Inhomogeneous magnetism and superconductivity

Lead Research Organisation: Royal Holloway, University of London
Department Name: Physics

Abstract

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.

Publications

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Kalenkov MS (2011) Triplet superconductivity in a ferromagnetic vortex. in Physical review letters

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Mironov V (2010) Antivortex state in crosslike nanomagnets in Physical Review B

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Shelly CD (2016) Resolving thermoelectric "paradox" in superconductors. in Science advances

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Wren T (2015) Phase diagram of magnetic states in nickel submicron disks in Journal of Applied Physics

 
Description 1. We have demonstrated that triplet superconductivity can be conveniently realized in hybrid superconducting-ferromagnetic (S-F) structures with a ferromagnetic vortex, where magnetisation rotates in a spiral fashion. We have shown that due to proximity-induced long-range triplet superconducting pairing such S-F-S junctions can sustain appreciable supercurrent which can be directly measured in experiments.
2. We reported on results of computer micromodelling of anti-vortex states in asymmetrical cross-like ferromagnetic nanostructures and their practical realization. The arrays of Cobalt crosses were fabricated using e-beam lithography and ion etching. The stable formation of anti-vortex and quasi-uniform magnetic states in the nanostructures during magnetization reversal was demonstrated experimentally using magnetic force microscopy.
3. For almost a century, thermoelectricity in superconductors has been one of the most intriguing topics in physics. During its early stages in the 1920s, the mere existence of thermoelectric effects in superconductors was questioned. In 1944, it was demonstrated that the effects may occur in inhomogeneous superconductors. Theoretical breakthrough followed in the 1970s, when the generation of a measurable thermoelectric magnetic flux in superconducting loops was predicted; however, a major crisis developed when experiments showed puzzling discrepancies with the theory. Moreover, different experiments were inconsistent with each other. This led to a stalemate in bringing theory and experiment into agreement. With this work, we resolve this stalemate, thus solving this long-standing "paradox," and open prospects for exploration of novel thermoelectric phenomena predicted recently.
Exploitation Route With this work, we resolve long-standing thermoelectric "paradox" in superconductors and open prospects for exploration of novel thermoelectric phenomena predicted recently.
Sectors Education,Electronics,Healthcare

URL https://www.royalholloway.ac.uk/aboutus/newsandevents/news/newsarticles/research-at-royal-holloway-launches-next-generation-of-brain-scanners.aspx
 
Description The next generation of magnetoencephalography (MEG) brain scanners is now under development and one of the key enablers to produce lower cost, high performance machines is a nano electronic device designed using results of this research. The MEG scanner development is being undertaken by a new UK company, York Instruments Ltd. Backed financially by US based investment, the development will enable MEG scanners to be produced at much reduced and affordable cost, providing wider accessibility.
Sector Electronics,Healthcare
Impact Types Policy & public services