铁电纳米结构中退极化场屏蔽机理的多尺度研究

发布时间：2018-01-29 01:21

本文关键词： 铁电材料第一性原理计算相场模拟无约束薄膜纳米管二维电子气隧穿结　出处：《浙江大学》2015年博士论文　论文类型：学位论文

【摘要】：铁电材料在功能器件、智能结构等领域有着广泛的应用。随着器件的小型化和微型化,铁电材料纳米结构的研究受到了学术界和工业界越来越多的关注。其中,退极化场的消除机理关系到铁电材料的尺寸效应(区别于块体材料的特殊性质,例如,纳米尺寸下的电畴形貌),临界尺寸(小于临界尺寸铁电性将消失)等问题。因此,对退极化场消除机理的研究不仅对物理、材料领域的基础研究有着重要的学术意义,而且对工业应用等也可以给出重要的指导。本文主要利用第一性原理计算,在不同的力学、电学边界条件下,探索和讨论了多种退极化场的屏蔽机理。同时,以相场模拟方法作为第一性原理计算的外延,讨论较大尺寸的铁电纳米结构。(1)本文讨论的第一种退极化场屏蔽机理为“多畴结构”。多畴结构尽可能的减少了极化在自由表面的不连续性,降低了退极化场的强度,因而可以在电畴内部保持铁电极化。通过对PbTiO3无约束薄膜的模拟,我们讨论了其稳定的闭合电畴结构。并且通过和BaTiO3薄膜的比较,解释了PbTi03纳米薄膜中铁电临界厚度消失的机理。研究发现,应变梯度起了重要的作用。为了验证这一点,我们设计了BaTiO3纳米管的算例,并在其中验证了上述机理所预测的挠曲电效应。(2)在无约束铁电薄膜中,本文进一步利用相场模拟方法,在更大尺寸范围内针对铁电薄膜中带状180。电畴结构的Kittel law和generalized scaling law,做了探索性的研究。相场模拟方法作为一种连续介质理论,高度依赖于其采用的能量形式和参数。然而,其梯度能系数在以往的文献中并不确定,只是限于一个取值范围。本文结合以上两点,在带状180。电畴结构的模拟中考虑了不同的梯度能系数,以揭示其对Kittel law的影响。并且,通过进一步考虑畴壁厚度,在数值计算的层面上验证并讨论了generalized scaling law。(3)本文阐述的第二种退极化场屏蔽机理为“二维电子气”。基于LaAlO3/SrTiO3界面上二维电子气的实验发现,本文尝试将其与铁电薄膜相结合,利用铁电极化来代替极性材料(例如LaAlO3)中的内在极化,并且从热力学模型预测出了该机理的可行性。根据热力学模型,当铁电薄膜的厚度大于临界厚度时,二维电子气可以稳定存在于薄膜的界面或表面,并几乎完全抵消了退极化场,从而稳定了铁电薄膜中的面外单畴构型。根据这一预测,本文对PbTiO3/SrTiO3异质结构进行了第一性原理计算,计算结果证实了该机理。(4)本文讨论的第三种退极化场屏蔽机理为“金属电极”。已有的实验就多铁性隧穿结Fe/BaTiO3/Fe的界面原子结构给出了不同的报道。本文利用第一性原理计算,结合不同的界面结构,对两种隧穿结进行了系统的研究,包括铁电性、铁磁性和界面磁电耦合效应。并揭示了由于不稳定化学键的形成所导致的结构整体铁电性的提高和界面磁电耦合效应的增强。
[Abstract]:Ferroelectric materials have been widely used in functional devices, intelligent structures and other fields. With the miniaturization and miniaturization of devices, the research of ferroelectric nanostructures has attracted more and more attention in academia and industry. The elimination mechanism of depolarization field is related to the size effect of ferroelectric materials (different from the special properties of bulk materials, such as domain morphology in nanometer size). Therefore, the study of the mechanism of depolarization field elimination is of great academic significance not only to the basic research in the field of physics and materials. In this paper, we mainly use the first principle calculation, under different mechanical and electrical boundary conditions, explore and discuss the shielding mechanism of a variety of depolarization fields. The phase field simulation method is used as the extension of first-principle calculation. The first kind of depolarization field shielding mechanism discussed in this paper is "multi-domain structure". Multi-domain structure reduces polarization discontinuity on free surface as far as possible. The intensity of the depolarization field is reduced, so that the ferroelectrode can be kept in the domain. The simulation of the PbTiO3 unconfined film is carried out. The stable closed domain structure is discussed and the mechanism of the disappearance of ferroelectric critical thickness in PbTi03 nanocrystalline films is explained by comparison with BaTiO3 films. Strain gradient plays an important role. In order to verify this, we have designed an example of BaTiO3 nanotubes. It is verified that the flexural effect predicted by the above mechanism is further simulated in the unconstrained ferroelectric thin films by using the phase field simulation method. In a larger range, Kittel law and generalized scaling law with band-shaped 180. domain structure in ferroelectric thin films are obtained. The phase field simulation method, as a continuum theory, is highly dependent on the energy forms and parameters used. However, its gradient energy coefficient is uncertain in previous literatures. Considering the above two points, different gradient energy coefficients are considered in the simulation of the zonal 180. domain structure to reveal its influence on the Kittel law. The thickness of domain wall is further considered. In this paper, generalized scaling law. 3) is verified and discussed at the level of numerical calculation. The second depolarization field shielding mechanism described in this paper is "two-dimensional electron gas". Based on the experimental discovery of two-dimensional electron gas at LaAlO3/SrTiO3 interface. In this paper, we try to combine it with ferroelectric thin film and use iron electrode instead of internal polarization in polar materials (such as LaAlO _ 3). According to the thermodynamic model, when the thickness of ferroelectric film is larger than the critical thickness, the two-dimensional electron gas can stably exist on the interface or surface of the film. The depolarization field is almost eliminated, and the out-of-plane single-domain configuration in ferroelectric thin films is stabilized. According to this prediction, first-principles calculations of PbTiO3/SrTiO3 heterostructures are carried out. The calculated results confirm the mechanism. The third depolarization field shielding mechanism discussed in this paper is "metal electrode". Different reports have been given on the interface atomic structure of polyferroelectric tunneling junction (Fe/BaTiO3/Fe). First principle calculations are used in this paper. Combined with different interface structures, two tunneling junctions were systematically studied, including ferroelectric properties. The ferromagnetic and interface magnetoelectric coupling effects, and the enhancement of the ferroelectric properties of the structure and the magnetoelectric coupling effect of the interface due to the formation of unstable chemical bonds are also revealed.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.1

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