Generation, Imaging and Control of Novel Coherent Electronic States in Artificial Ferromagnetic-Superconducting Hybrid Metamaterials and Devices

Lead Research Organisation: University of Bath
Department Name: Physics

Abstract

Condensed matter physicists are constantly looking for interesting new states of matter and new materials with unique exploitable properties which reflect the complex quantum mechanical interactions of the electrons that bind them together. These electronic properties can sometimes be approximated to the sum of individual electrons moving within the material, such as the electrical conduction of electrons in simple metals or semiconductors like silicon. The properties are on the whole more interesting, however, when an accurate description of the material needs to account for the collective and correlated motion of large numbers of electrons. Such collective physics leads to the familiar ferromagnetic properties of materials like iron and, less familiarly, the phenomenon of superconductivity. In the latter electrons are able to flow through the bulk of a superconducting material without generating heat and extraordinarily high electrical currents become possible. This finds application, for example, in the large superconducting magnets required for MRI body scanners. Magnetism and superconductivity are often antagonistic phenomena, since they involve different arrangements of the spins of electrons. Spin is a quantum property of electrons that gives rise to intrinsic magnetic fields - it can be visualised as a tiny compass needle attached to each electron. In ferromagnetism all the spins point in the same direction, but in conventional superconductivity the electrons form pairs (Cooper pairs) in which the spins point in opposite directions. Ferromagnetism therefore normally destroys these Cooper pairs, and hence superconductivity, by causing their spins to align parallel.
Condensed matter physicists often look for new materials where ferromagnetism and superconductivity coexist, since this can suggest the presence of some exotic new form of superconductivity (or other novel quantum state). One example is spin-triplet superconductivity, in which the spins of a Cooper pair prefer to be aligned parallel rather than antiparallel. One way to try and engender such exotic states of matter is to grow artificial materials by depositing very thin superconducting and ferromagnetic films (a few nm thick) on top of one another. In this way the properties of the final structure can be tuned, sometimes leading it to exhibit behaviours that have never been found in naturally occurring bulk materials. An example of this is a unique kind of spin-triplet (so called odd-frequency) superconductivity that has recently been demonstrated in this type of thin film structure. The production of artificial materials can be taken a step further if one also patterns such thin film materials in the plane of the film using advanced electron beam lithography technology, to produce additional patterns and structures on the nanoscale. Such an approach could lead not only to the discovery of interesting new quantum phases, but could also to useful properties that could be exploited in future technologies, such as quantum computing.
In this project we bring together a team of experts with a diverse range of skills that can grow, pattern, measure and undertake theoretical studies on the type of nanostructured materials discussed above. One particularly novel aspect of our approach is the use of powerful imaging techniques involving neutron, muons, X-rays and bespoke scanning magnetic sensors to gain unique insights into both the basic physics at play in systems exhibiting odd-frequency triplet superconductivity as well as the relation between the detailed magnetic and physical structures and their exotic properties. Although the basic aim of this project is the pursuit of new scientific knowledge, we will be looking for interesting effects and properties that might find future applications.

Publications

10 25 50
 
Description This critical mass grant is supporting blue skies curiosity-driven research. In particular we are trying to understand the physics surrounding superconducting correlations in ferromagnet-superconductor hybrid structures, including the generation of triplet (equal spin paired) in non-collinear magnetic systems.

The project ran over 4 years and we have made significant progress with regard to our original 10 objectives, with a number of important results either published or currently being prepared for submission to leading physics journals. During the grant period we have established good partnerships with other leading groups in the field (Cambridge, Leiden, Konstanz, Paris, NTNU, Imperial) and have produced joint publications with these partners. One of us (Eschrig) now participates in a programme grant on the development of a superconducting spintronics demonstrator device together with University of Cambridge, UCL, and Imperial College. In addition, two of us (Burnell, Marrows) are involved in the related platform grant on magnetic metamaterials "Spintronics at Leeds".

The consortium associated with this critical mass grant originally set out an ambitious programme of objectives, and we have made significant progress across all fronts. Below we list 4 examples.

1. Spatially probe the novel spin-triplet state to detect spin-polarisation of Cooper pairs directly and explore new device architectures to enhance the formation and control of spin triplet pairs:
Experimentally we have placed limits on the size of this effect in the ferromagnetic and superconducting layer using polarized neutron reflectometry (PNR), XMCD and muon spectroscopy in a range of samples from our own and other leading groups. We developed highly functional spin valve devices based on a Co layer pinned by IrMn. We have also explored magnetic Skyrmion-based structures and the use of epitaxial Er in spin valve devices. Work on Er/Nb samples that is currently under peer review demonstrates that the superconducting transition temperature of a thin film can be controlled by the magnetic state of a helical magnet that can be programmed by an external magnetic field, constituting a prototype spintronics device. We derived a new theory for superconductor-ferromagnet interfaces appropriate for strongly spin-polarized magnets and diffusive materials, the case most relevant for applications. This solves a long-standing theoretical problem, sought since 1999, of how to implement boundary conditions for Usadel theory in spin-polarized systems. We also implemented Bogoliubov-de-Gennes theory for spin-valves and applied it to experiments within the consortium. A theoretical model of how the transition temperature of a thin film may be controlled by the magnetic state of a helical magnet is under current investigation (see arXiv:1702.08828). In addition we explored new device architectures involving weak link superconductor-ferromagnet structures, in a theoretical work involving a Euromaster student at RHUL (Jan Gelhausen), published in PRB. We also explored the role spin triplet pairs generated at superconductor-ferromagnet interfaces play for the thermoelectric power generation, with a detailed theoretical study in collaboration with Konstanz, leading to a highly cited PRL and a New J Phys article.

2. Investigate the spin-polarisation of triplet pairs injected into N, conventional S and other materials:
Extensive progress was made in this area with a key publication in the field. We obtained the dramatic finding of an anomalously large magnetisation induced in a normal magnetic layer. The role of m=0 or m=+/-1 triplets in this process is not yet completely clear but provides a challenge to theory. The most recent experiments demonstrate that the magnetisation induced in a normal metal layer by the superconductor can be enhanced by the presence of only a single ferromagnetic layer placed adjacent to the superconductor. These results, which cannot be explained by current theoretical models typically applied to these system, are currently under peer review. Further results exploring the possible role of spin-orbit interactions are currently in preparation for submission. We performed extensive theoretical studies of spin-polarisation of triplet pairs injected into a normal metal or a superconductor, and elucidated in particular the role of spin-polarized Andreev bound states at superconductor-ferromagnet interfaces, work that involved collaboration with Imperial College (Leslie Cohen), and Leiden (Jan Aarts), published in Europhys. Lett.

3. Explore generic aspects of the interaction of magnetic and superconducting order at buried interfaces:
Using advanced characterisation to understand in detail the structure-function relation of existing spin-triplet devices, extensive data has been collected and is either published or undergoing analysis. We have carried out work both on our own samples and on those from leading groups in the field (Cambridge, Leiden, Hitachi GST), providing useful information on magnetic states and interfacial quality. These have been measured using muons (including Py/Nb, Ho/Nb, Co/Nb spin valves, Er/Nb, Co/Nb bilayers, NiTi/CoPd), PNR (including Ho/Co/Nb, Co/Nb spin valves, NiNb multilayer), and XMCD (including Co/Nb and Fe/Nb spin valves with delta-doped Mn). We have recently published an exploration of the influence of the superconducting state on the magnetic state of a spin valve, showing that a vortex mediated mechanism can be used to pin magnetic domains. Another work currently in preparation describes a system where a continuously tunable critical current can be achieved in a superconductor by choosing the magnetic state of an adjacent ferromagnetic Ni layer. We performed extensive calculations for induced magnetisation in a superconductor coupled to a ferromagnet at a buried interface, involving an externally funded PhD student (Roland Grein), and published in PRB.

4. Improve routes to the reproducible production of exotic hybrid superconducting-magnetic systems over areas large enough to be amenable to a wide range of quantitative techniques:
We have been highly successful in producing working devices suitable for investigation using a number of techniques systems including XMCD, muons, PNR etc. Some of us have also recently extended this methodology into other areas including collaborations to explore novel superconducting and spin transfer effects using hybrid devices that include C60 layers. We theoretically explored the role of spin-orbit coupling for the creation of spin-triplet pairs and for singlet-triplet mixing in materials that lack a center of inversion, and performed a detailed study of such materials, supported by a locally funded PhD student at RHUL (Niclas Wennerdal). This work is published in a book chapter and a recent PRB article. In addition we studied the interplay between odd-frequency superconductivity in superconductor-ferromagnet structures with the Meissner response, which includes a theoretical work in collaboration with groups in Russia and Japan, published in PRB.
Exploitation Route In the longer term out results could unite the fields of superconductivity and spintronics to realise novel devices based on dissipationless spin currents ("super spintronics").
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Security and Diplomacy

URL http://www.st-andrews.ac.uk/physics/SFMConsortium/
 
Description Over the course of the project we have obtained exciting results on an induced magnetic moment in a non-magnetic metallic layer adjacent to a superconducting spin valve structure. In the longer term this promises to be the first step towards establishing the entirely new field of superconducting spintronics. This work has lead to a number of follow-up studies, in particular the use of ferromagnetic resonance by the Cambridge Group led by Mark Blamire to demonstrate enhanced spin currents in superconducting heterostructures containing 'spin sink' layers with strong spin-orbit coupling. The details of the spin transport processes in these structures remains under discussion. We have also investigated the subtle interplay between ferromagnetism and superconductivity in hybrid structures and shown how a superconductor can be used to control the magnetic reversal of a ferromagnet. This could have important memory applications as an adjunct to superconducting qubits.
First Year Of Impact 2015
Sector Digital/Communication/Information Technologies (including Software),Electronics
Impact Types Economic

 
Title Data for the publication: Reconfigurable superconducting vortex pinning potential for magnetic disks in hybrid structures 
Description This dataset contains raw data and images from Marchiori, E. et al. Reconfigurable superconducting vortex pinning potential for magnetic disks in hybrid structures. Sci. Rep. 7, 45182; doi: 10.1038/srep45182 (2017). 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Data underpinning:Remotely induced magnetism in a normal metal using a superconducting spin-valve 
Description  
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
 
Title Dataset for "Continuously Tuneable Critical Current in Superconductor-Ferromagnet Multilayers" 
Description Datasets underpinning the four Figures for "Continuously Tuneable Critical Current in Superconductor-Ferromagnet Multilayers" which is accepted for publication in Applied Physics Letters. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Dataset for "High quality hydrogen silsesquioxane encapsulated graphene devices with edge contacts" 
Description Datasets underpinning the four figures for "High quality hydrogen silsesquioxane encapsulated graphene devices with edge contacts" in Materials Letters. The primary data files are transmission line measurements for edge contacts and Hall mobilities of devices with and without hydrogen silsesquioxane (HSQ) encapsulation. Also included are a device processing chart and atomic force microscopy (AFM) profiles. Raman data presented in the Supporting Information are also included. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title Dataset for "Tuning the structure of the Josephson vortex lattice in Bi2Sr2CaCu2O8+d single crystals with pancake vortices" 
Description Datasets underpinning the 6 Figures for "Tuning the structure of the Josephson vortex lattice in Bi2Sr2CaCu2O8+d single crystals with pancake vortices" in Scientific Reports. The primary data files are scanning Hall microscopy (SHM) images of pancake vortices catured at a temperature of 85K. Also included are graphs of vortex chain spacing and the estimated effective anisotropy extracted from the SHM images as a function of applied magnetic field. Simulation results for the vortex chain spacing as a function of applied magnetic field arising from the model described in the Supporting Materials are also included. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Dataset for: "Mapping the flux penetration profile in a 2G-HTS tape at the microscopic scale: deviations from a classical critical state model" 
Description Datasets underpinning the 6 Figures for "Mapping the flux penetration profile in a 2G-HTS tape at the microscopic scale: deviations from a classical critical state model" in Superconductor Science and Technology. The data files consist of .txt files for each figure organised in individual folders and presented in labelled columns (tab delimited). The data was acquired by scanning Hall microscopy (SHM) in two operation modes: the 'local' magnetometry mode where 'local' magnetic induction is measured, and the rapid 'flying' mode that makes a rapid 2D scan of the maximum field of view. The data files consist of individual graph curves and SHM images. A critical state model was used to fit the experimental data and estimate the superconducting critical temperature at different temperatures. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title Dataset for: "Nanoscale graphene Hall sensors for high-resolution magnetic imaging" 
Description This dataset contains data from the characterisations of chemical vapour deposition (CVD) graphene Hall sensors with wire widths between 50nm and 1500nm. Characterisations include noise amplitude at various drive currents and back gate voltages, Hall voltage measurements at various magnetic fields and back gate voltages, 2-wire voltage measurements with back gate voltage, and an SEM image used in publications/dissemination. The data files are named according to a convention that indicates to which wire width each file corresponds. Data in figures 7 and 8b of the associated paper are based on all of the complete sets of data for each of the wire widths. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title Irreversible Magnetization Switching at the onset of Superconductivity in a Superconductor Ferromagnet Hybrid 
Description The archive contains a separate folder for each figure in the article. A single README.txt file describes the files contained in each folder. 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
 
Title Spontaneous symmetry breaking in vortex systems with two repulsive lengthscales 
Description This dataset contains data from Curran, P. J. et al. Spontaneous symmetry breaking in vortex systems with two repulsive lengthscales. Sci. Rep. 5, 15569; doi: 10.1038/srep15569 (2015). 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
 
Description SFM Consortium 
Organisation Royal Holloway, University of London
Country United Kingdom 
Sector Academic/University 
PI Contribution This is a critical mass consortium investigating triplet correlations in non-collinear magnetisation ferromagnet-superconductor structures. We are performing scanning Hall probe characterisation of samples and fabricating and characterising magnetotransport structures.
Collaborator Contribution Neutron diffraction, X-ray scattering, transport, thin film sputtering, theory.
Impact This is a single discipline collaboration but involving multiple complementary techniques.
Start Year 2013
 
Description SFM Consortium 
Organisation Rutherford Appleton Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution This is a critical mass consortium investigating triplet correlations in non-collinear magnetisation ferromagnet-superconductor structures. We are performing scanning Hall probe characterisation of samples and fabricating and characterising magnetotransport structures.
Collaborator Contribution Neutron diffraction, X-ray scattering, transport, thin film sputtering, theory.
Impact This is a single discipline collaboration but involving multiple complementary techniques.
Start Year 2013
 
Description SFM Consortium 
Organisation University of Leeds
Country United Kingdom 
Sector Academic/University 
PI Contribution This is a critical mass consortium investigating triplet correlations in non-collinear magnetisation ferromagnet-superconductor structures. We are performing scanning Hall probe characterisation of samples and fabricating and characterising magnetotransport structures.
Collaborator Contribution Neutron diffraction, X-ray scattering, transport, thin film sputtering, theory.
Impact This is a single discipline collaboration but involving multiple complementary techniques.
Start Year 2013
 
Description SFM Consortium 
Organisation University of St Andrews
Country United Kingdom 
Sector Academic/University 
PI Contribution This is a critical mass consortium investigating triplet correlations in non-collinear magnetisation ferromagnet-superconductor structures. We are performing scanning Hall probe characterisation of samples and fabricating and characterising magnetotransport structures.
Collaborator Contribution Neutron diffraction, X-ray scattering, transport, thin film sputtering, theory.
Impact This is a single discipline collaboration but involving multiple complementary techniques.
Start Year 2013
 
Description University of Antwerp 
Organisation Antwerp University Hospital
Country Belgium 
Sector Hospitals 
PI Contribution Performed experiments in mesoscopic superconductivity.
Collaborator Contribution Milorad Milosevic performed Ginzburg-Landau and analytic calculations to understand our experiments.
Impact Single discipline collaboration.
Start Year 2006
 
Description Press release - Aerospace and Defence News 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Magnetised gold heralds new generation of electronics
Aerospace & Defence News
Year(s) Of Engagement Activity 2015
URL http://aerospace.firetrench.com/2015/10/magnetised-gold-heralds-new-generation-of-electronics/
 
Description Press release - Dundee Courier 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Kings of spin scientists have the golden touch
Courier (Dundee) - Online, The
Year(s) Of Engagement Activity 2015
URL http://www.thecourier.co.uk/news/local/fife/kings-of-spin-scientists-have-the-golden-touch-1.903787
 
Description Press release - Electronics Weekly 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Turning gold into spintronics
Electronics Weekly
Year(s) Of Engagement Activity 2015
URL http://www.electronicsweekly.com/news/design/university-electronics/turning-gold-into-spintronics-20...
 
Description Press release - FIRE News 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Magnetised gold heralds new generation of electronics
FIRE News
Year(s) Of Engagement Activity 2015
URL http://ftnews.firetrench.com/2015/10/magnetised-gold-heralds-new-generation-of-electronics/
 
Description Press release - Fresh Ghana.com 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Magnetised bullion heralds new era of electronics
Fresh Ghana.com
Year(s) Of Engagement Activity 2015
URL http://news.vocus.com/?a=22944469161&p=19f&v=1&x=BRtciZWJNd_EQfv31zCo8Q
 
Description Press release - ITV Network 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Scientists discover 'ground-breaking' magnetic capabilities of gold
ITV Network Ltd - Online
Year(s) Of Engagement Activity 2015
URL http://www.itv.com/news/west/2015-10-06/scientists-discover-ground-breaking-magnetic-capabilities-of...
 
Description Press release - International Business Times Australia 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Magnetised gold could change computer technology, research reveals
International Business TImes Australia - Online
Year(s) Of Engagement Activity 2015
URL http://www.ibtimes.com.au/magnetised-gold-could-change-computer-technology-research-reveals-1472950
 
Description Press release - NOODLS 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact University of Bath (via noodls) / Magnetised gold heralds new generation of electronics
NOODLS
Year(s) Of Engagement Activity 2015
URL http://www.noodls.com/viewNoodl/30193150/university-of-bath/magnetised-gold-heralds-new-generation-o...
 
Description Press release - NOODLS 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Magnetised gold is first step
NOODLS
Year(s) Of Engagement Activity 2015
URL http://www.noodls.com/viewNoodl/30193155/university-of-bath/spintronics-magnetised-gold-is-first-ste...
 
Description Press release - Phys.org 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Superconductivity trained to promote magnetization
Phys.org
Year(s) Of Engagement Activity 2015
URL http://phys.org/news/2015-10-superconductivity-magnetization.html
 
Description Press release - Physics World 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Superconductor induces magnetism in non-magnetic gold
Physics World
Year(s) Of Engagement Activity 2015
URL http://physicsworld.com/cws/article/news/2015/oct/05/superconductor-induces-magnetism-in-non-magneti...
 
Description Press release - The National, Scotland 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Research on magnetising gold could improve computers, say St Andrews University scientists
National
Year(s) Of Engagement Activity 2015
URL http://www.thenational.scot/news/research-on-magnetising-gold-could-improve-computers-say-st-andrews...
 
Description Press release - The Scotsman 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Scientist discovery 'could make computers faster'
scotsman.com
Year(s) Of Engagement Activity 2015
URL http://www.scotsman.com/news/sci-tech/scientist-discovery-could-make-computers-faster-1-3909167
 
Description Press release - The Stack 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Scientists reveal next generation spintronics to power faster, smaller computers
The Stack
Year(s) Of Engagement Activity 2015
URL https://thestack.com/world/2015/10/06/machiel-flokstra-spintronics-superconductor-magnetised-golf/
 
Description Press release - Western Daily Press 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Scientists have managed to magnetise gold for the first time ever - and it will change your life
Western Daily Press
Year(s) Of Engagement Activity 2015
URL http://www.westerndailypress.co.uk/Scientists-managed-magnetise-gold-time-change/story-27930012-deta...
 
Description Press release - World News 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Magnetised gold heralds new generation of electronics
World News
Year(s) Of Engagement Activity 2015
URL http://article.wn.com/view/2015/10/06/Magnetised_gold_heralds_new_generation_of_electronics_Univer/
 
Description Talk to Bath Royal Literary and Scientific Institution 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact An invited talk entitled "High Temperature Superconductors come under pressure" to the general public at the Bath Royal Literary and Scientific Institute.
Year(s) Of Engagement Activity 2019
URL https://www.brlsi.org/events-proceedings/events/high-temperature-superconductors-come-under-pressure