Addressing Current Issues in Multiferroics

现代社会的顺利运行取决于我们可靠地存储和访问数据的能力。几乎所有我们需要的东西，从银行账户的精确管理到数字娱乐的灵活性，都需要读取和解释二进制“1”或“0”字符串。在数据存储的核心，这种二进制数通常以电荷的极性（在DRAM、闪存或FRAM中）或磁化方向（在磁性硬盘驱动器中）的形式存在。电荷存储设备和磁存储设备在结构或操作上都有缺点，因此多年来人们一直对开发一种结合了两者优点的存储元件感兴趣，使信息的“写入”可以用电来完成，而“读取”可以用磁来完成。同时具有铁磁性和铁电性的材料对于这种应用来说是非常理想的，因此，所谓的“多铁性”已经成为最近研究兴趣的一个话题。不幸的是，很少有已知的多铁性系统，迄今为止还没有发现可以容易地制造并同时显示大极化和大磁化的系统。因此，该提案的第一个元素是探索两组相对较新的多铁氧体（双松弛体和晶格应变EuTiO3），看看它们是否能提供优于目前最知名的多铁氧体（铋铁氧体）的性能。在存储元件中使用多铁性材料需要对磁性和铁电区域（称为畴）进行操作。虽然对铁磁体和铁电体的畴行为分别了解很多，但对多铁畴的静态和动态行为知之甚少。中尺度和纳米尺度物体（与高密度存储器相关的尺寸）领域的探索将在小型单晶上进行，使用由申请人独特开发的基于聚焦离子束的方法从高纯度块状材料中切割。到目前为止，这已经给出了关于铁电体的非常清晰的信息，对于多铁畴性质的基础研究应该是理想的。除了对多铁性存储器的兴趣外，研究人员对多铁性存储器在更多特殊应用中的潜在用途越来越感兴趣-铋铁氧体中的畴壁已被发现可以作为平面导体，并且已经显示出巨大的光伏效应。到目前为止，这种效应只在脉冲激光沉积生长的薄膜中被探测到。虽然这是一种有用且灵活的生长技术，但它有引入显著缺陷的倾向，这些缺陷可能导致材料的外在特性，而不是材料的内在特性。我们希望研究使用上述已建立的聚焦离子束工艺制备的铋铁氧体单晶薄膜（以及后来的双弛豫器）在这种奇异畴壁和光伏效应下的性能。重要的是，使用这种方法应该允许不同的观点，这可能与迄今为止仅通过PLD生长薄膜获得的信息相证实或冲突。

The smooth operation of the modern world depends on our ability to store and access data reliably. Almost everything we need, from the accurate management of bank accounts to the flexibility of digital entertainment, requires the reading and interpretation of strings of binary '1's or '0's. At the heart of data storage, such binary numbers usually exist in the form of either the polarity of electrical charge (in DRAM, Flash or FRAM) or the orientation of magnetisation (in magnetic hard-drives). Charge-storage devices and magnetic storage devices both have negative aspects about their architectures or operation, and so for some years there has been interest in developing a memory element that combines the positive features of each, allowing 'writing' of information to be done electrically, and 'reading' to be done magnetically. Materials that are both ferromagnetic and ferroelectric would be highly desirable for such applications and, as a result, so-called 'multiferroics' have become a topic of great recent research interest. Unfortunately, there are very few known multiferroic systems and none has been discovered to date which can readily be made and simultaneously displays both large polarisation and magnetisation. The first element of this proposal is therefore to explore two relatively new groups of multiferroics (birelaxors and lattice strained EuTiO3) to see if they can offer properties that are superior to the best known multiferroic currently available (bismuth ferrite). The use of a multiferroic in a memory element requires the manipulation of magnetic and ferroelectric regions, known as domains. While a great deal is known about domain behaviour in ferromagnets and in ferroelectrics separately, much less is known about the static and dynamic behaviour of multiferroic domains. Exploration of domains in meso and nanoscale objects (dimensions relevant to high density memory) will be performed on small scale single crystals, cut from high purity bulk material using a Focused Ion Beam-based methodology uniquely developed by the applicants. To date this has given extremely clear information on ferroelectrics and should be ideal for fundamental investigations into multiferroic domain properties.In addition to interest in multiferroic memory, researchers have become increasingly excited by the potential use of multiferroics in more exotic applications - the domain walls in bismuth ferrite have been found to act as planar conductors and large photovoltaic effects have been displayed. To date, such effects have only been probed in thin films grown by pulsed laser deposition. While this is a useful and flexible growth technique it has a tendency to introduce significant levels of defects that can lead to properties which are extrinsic, rather than intrinsic to the material. We wish to examine the properties of single crystal thin films of bismuth ferrite (and later birelaxors) made using the established Focused Ion Beam process mentioned above for such exotic domain wall and photovoltaic effects. Importantly, using this approach should allow a different view, which may corroborate or conflict with information to date only obtained through PLD grown films.