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

发布时间：2018-04-23 21:31

本文选题：BiFeO_3 + 铁电性　；参考：《华东师范大学》2014年博士论文

【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。
[Abstract]:Multiferroic material is a kind of multi - functional material with two or more ferroic characteristics ( ferroelectric , ferromagnetic , iron elastic or iron eddy ) , and there is a coupling relation between the various iron . As a research pioneer of multiferroic materials , BiFeO3 ( BFO ) is a few single - phase multiferroic materials with antiferromagnetic and ferroelectric properties at room temperature .

It is very difficult to explain the ferroelectric polarization intensity of BFO and the coupling relation between the polarization direction and the easy axis of magnetization . The paper uses the first principle to calculate the ferroelectric origin of BFO and reveals the physical mechanism of the related electric control magnetism . The main contents and innovations are as follows :

The OSEP method can accurately control the movement of the specific atomic orbit , which can help to study the influence of the atomic orbit on the physical and chemical properties of the material .

In the indirect magnetic exchange system , a new mechanism of ferroelectric formation is found , and the magnetic interaction is beneficial to the formation of ferroelectric phase . If the magnetic property has the condition of overcoming the elastic energy , the system will exhibit the ferroelectric phase . In the classical antiferromagnetic material MnO as an example , the biaxial compressive stress can be calculated to increase the magnetic energy and reduce the elastic energy , thus leading to the appearance of the ferroelectric phase .

It is found that the Fe - 3d states contribute to the ferroelectric properties , including indirect superswitching and Fe - O bonding . Especially in the tetragonal phase BFO , the influence of Fe - O - Fe super - exchange is more obvious , and the ferroelectric polarization intensity is enhanced .

In this paper , we find that the antiferroic order of the quasi - tetragonal phase BFO is changed from the C1 to G type in the vicinity of the lattice constant of 3.91A , and this change can be explained by the competition of the integral constants Jc and J2c . In the meantime , because of the linear and nonlinear optical properties of the quasi - tetragonal phase BFO , it is found that the second harmonic coefficient 蠂 zzz ( 2 ) changes from the insulating state to the state of the state .

The magnetic anisotropy energy and net magnetic moment of BFO have been studied . It is found that the magnetic anisotropy energy will increase by an order of magnitude when the polarization of iron is transferred from 111 掳 to ~ 001 掳 direction , which enhances the interaction of Dzyaloshinskii in the system . By taking into account the non - collinear magnetic ordering of BFO , the magnetic moment deflection angle and net magnetic moment can be increased obviously , thus reflecting the effect of electric control net magnetic moment .

【学位授予单位】：华东师范大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM221

【参考文献】