Intrinsic Pinning in Magnetic Iron-Based Superconductors; a Route to High Critical Current Conductors at High Magnetic Fields

1911年，卡默林·昂尼斯发现了超导现象，这是20世纪最了不起的发现之一。超导体是一种可以无阻力地传输大电流的材料，也就是说，它不会损失任何能量。尽管超导体为电力传输提供了明显的好处，但在它们找到主流应用之前，还有许多问题需要解决。第一个被发现的超导体必须被冷却到极低的温度，在开尔文尺度上接近绝对零度，这就需要开发合适的低温冷却系统，而这种系统本身就有很大的能量损失。自从1986年发现氧化铜基超导体和2006年发现工作温度更高的铁基超导体以来，这个问题已经在很大程度上得到了解决。然而，我们现在知道，只有某些类型的超导材料，即所谓的“II型”材料，能够在医疗磁共振成像或聚变反应堆中的磁约束所需的极高磁场下运行。然而，这些“II型”材料只能通过允许被称为涡流的微小磁场管进入它们来实现这一点，如果它们被大流动的超电流驱动进入运动，就会开始产生热量（并失去能量）。幸运的是，大自然已经找到了解决这个问题的办法，漩涡可以被困在材料中存在的缺陷中，阻止它们移动和失去能量。因此，开发高电流超导导线需要在不显著降低其他有用超导性能的前提下，在材料中引入尽可能多的缺陷。即使如此，超导体在非常高的磁场下（当它们被许多漩涡淹没时）的载流能力对于预期的应用来说仍然太低。在这个项目中，我们将研究最近发现的磁性和超导性共存的新型铁基超导体。这种行为是非常不寻常的，因为磁性和超导性通常是对抗现象；它们涉及电子量子自旋的相反排列。每个电子都可以被想象成带有一个微小的指南针“针”（自旋）；在铁磁体中，所有的“针”都指向同一个方向，而在传统的超导体中，电子形成成对，“针”指向相反的方向。值得注意的是，在这些新的铁基材料中，铁磁性的存在并没有破坏超导性，并且在超导状态下，磁性“针”指向不同方向的区域被称为区域的拼凑。这些磁畴和它们之间的边界代表了一种新型的缺陷，它可以强烈地捕获涡流，即使在非常高的磁场中也能增强载流能力。在这个项目中，我们将汇集一个具有不同技能的专家团队，他们可以对磁性铁基超导体进行生长，模式，测量和理论研究。我们将仔细研究磁畴的拼凑如何捕获超导涡旋并控制其动态特性，并将开发先进的理论模型来理解我们的结果。一旦在高磁场下实现最高电流密度的条件建立起来，我们将把它们应用到我们在卡尔斯鲁厄（德国）的合作伙伴种植的铁基超导薄膜上，最终目标是通过非常低成本的方法生产高性能的商业电线。虽然该项目的主要动机是开发满足关键应用要求的新材料，但我们也将产生许多新的科学知识，这些知识将对更广泛的研究超导材料的研究团体有很大的价值。

The discovery of superconductivity by Kamerlingh Onnes in 1911 was one of the most remarkable discoveries of the 20th century. A superconductor is a material that can carry large electrical currents without any resistance, that is without losing any energy. Although superconductors offered clear benefits for the transmission of electrical power, many problems needed to be solved before they could find mainstream applications. The first discovered superconductors had to be cooled to extremely low temperatures, close to absolute zero on the Kelvin scale, requiring the development of suitable cryogenic cooling systems which themselves had significant energy losses. Since the discovery of copper oxide-based superconductors in 1986 and iron-based superconductors in 2006 with much higher operation temperatures, this problem has been largely solved. However, we now know that only certain types of superconducting materials, so-called 'type II' ones, are capable of operation at the very high magnetic fields needed for medical magnetic resonance imaging or magnetic confinement in fusion reactors. However, these 'type II' materials are only able to achieve this by allowing tiny tubes of magnetic field called vortices to enter them which start to generate heat (and lose energy) if they are driven into motion by large flowing supercurrents. Fortunately nature has found a solution to this problem, and vortices can become trapped at defects present in the material preventing them from moving and losing energy. Developing high current superconducting wires therefore requires introducing as many defects into the material as possible without significantly degrading other useful superconducting properties. Even then the current carrying capacity of superconductors at very high magnetic fields (when they become flooded with many vortices) can still be too low for intended applications. In this project we will investigate new types of iron-based superconductors that have recently been discovered in which magnetism and superconductivity coexist. This behaviour is very unusual as magnetism and superconductivity are normally antagonistic phenomena; they involve opposite arrangements of the quantum spins of electrons. Each electron can be visualised as having a tiny compass 'needle' (the spin) attached to it; in ferromagnets all the 'needles' point in the same direction, while in conventional superconductors the electrons form pairs in which the 'needles' point in opposite directions. Remarkably, in these new iron-based materials the presence of ferromagnetism does not destroy superconductivity, and a patchwork of regions called domains where the magnetic 'needles' point in different directions coexists with the superconducting state. These magnetic domains and the boundaries between them represent a new type of defect that can strongly trap vortices, leading to enhanced current carrying capacities, even in very high magnetic fields.In this project we will bring together a team of experts with a diverse range of skills that can grow, pattern, measure and undertake theoretical studies on magnetic iron-based superconductors. We will carefully investigate how the patchwork of magnetic domains present can trap superconducting vortices and control their dynamic properties and will develop advanced theoretical models to understand our results. Once the conditions have been established for achieving the highest current densities at high magnetic fields we will apply them to iron-based superconducting thin films grown by our partner in Karlsruhe (Germany) with the ultimate goal of realising high performance commercial wires that can be produced by very low-cost methods. Although the main motivation for this project is to develop new materials that meet the requirements for key applications, we will also generate a lot of new scientific knowledge that will be of great value to the wider research community working on superconducting materials.