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，但仍远未被完全理解。我建议使用新的，到目前为止，完全未开发的实验技术来了解磁性相互作用使磁性状态稳定的材料；但只是刚刚稳定！这意味着环境的微小变化会导致磁性的剧烈变化。我建议用μ子来研究这些效应。这些是亚原子粒子，可以植入材料中，充当微观磁力计。在固体中，原子通过附着在原子上的电子之间的静电力相互作用。这些力是短程的，所以原子只和它的邻居说话。电子有一种被称为自旋的特性，最好把它想象成附着在每个电子上的箭头。在高温下，自旋是随机排列的，但当我们降低温度时，静电相互作用会使自旋与邻近的自旋排列在一起。令人惊讶的是，近距离的力使固体中所有的旋转对齐。从局部原子只对它们的邻居说话，我们创造了远距离秩序形式的集体行动。长期秩序在自然界随处可见，这种秩序的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。磁性的一个关键因素是原子间相互作用传递信息的方式（比如排列自旋）。在某些情况下，相互作用可能只沿原子线（一维）或原子平面（二维）发生作用。这个维度是所有远程有序系统行为的根源。这远不是一个理论上的抽象概念——在实验室里制造1D和2D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。这些分子磁铁是自组装的纳米结构，由磁性金属原子网络组成，这些金属原子通过有机分子连接在一起。大量的有机分子使我们能够对磁铁的结构进行微小的改变，从而获得具有所需性能的定制材料。另一类重要的磁铁产生于发送给原子的信息发生冲突时，这种现象被称为挫败感。如果每个原子都收到相互矛盾的指示，告诉它应该朝哪个方向排列，那么它会服从哪个就不明显了。因此，很难预测系统的基态（即在非常低的温度下采用的状态）。对这些系统的研究提供了洞察为什么材料采用它们所做的状态。为什么某种材料是铁磁体，而另一种材料在低温下保持无序状态？我们甚至可以深入了解为什么固态本身是稳定的。我建议用μ子来研究受挫的低维材料。这是一种亚原子粒子，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无法看到的特性。受挫材料和低维材料都倾向于存在于稳定的边缘，因此外部环境的微小变化会导致它们的行为发生巨大变化。这意味着对这些磁体中的一个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信息。最近发展了新的实验技术，可以应用微扰和同时测量μ子。这些，到目前为止，在前线研究中还完全没有被利用，这是它们的第一次部署，形成了我工作的基础。

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.