DEFECTS IN FRUSTRATED SYTEMS

在一个由许多较小单元组成的复杂系统中，每个元素都将与所有相邻元素相互作用，系统试图安排自己，以便与每个相邻元素形成最有利的联系。然而，有时邻居的要求是相互不相容的，必须找到一个折衷方案。如果是这种情况，我们将该系统描述为受挫。挫折感在自然界中广泛存在，并且被认为对我们理解大脑如何工作等问题至关重要。蛋白质如何折叠所描述的受挫折的生物系统是如此复杂和如此重要，挫折科学已经成为一个主要的研究领域，并且对更简单的模型系统有很大的需求，其中可以调整相互作用强度，可以改变模型系统的大小，可以以受控的方式引入缺陷，并且可以操纵、移除单个元件或记录它们的单个状态。在这样一个理想的系统中，人们可以将理论和实验结合起来，开始理解这种复杂性中的潜在物理学。磁阻挫已被证明是寻找模型系统最成功的领域。传统上，这些是通过固态化学制备的磁性晶体。然而，最近已经表明，可以使用纳米技术使磁棒阵列足够小并且足够靠近，使得它们之间的磁相互作用变得非常显著，并且可以设计新颖的几何形状，其中磁相互作用不能全部满足。这一发展开辟了广阔的新途径的研究模型受挫的系统。在固态化学中，人们在可能的几何排列中受到自然的限制，而在纳米技术中，任何将镶嵌的图案都可以被制造成阵列，在任何长度尺度上，直到光刻的最小特征尺寸。在这里，我建议研究这种理想的系统，是基于受抑的磁性纳米结构。我们从受抑磁性化学结构中获得的经验告诉我们，三角形和六边形是有利于磁性受抑的构建块。最初的工作是在相互隔离的磁棒阵列上完成的，但我计划把重点放在电连续的晶格上，磁性材料的电性能对磁性结构很敏感，因此这提供了对阻挫结构的直接探测，并且可以研究其对温度变化的动态响应和磁场。磁力显微镜（MFM）和扫描霍尔探针成像将用于在这些实验中的磁结构的图像。这些原位测量将允许电响应的变化与磁结构的变化直接相关，并将提供铁磁金属的磁和电特性之间的耦合性质的重要信息，以及拓扑结构的作用，这对于新的基于自旋的电子学或自旋电子学技术是非常重要的。除了提高对其他不同领域的知识之外，我将制作的磁性阵列本身也是令人兴奋的。它们对磁场的不寻常和敏感的反应可能在传感器中有用。此外，所有元素之间的强耦合，以及单个元素的磁状态可以写入（通过施加磁场改变）和读取的事实，意味着它们可能用于新型计算，通常被描述为神经网络，因为它们更像大脑而不是传统计算机。

In a complex system made up of many smaller units, each element will interact with all of its neighbours, and the system tries to arrange itself so that the most favourable bond is formed with each neighbour. However, sometimes the neighbours have requirements that are mutually incompatible and a compromise must be found. If this is the case we describe the system as being frustrated. Frustration occurs widely in nature and is thought to be critical to our understanding of such questions as how do our brains work? and how do proteins fold? The frustrated biological systems described are so complex and so important that the science of frustration has become a major research area and there is great demand for simpler model systems where the interaction strength can be tuned, the model system size can be varied, defects can be introduced in a controlled manner and individual elements can be manipulated, removed or their individual state recorded. In such an ideal system one could unite theory and experiment and begin to understand the underlying physics within this complexity. Magnetic frustration has proved to be the most successful area for finding model systems. Traditionally these were magnetic crystals prepared by solid-state chemistry. However it has recently been shown that it is possible to use nanotechnology to make arrays of magnetic bars sufficiently small and sufficiently close together that the magnetic interactions between them becomes very significant, and that novel geometries can be designed where the magnetic interactions cannot all be satisfied. This development opens up broad new avenues of research in model frustrated systems. In solid-state chemistry one is limited by nature in the geometrical arrangements that are possible, whereas with nanotechnology any pattern that will tessellate can be fabricated into an array, on any length-scale down to the minimum feature size of the lithography. Here I propose to study such ideal systems that are based on frustrated magnetic nanostructures. Our experience from frustrated magnetic chemical structures tells us that triangles and hexagons are the building blocks that favour magnetic frustration. The initial work was done on arrays of magnetic bars that were isolated from one another, but I plan to focus on electrically continuous lattices, such as the hexagonal honeycomb structure so that electrical current can pass through it. The electrical properties of magnetic materials are sensitive to the magnetic structure and so this gives a direct probe of the frustrated structure and one can study its dynamic response to changes in temperature and magnetic field. Magnetic force microscopy (MFM) and scanning Hall probe imaging will be used to image the magnetic structure during these experiments. These in-situ measurements will allow the change in electrical response to be correlated directly with the change in magnetic structure, and will provide important information of the nature of the coupling between the magnetic and electrical properties of ferromagnetic metals, and the role of topology, which is currently very important for new spin-based electronics or spintronics technology. In addition to improving knowledge of diverse other fields, the magnetic arrays that I will make are exciting in their own right. Their unusual and sensitive response to magnetic fields might be useful in sensors. Furthermore the strong coupling between all the elements, and the fact that the magnetic state of individual elements can be both written (changed by applying a magnetic field) and read, means they could potentially be used for novel types of computation, often described as neural networks because they work more like the brain than like a conventional computer.