤对电子对铁电性有影响之外,Fe-3d态对铁电性也有贡献,包括间接超交换作用和Fe-O成键。尤其是在四方相BFO中,由于Fe-O-Fe超交换作用的增强,使得Fe-3d态对铁电性的影响更加明显,铁电极化强度也随之增强。
4研究了类四方相BFO的电控磁有序和金属-绝缘相变。结果发现[001]极化态的BFO在晶格常数为3.91A附近其反铁磁序出现从C1型向G型的转变,而这种改变可以用海森堡交换积分常数Jc和J2c的竞争来解释。同时,由于[111]极化态一直保持在G型反铁磁序,因此在一定条件下外加电场使得铁电极化从[001]转到[111]方向,相应的反铁磁序从C1型转变为G型,从而实现了电控磁有序。而通过计算类四方相BFO的线性和非线性光学性质,发现在低频处二次谐波系数χzzz(2)对C1型反铁磁序的响应要比G型反铁磁序强很多,因此可以通过探测χzzz(2)来区别C1型和G型反铁磁序。应力和铁电极化方向不仅能改变磁有序,也能诱导金属-绝缘相变,当晶格常数逐渐增加时,[001]极化态从绝缘态变为金属态,而[110]极化态却从金属态变为绝缘态,因此在一定的晶格常数下外加电场可以改变铁电极化方向来引起金属-绝缘相变的出现。
5研究了BFO的磁晶各向异性能和净磁矩。计算发现当铁电极化从[111]转到[001]方向时,其磁晶各向异性能将会增加一个数量级,这增强了体系中Dzyaloshinskii-Moriya相互作用。通过考虑BFO的非共线磁序,计算得到磁矩偏角以及净磁矩都会出现明显的增加,从而体现出一种电控净磁矩的效应。
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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