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

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.