Magnetism at the Edge of Stability Probed with Advanced Muon Spectroscopy

材料中的磁性是最古老的科学发现之一，但仍远未完全被理解。我提议使用新的、迄今完全未被开发的实验技术来了解材料，在这些材料中，磁相互作用使磁态稳定；但只是刚刚稳定！这意味着环境中的微小变化会导致磁性的剧烈变化。我建议用介子来研究这些效应。这些是亚原子粒子，可以植入材料中，在那里它们充当微观磁力计。在固体中，原子之间通过附着在原子上的电子之间的静电力相互作用。这些力都是短程的，所以一个原子只能和它的邻居说话。电子有一种被称为自旋的性质，最好被认为是每个电子上都有一个箭头。在高温下，自旋是随机排列的，但当我们降低温度时，静电相互作用会导致自旋与它们相邻的自旋排成一列。令人惊讶的是，短程作用力会使固体中的所有自旋对齐。从只与邻居对话的本地原子，我们以长期秩序的形式创造了集体行动。长期有序在自然界中随处可见，这种有序理论解释了星系的聚集、地震的分布、疾病的传播，甚至宇宙本身的存在。磁性中的一个关键因素是相互作用在原子之间传递信息的方式(就像这样排成一列自旋)。可能存在这样的情况，即相互作用仅沿原子线(一维)或原子平面(二维)起作用。这种维度是所有长程有序系统行为的根源。这远远不是理论上的抽象--在实验室里制造一维和二维材料是可能的。在这里，分子通常被用作材料的构建块，而不是单个原子。这些分子磁体是自组装的纳米结构，由磁性金属原子组成的网络使用有机分子连接在一起。大量的有机分子允许我们对磁铁的结构进行微小的改变，从而制造出具有所需性能的量身定制的材料。另一类重要的磁铁是当发送给原子的信息发生冲突时产生的，这种现象被称为挫折。如果每个原子都收到了关于哪个方向应该对齐的相互冲突的指令，那么它将服从哪个方向就不明显了。因此，很难预测系统的基态(即在极低温度下采用的状态)。对这类系统的研究提供了对材料为什么会采用它们所采用的状态的见解。为什么某种物质应该是铁磁体，而另一种材料却在低温下保持无序？我们甚至可以洞察为什么固态本身是稳定的。我建议使用介子对受挫和低维材料进行研究。这些是一种亚原子粒子，可以植入材料中以测量内部磁场。对µ子的研究揭示了其他更传统的实验技术所看不到的特性。受挫和低维的材料往往都处于稳定的边缘，因此它们外部环境的微小变化会导致它们的行为发生戏剧性的变化。这意味着，对这些磁铁施加微小扰动的实验往往会产生关于它们行为的许多有趣的信息。最近开发了新的实验技术，其中可以应用微扰并用Muon同时进行测量。这些，到目前为止，在一线研究中完全没有被利用，这是它们的第一次部署，构成了我工作的基础。

Magnetism in materials is one of the oldest scientific discoveries, but is still far from being completely understood. I am proposing to use new and, as yet, completely unexploited experimental techniques to learn about materials where the magnetic interactions act to make the magnetic state stable; but only just stable! This means that small changes in the environment can cause dramatic changes in the magnetic properties. I propose to investigate these effects with muons. These are subatomic particles that may be implanted into materials where they act as microscopic magnetometers. In a solid, the atoms interact with each other through electrostatic forces between the electrons attached to the atoms. These forces are short range, so an atom is only on speaking terms with it neighbours. Electrons have a property known as spin, which is best thought of as an arrow attached to each electron. At high temperatures the spins on are randomly aligned, but as we reduce the temperature the electrostatic interactions cause the spins to line up with those of their neighbours. Amazingly, short range forces act to make all of the spins in the solid align. From local atoms speaking only to their neighbours, we have created collective action in the form of long-range order. Long-range order is seen throughout nature and the theory of such order explains the clustering of galaxies, the distribution of earthquakes, the spread of disease and even the very existence of the universe itself. A crucial factor in magnetism is the way in which interactions pass information (like line up spins this way'') between atoms. There may be situations where the interactions only act along a line of atoms (one-dimension) or in a plane of atoms (two-dimensions). This dimensionality is at the root of the behaviour of all long-range ordered systems. This is far from being a theoretical abstraction - it is possible to make 1D and 2D materials in the laboratory. Here, molecules are often employed as the building blocks of the materials rather than individual atoms. These molecular magnets are self assembled nanostructures, formed from networks of magnetic metal atoms which are linked together using organic molecules. The great number of organic molecules allow us to make small changes to the structure of magnets leading to tailor made materials with desired properties.Another important class of magnet results when messages sent to an atom conflict, a phenomenon known as frustration . If each atom is receiving conflicting instructions as to which direction is should align, it is not obvious which it will obey. It is therefore difficult to predict the ground state of the system (that is, the state adopted at very low temperatures). The investigation of such systems provide insights into why materials adopt the states that they do. Why should a certain material be a ferromagnet while another stays disordered down to low temperature? We can even gain an insight into why the solid state itself is stable.I propose to carry out research into frustrated and low-dimensional materials using muons. These are a subatomic particle that may be implanted in a material in order to measure the internal magnetic field. Investigations with muons reveal properties invisible to other, more conventional, experimental techniques. Both frustrated and low-dimensional materials tend to exist at the edges of stability, so that small changes in their external environment lead to dramatic changes in their behaviour. This means that experiments where small perturbations are applied to on of these magnets tend to yield much interesting information about their behaviour. New experimental techniques have recently been developed where perturbations may be applied and simultaneous measurements made with muons. These, as yet, have been completely unexploited in front line research and it is their first deployment that forms the basis of my work.