Inhomogeneous magnetism and superconductivity

在过去的十五年里，人们对超导和磁性的耦合进行了大量的研究。这两种效应都是通过电子之间的耦合来实现的，但铁磁性导致了自旋的平行排列，而传统的(所谓的自旋单态)超导电性需要反平行的自旋排列。因此，超导对铁磁体的耦合通常比对非磁性金属的耦合弱得多(所谓的邻近效应)。然而，在很短的范围内(几纳米)，两种材料界面上的超导和铁磁性之间的耦合导致了不同于任何一种材料的复杂行为。最值得注意的是，负责超导的电子对在铁磁体中有一个快速振荡的相，这可能导致在薄铁磁势垒分隔两个超导体的设备中出现负的而不是正的超导电流。基于这种效应的装置目前正在开发用于量子计算。更具争议的是，最近的几个实验检测到了一种更远距离的邻近效应，在这种效应中，超导可以穿透几百纳米的距离的铁磁体。这一效应似乎证实了理论上的预测，即如果磁场是不均匀的(即，所有自旋并不指向一个方向)，或者电子是100%自旋极化的，那么应该出现所谓的超导自旋三重态。我们提议的项目的目的是仔细研究形成这种自旋三重态所需的条件，并了解如何控制它，以便开发潜在的应用。特别是，我们将研究具有螺旋而不是线性磁序的铁磁体的类别，我们还将生长人工磁结构，其中这种螺旋可以通过施加磁场来改变，我们将探索磁畴壁(即材料中磁性改变方向的区域)的存在如何影响超导性质。

The past fifteen years has seen considerable research into the coupling of superconductivity and magnetism. These two effects are both mediated by coupling between electrons, but ferromagnetism leads to the parallel alignment of spins while conventional (so called spin-singlet) superconductivity requires anti-parallel spin alignment. As a result the coupling of superconductivity into ferromagnets is generally much weaker than the coupling into non-magnetic metals (the so-called proximity effect). However, at very short-range (a few nanometres) the coupling between superconductivity and ferromagnetism at the interface between the two materials results in complex behaviour which is distinct from that of either material. Most notably, the pairs of electrons which are responsible for superconductivity have a rapidly oscillating phase in the ferromagnet which can lead to negative rather than positive supercurrents appearing in devices in which a thin ferromagnetic barrier separates two superconductors. Devices based on this effect are currently being developed for quantum computation. More controversially, a few very recent experiments have detected a much longer-ranged proximity effect in which superconductivity can penetrate a ferromagnet over distances of hundreds on nanometres. This effect seems to be confirmation of theoretical predications that if the magnetism is inhomogeneous (i.e. all the spins do not point in a single direction) or the electrons are 100% spin polarised then a so-called spin-triplet state of superconductivity should appear. The aim of our proposed project is to investigate carefully the conditions required for the formation of this spin-triplet state and to understand how to control it so that potential applications can be developed. In particular we will look at classes of ferromagnet which have a spiral rather than linear magnetic order, we will also grow artificial magnetic structures in which such spirals can be changed by applying a magnetic field and we will explore how the presence of magnetic domain walls (which are regions in which the magnetism changes direction in a material) affects the superconducting properties.