Inhomogeneous magnetism and superconductivity

Lead Research Organisation: Imperial College 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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Branford WR (2009) Coexistence of universal and topological anomalous hall effects in metal CrO2 thin films in the dirty limit. in Physical review letters

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Magnus F (2008) A d.c. magnetic metamaterial. in Nature materials

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Moore J (2008) Evidence for supercurrent connectivity in conglomerate particles in NdFeAsO 1-d in Superconductor Science and Technology

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Salem-Sugui S (2017) Effects of proton irradiation on flux-pinning properties of underdoped Ba(Fe0.96Co0.04)2As2 pnictide superconductor in Journal of Alloys and Compounds

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Usman I (2011) Evidence for spin mixing in holmium thin film and crystal samples in Physical Review B

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Yates K (2010) Evidence for nodal superconductivity in Sr 2 ScFePO 3 in Superconductor Science and Technology

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Yates K (2013) Andreev spectroscopy of CrO 2 thin films on TiO 2 and Al 2 O 3 in EPL (Europhysics Letters)

 
Description In a number of recent experiments, holmium has been shown to promote spin triplet pairing when in proximity to a spin singlet superconductor. The condition for the support of spin triplet pairing is that the ferromagnet should have an inhomogeneous magnetic state at the interface with the superconductor. The principle result from the Imperial College project studentship work is that we have used Andreev reflection spectroscopy to study the properties of single ferromagnet/superconductor interfaces formed of holmium and niobium, as a function of the contact resistance of the junction between them. We found that both single crystal and c-axis oriented thin film holmium show unusual behavior for low junction contact resistance, characteristic of spin mixing type properties, which are thought necessary to underpin spin triplet formation. We also explored whether this signature was observed when the junction was formed of Ni19Pd81 and niobium. This work provided some specific signatures of the conditions necessary for spin triplet pairing. In addition to this work we also studied the Point contact Andreev reflection spectra as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr(2)ScFePO(3). A zero bias conductance peak which disappears at the superconducting transition temperature dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7 T at 2 K and a flattening of the feature is observed in some contacts. We examined whether these observations could be interpreted within a d-wave, or nodal order parameter, framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.
Exploitation Route Our work contributed to the development of a new field called superconducting spintronics helping to realise the possibility of new types of devices for information storage and memory applications. This grant has let to a number of follow on grants, including the Cambridge Materials department lead program grant on superconducting spintronics aiming to develop device concepts using the ideas developed in this original grant.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Other
URL http://www.imperial.ac.uk/people/l.cohen/research.html