Meissner screening as a probe for inverse superconductor-ferromagnet proximity effects

Funding: We acknowledge the support of the EPSRC through Grants No. EP/I031014/1, No. EP/J01060X, No. EP/J010634/1, No. EP/L015110/1, No. EP/R031924/1, and No. EP/R023522/1. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 743791 (SUPERSPIN). R.S. acknowledges funding under ETH Zurich Postdoctoral Fellowship 20-1FEL-36.We present experimental results on the observed flux screening in proximity coupled superconductor-ferromagnet thin film structures using Nb and Co as the superconductor and ferromagnet respectively. Using the low-energy muon-spin rotation technique to locally probe the magnetic flux density, we find that the addition of the ferromagnet (F) increases the total flux screening inside the superconductor. Two contributions can be distinguished. One is consistent with the predicted spin-polarization (or magnetic proximity) effect, while the other is in line with the recently emerged electromagnetic (EM) proximity models. Furthermore, we show that the addition of a few nanometers of a normal metallic layer between the Nb and the Co fully destroys the contribution due to electromagnetic proximity. This is unanticipated by the current theory models in which the magnetization in the F layer is assumed to be the only driving force for the EM effect and suggests the role of additional factors. Further experiments to explore the influence of the direction of the F magnetization also reveal deviations from theory. These findings are an important step forward in improving the theoretical description and understanding of proximity coupled systems.Publisher PDFPeer reviewe

The role of ion dissolution in metal and metal oxide surface inactivation of SARS CoV-2

Funding: This work was funded by UKRI-NIHR (MRC MR/V028464/1) COVID-19 Rapid Response Initiative.Anti-viral surface coatings are under development to prevent viral fomite transmission from high-traffic touch surfaces in public spaces. Copper’s anti-viral properties have been widely documented, but the anti-viral mechanism of copper surfaces is not fully understood. We screened a series of metal and metal oxide surfaces for anti-viral activity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease (COVID-19). Copper and copper oxide surfaces exhibited superior anti-SARS-CoV-2 activity; however, the level of anti-viral activity was dependent on the composition of the carrier solution used to deliver virus inoculum. We demonstrate that copper ions released into solution from test surfaces can mediate virus inactivation, indicating a copper ion dissolution-dependent anti-viral mechanism. The level of anti-viral activity is, however, not dependent on the amount of copper ions released into solution per se. Instead, our findings suggest that degree of virus inactivation is dependent on copper ion complexation with other biomolecules (e.g., proteins/metabolites) in the virus carrier solution that compete with viral components. Although using tissue culture-derived virus inoculum is experimentally convenient to evaluate the anti-viral activity of copper-derived test surfaces, we propose that the high organic content of tissue culture medium reduces the availability of “uncomplexed” copper ions to interact with the virus, negatively affecting virus inactivation and hence surface anti-viral performance. We propose that laboratory anti-viral surface testing should include virus delivered in a physiologically relevant carrier solution (saliva or nasal secretions when testing respiratory viruses) to accurately predict real-life surface anti-viral performance when deployed in public spaces.PostprintPeer reviewe

Thermodynamic phase transitions in a frustrated magnetic metamaterial

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 290605 (COFUND: PSI-FELLOW) and from the EPSRC (grant EP/J01060X).Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour. Here we create a thermally active artificial kagome spin ice that is made up of a large array of dipolar interacting nanomagnets and undergoes phase transitions predicted by microscopic theory. We use low energy muon spectroscopy to probe the dynamic behaviour of the interacting nanomagnets and observe peaks in the muon relaxation rate that can be identified with the critical temperatures of the predicted phase transitions. This provides experimental evidence that a frustrated magnetic metamaterial can be engineered to admit thermodynamic phases.Publisher PDFPeer reviewe

Control of superconductivity with a single ferromagnetic layer in niobium/erbium bilayers

Superconducting spintronics in hybrid superconductor{ferromagnet (S{F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, Tc, of the S-layer can be controlledby the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using eld history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both Tc and the shape of the resistive transition, R(T), to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the e ective exchange eld of the ferromagnet and link this, using conventional S{F theory, to the suppression of Tc. We observe stepped features in the R(T) which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description

Emergent magnetism at transition-metal–nanocarbon interfaces

Charge transfer at metallo–molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc–C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo–carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2−π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz–π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices

Continuously tuneable critical current in superconductor-ferromagnet multilayers

We demonstrate that the critical current of superconducting Nb/Ni multilayers can be continuously tuned by up to a factor of three during magnetization reversal of the Ni films under an applied in-plane magnetic field. Our observations are in reasonably good agreement with a model of vortex pinning by Bloch domain walls that proliferate in the samples during magnetization reversal, whereby each vortexinteracts with at most one wall in any of the Ni layers. Our model suggests ways in which the controllable pinning effect could be significantly enhanced, with important potential applications in tuneable superconducting devices

The role of ion dissolution in metal and metal oxide surface inactivation of SARS-CoV-2

Antiviral surface coatings are under development to prevent viral fomite transmission from high-traffic touch surfaces in public spaces. Copper's antiviral properties have been widely documented; but the antiviral mechanism of copper surfaces is not fully understood. We screened a series of metal and metal oxide surfaces for antiviral activity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease (COVID-19). Copper and copper oxide surfaces exhibited superior anti-SARS-CoV-2 activity; however, level of antiviral activity was dependent upon the composition of the carrier solution used to deliver virus inoculum. We demonstrate that copper ions released into solution from test surfaces can mediate virus inactivation, indicating a copper ion dissolution-dependent antiviral mechanism. Level of antiviral activity is, however, not dependent on the amount of copper ions released into solution per se. Instead, our findings suggest that degree of virus inactivation is dependent upon copper ion complexation with other biomolecules (e.g., proteins/metabolites) in the virus carrier solution that compete with viral components. Although using tissue culture-derived virus inoculum is experimentally convenient to evaluate the antiviral activity of copper-derived test surfaces, we propose that the high organic content of tissue culture medium reduces the availability of "uncomplexed" copper ions to interact with the virus, negatively affecting virus inactivation and hence surface antiviral performance. We propose that laboratory antiviral surface testing should include virus delivered in a physiologically relevant carrier solution (saliva or nasal secretions when testing respiratory viruses) to accurately predict real-life surface antiviral performance when deployed in public spaces

Intrinsic Paramagnetic Meissner Effect due to s-wave Odd-Frequency Superconductivity

In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux, the diamagnetic Meissner effect, from the interior of superconducting lead. This discovery was crucial in formulating the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a superconductor-magnet hybrid system where magnetic ordering breaks time-reversal symmetry of the superconducting condensate and results in the stabilisation of an odd-frequency superconducting state. It has been predicted that under appropriate conditions, odd-frequency superconductivity should manifest in the Meissner state as fluctuations in the sign of the magnetic susceptibility meaning that the superconductivity can either repel (diamagnetic) or attract (paramagnetic) external magnetic flux. Here we report local probe measurements of faint magnetic fields in a Au/Ho/Nb trilayer system using low energy muons, where antiferromagnetic Ho (4.5 nm) breaks time-reversal symmetry of the proximity induced pair correlations in Au. From depth-resolved measurements below the superconducting transition of Nb we observe a local enhancement of the magnetic field in Au that exceeds the externally applied field, thus proving the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.publishe

Dataset for "Continuously Tuneable Critical Current in Superconductor-Ferromagnet Multilayers"

Datasets underpinning the four Figures for "Continuously Tuneable Critical Current in Superconductor-Ferromagnet Multilayers" which is accepted for publication in Applied Physics Letters