Exploration of topological defects in ferroelectrics

铁电体(FeS)是一种在一定温度下表现出自发极化的材料，具有两个或两个以上稳定的极化状态，可通过外加电场进行可逆访问。以前的应用包括结构域--FE中具有一致偏振感的区域--作为材料的功能部分。最近的焦点转移到了域壁(DWS)上，这是一种将结构域分开的纳米尺度的膜状结构。研究表明，DW具有与其周围区域不同的性质。该项目旨在研究这一不断发展的领域中的两个领域：磁化壁导电性和FE摩尔微米。现在许多材料都证明了在DWS中以其他绝缘材料(1)增强的导电性。此外，由于极化可以通过施加外场而可逆，因此可以移动和控制DW，甚至可以创建和移除DW，从而产生了一种新的可重构电路的想法，其中磁区壁是一个可移动的电连接(2)。导电性是不同的，但表征导电性是实现磁壁纳米电子学的关键，也是该项目的主要目标之一。目前正在进行的工作是通过测量几何磁阻--施加磁场后电阻率的变化--来测量Nb酸锂(LN)中导电DWS的载流子迁移率。这种效应的产生是由于可以写在LN薄膜(1)中的DW独特的圆锥形几何结构，使得一种新的磁区壁迁移率的测量成为可能。此外，已经制定了计划，专门在进行DW S时对电子能带结构进行一些第一次直接测量。利用大型X射线源在DWS局部激发载流子，可以测量载流子的能量和动量。这些参数组成了电子能带结构，让我们深入了解电导增强背后的机制。该项目的另一个方面是研究与磁性天米子的有限元类比。磁性天米子是一种受限的自旋模式，被描述为受拓扑保护，这意味着它不能连续变形和移除，并且在某种程度上是稳定的。它们的前景来自纳米尺寸，使其成为高密度数据存储应用的理想选择，例如最近设想的赛道存储器(3)。在复杂的结构中，已经从理论上预测(4)和观察到了(5)用极化取代自旋的电子对应物，但一般情况还有待观察(4)。如果发现了FE Skyrmions，就可以用类似于上面提到的DWS的方式受场控制，从而有望成为功能元素(4)。该项目寻求使用聚焦离子束切割Fe钛酸铅薄片，写入有望成为天离子的磁区图案，并使用原子分辨率透射电子显微镜(与利默里克大学合作)来验证这一点。如果观察到，我们可以前进到使用新的原子力显微镜方法来编写、移动和控制这些具有外加电场的极化图案，研究它们的响应和传输特性，探索它们的物理起源，并最终表征它们的潜在应用1。Schröder，Mathias等人。“LiNbO_3单晶中的导电磁区壁。”先进功能材料22.18(2012年)：3936-3944.2。McQuaid、Raymond GP等人。“在不适当的铁电铜-氯方镁石中注入和控制导电磁畴壁的运动。”自然传播8(2017年)：15105.3。Tomasello、Riccardo等人。“设计Skyrmion赛道记忆的策略。”科学报告4(2014年)：6784.4。Gonçalves，MA Pereira等人。“创造和调整天际电气泡的理论指导方针。”科学进步5.2(2019)：平均7023.5。Das，Sujit等人。“对室温极地天象的观测。”《自然》568.7752(2019年)：368.

Ferroelectrics (FEs) are materials which exhibit spontaneous polarisation below a certain temperature, and have two or more stable polarisation states, reversibly accessible by application of external fields. Previous applications have involved domains -regions of the FE with a consistent sense of polarisation- as the functional part of the material. Recent focus has switched to the domain walls (DWs), the nanoscale membrane-like structures that separate domains. DWs have been shown to exhibit properties distinct from the domains surrounding them. This project aims to investigate two areas of this evolving field: domain wall conduction and FE skyrmions.Enhanced electrical conductivity at DWs in an otherwise insulating bulk(1) has now been demonstrated in many materials. Also, since the polarisation is reversible with the application of external fields, DWs can be moved and controlled, even created and removed, giving rise to the idea of a new kind of reconfigurable circuitry, where the domain wall is a mobile electrical connection(2). Conduction varies, but characterising the conduction is key to realising domain wall nanoelectronics, and is one of the main aims of the project. Work is under way to obtain a measurement for the carrier mobility of conducting DWs in lithium niobate (LN) by measuring the geometric magnetoresistance- a change in resistivity upon application of a magnetic field. This effect arises due to the unique conical geometry of the DWs that can be written in LN thin films(1), making a novel measurement of mobility in domain walls possible. Furthermore, plans are in place to make some of the first direct measurements of the electronic band structure specifically at conducting DWs. Using a large facility X-Ray source to excite carriers locally at the DWs, energy and momentum of carriers can be measured. These parameters comprise the electronic band structure, giving insight into the mechanism behind the enhanced conductivity.Another aspect of the project is to investigate a FE analogy to the magnetic skyrmion. A magnetic skyrmion is a confined spin pattern, described as being 'topologically protected', meaning it cannot be continuously deformed and removed, and is somewhat stable. Their promise comes from their nanometer size, making them ideal for high density data storage applications, such as the recently envisioned racetrack memory(3). An electric counterpart, where polarisation replaces spin, has been theoretically predicted(4) and observed(5) in complex structures, but the general case is yet to be seen(4). If found, FE skyrmions could be controlled by fields in a similar way to the DWs mentioned above, and thus hold promise as functional elements(4). This project seeks to use focussed ion beams to cut thin slices of FE lead titanate, write a domain pattern which is expected to be skyrmionic, and use atomic resolution transmission electron microscopy (in collaboration with University of Limerick), to verify this. If observed, we could move forward into using novel atomic force microscopy methods to write, move and control these polarisation patterns with applied fields, investigating their response and transport properties, exploring their physical origin and ultimately characterising them for potential application.1. Schröder, Mathias, et al. "Conducting domain walls in lithium niobate single crystals." Advanced Functional Materials 22.18 (2012): 3936-3944.2. McQuaid, Raymond GP, et al. "Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite." Nature communications 8 (2017): 15105.3. Tomasello, Riccardo, et al. "A strategy for the design of skyrmion racetrack memories." Scientific reports 4 (2014): 6784.4. Gonçalves, MA Pereira, et al. "Theoretical guidelines to create and tune electric skyrmion bubbles." Science advances 5.2 (2019):eaau7023.5. Das, Sujit, et al. "Observation of room-temperature polar skyrmions." Nature 568.7752 (2019): 368.