Artificial Spin Ice: Designer Matter Far From Equilibrium

我们的项目是两所大学和一个国家实验室之间的合作项目，它们在理论和实验相结合的项目中共同工作。实验是基于传统的实验室和大型设施。该工作计划还涉及与美国布鲁克海文国家实验室的同事继续进行国际合作，他们帮助我们在创造和理解纳米结构磁体方面取得了一些最新突破，并以独特的专业知识和设施的形式增加了新的突破。这两个实验室都是由美国能源部支持的美国国家实验室。我们的目标是了解和控制一类新的磁性材料的非平衡动力学：亚微米尺寸的强相关磁体阵列。最近已经研究了一些例子，并命名为“人造自旋冰”。这些材料是新的超材料的重要例子，具有天然磁性材料所没有的独特性质。人工磁冰是由强相互作用的磁矩组成的系统，这些磁矩是几何排列的，以便产生具有高度能量简并的亚稳态配置。我们将使用人工自旋冰作为系统探索非平衡动力学的范例。这是一个模型系统，其自由能由设计指定，因此完全确定，并且可以直接观察精确的微观状态及其随时间的演变，以便与数学预测进行详细比较。元件间的相互作用在很大程度上可以通过几何形状的控制由设计来指定。以这种方式，有可能在导致对所施加的磁场的复杂响应负责的几何挫折的排序之间产生竞争。由此产生的磁性能类似于传统的薄膜或块状磁系统，但具有对应用特别重要的关键差异。例如，在正方形冰中有两种基态构型，它们形成净矩为零的畴。磁荷（有时称为出射磁单极子）是一种特别感兴趣的物质，因为它们可以很容易地被检测到，并且可以通过施加的磁场在阵列中移动。我们将开发注入和检测电荷的方法，并控制它们通过阵列几何形状的流动。我们的目标是确定电路设计的结构和技术，通过这些电路，“磁性”可以流动，并有效地用于技术应用。我们的想法是利用热涨落来帮助磁荷迁移。我们可以通过在我们的阵列中使用纳米级颗粒来做到这一点，使得颗粒接近它们的超顺磁性阻挡温度。与先前工作的一个重要区别是，我们将使用具有接近室温的相变的材料，以允许我们在热平衡和非热非平衡状态之间简单地进行调谐。这将意味着各个力矩可以自发地反转，从而使磁荷通过波动阵列的热驱动运动成为可能。通过结合应用领域和温度控制，我们将能够启动，停止和直接磁荷动力学。这些系统也可能给我们新的实验模型的临界动力学的研究在相变，因为它们可以由众所周知的完全可溶伊辛系统建模，以及提供新的范例信息处理架构。

Our project is a collaborative one between two Universities and a national laboratory working together across a combined theoretical and experimental programme. The experiments are based in both conventional laboratories and large-scale facilities. The work programme also involves continued international collaboration with colleagues in the US at Brookhaven National Laboratory, who have helped us make some of our most recent breakthroughs in creating and understanding nanostructured magnets, as well as adding new ones in the form of the unique expertise and facilities available for transmission x-ray imaging at the Advanced Light Source in Berkeley. Both of these are DOE-supported US national laboratories.Our goal is to understand and control non-equilibrium dynamics in a new class of magnetic materials: strongly correlated arrays of sub-micron sized magnets. Examples have been studied recently, and given the name "artificial spin ice". These materials are important examples of new metamaterials with unique properties not realised in naturally occurring magnetic materials. The artificial magnetic ice are systems composed of strongly interacting magnetic moments, and the moments are arranged geometrically in order to produce metastable configurations with a high degree of degeneracy in energy. We will use artificial spin ice as a paradigm for a systematic exploration of non-equilibrium dynamics. This is a model system for which the free energy is specified by design and hence completely determined, and the exact microstate--and its evolution in time--can be observed directly for detailed comparison with mathematical predictions. Inter-element interactions can be specified to a large extent by design through control of geometry. In this way it is possible to create competitions between ordering that result in geometrical frustrations responsible for complex response to applied magnetic fields. The resulting magnetic properties are analogous to those of traditional thin film or bulk magnetic systems, but with key differences that can be of especial importance for applications. For example, there are two ground state configurations in a square ice that form domains with zero net moment. These are separated by magnetised boundary walls along which magnetic charges can move.Magnetic charges (sometimes called emergent monopoles) are of particular interest since they can be readily detected and can be moved through an array by applied magnetic fields. We will develop methods to inject and detect charges, and control their flow through array geometries. Our goal is to identify structures and techniques for the design of circuits through which "magnetricity" can flow and be usefully employed for technological applications. Our idea is to use thermal fluctuations to aid magnetic charge mobility. We can do this by using nanoscale particles in our arrays, such that the particles are near their superparamagnetic blocking temperature. An important distinction with prior work is that we shall use materials with phase transitions close to room temperature to allow us to tune simply between thermally equilibrated and athermal non-equilibrium states. This will mean that the individual moments can reverse spontaneously, thus enabling thermally driven motion of magnetic charges through a fluctuating array. Through a combination of applied fields and temperature control we will be able to start, stop, and direct magnetic charge dynamics. These systems may also give us new experimental models for studies of critical dynamics at phase transitions since they can be modelled by well known exactly soluble Ising systems, as well as providing new paradigms for information processing architectures.