Ferrotoroidic structures: polar flux-closure, vortices and skyrmions

现代社会每天产生的海量数据需要更高效的信息存储。目前的能力不仅需要增加以满足需求，而且还需要从根本上改变以提供更好的密度，功耗，访问速度和时间稳定性。现有的数据编码是基于铁磁和铁电材料中分别存在的磁化和极化等铁序参数的切换。有限大小的影响决定了当前技术正在迅速接近的数据密度和保留的基本限制。因此，迫切需要新的数据编码机制。另一种选择是环形铁结构，它显示出与手性和圈数等性质相关的多阶参数，可用于编码额外信息。这些附加参数不仅增加了可实现的信息密度，而且预测它们精确地存在于经典铁参数不再对数据编码有效的特征长度处。因此，类铁提供了一种克服传统数据存储限制的替代方法。从理论上预测了类铁结构，但直到最近才在实验中观察到它们。除了巨大的应用潜力，特别是在非易失性存储器中，这些奇异的极性实体可能需要新的物理学才能完全理解。本研究的目的是通过一个全面的工作计划，通过实验来阐明环形铁结构的起源。通过对这一复杂现象的理解，我们将在密度、手性和空间定位方面控制和调整铁的环铁性。我们特别针对氧化物极性类铁，其中自发极化的重新取向是原子位移的结果。由于这个原因，通过测量原子之间的相对位移，透射电子显微镜是在纳米尺度上确定氧化物性质的首选技术。原位电子显微镜将提供实时信息来研究电场和极性实体之间的有效相互作用，以及潜在地切换环面矩手性来演示数据编码。

The impressive amount of data produced daily by modern society requires more efficient information storage. Current capabilities need not only to be increased to meet demand, but also to be fundamentally changed to offer better density, power consumption, access speed and time stability. Existing data encoding is based on switching of ferroic order parameters such as magnetisation and polarisation that exist in ferromagnetic and ferroelectric materials, respectively. The effect of finite size sets a fundamental limit of the data density and retention that is being rapidly approached by current technologies. Therefore, there is critical need for novel data encoding mechanisms. One alternative is offered by ferrotoroidic structures that show multiple order parameters, related to properties such as chirality and winding number, which can be used to encode extra information. Not only do these additional parameters multiply the achievable information density, but they are predicted to exist exactly at the characteristic length where classical ferroic parameters are no longer effective for data encoding. Thus, ferrotoroidics provide an alternative way to overcome the limits of classical data storage. Ferrotoroidic structures have been theoretically predicted but only very recently have they been experimentally observed. Besides the enormous application potential, especially in non-volatile memories, these exotic polar entities may require new physics to be fully understood. The present research aims to experimentally elucidate the origin of the ferrotoroidic structures through a comprehensive program of work. By understanding this complex phenomena, we will gain control of and tune the ferrotoroidicity in terms of density, chirality and spatial positioning. We are especially targeting oxide polar ferrotoroidics in which the reorientation of the spontaneous polarization is a result of atomic displacement. For this reason, transmission electron microscopy is the technique of choice to determine the oxide properties at nanoscopic scale by measuring the displacement of atoms relative to each other. In-situ electron microscopy will provide real time information to investigate the effective interactions between electric fields and polar entities as well as potentially switch the toroidal moment chirality to demonstrate data encoding.