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Theoretical and experimental analysis of the critical phenomena in magnetically nano-engineered superconductors

Final Activity Report Summary - SC-FM HYBRIDS (Theoretical and experimental analysis of the critical phenomena in magnetically nano-engineered superconductors)

Superconductivity has been one of the most exciting and active research directions over the last century. The fascinating properties of (i) perfect conductivity (zero resistance), and (ii) intrinsic diamagnetism (expulsion of applied magnetic field), make superconducting structures very promising for applications in both electronics and data storage. In the last decade, hybrid nanostructures incorporating superconducting (SC) and ferromagnetic (FM) metal components have become one of the most fascinating systems to study. The exciting new feature of SC/FM hybrids is the competition between ferromagnetism and superconductivity, representing two antitheses in condensed matter physics, which is of abiding interest in fundamental and applied materials science. Nowadays, the aim is to produce artificially nanostructured hybrid composite materials, such as SC-films with embedded magnetic nano-clusters, SC-FM multilayers, and submicron size hybrid circuits. Recent advances in electron-beam lithography have made it possible to create SC-FM structures practically at will, in a controlled and well-defined way.

The main aspects of this project were (i) the enhancement of the critical properties of hybrid superconducting structures, and (ii) the link of the possible enhancement with so-called 'vortex matter'. Large magnetic fields are able to overcome the screening from the superconductor, and gradually penetrate the sample in terms of individual flux quanta, called vortices. If additional electrical current is driven through the sample, vortices are easily driven into motion by the Lorentz force exerted. This flux flow dissipates energy, and eventually destroys superconductivity. This problem is resolved by artificial pinning of vortices, by e.g. regular lattice of small magnetic dots on top of the superconductor, and results in a significant enhancement of the critical current. On the other hand, the critical field of the sample is also changed in magnetically textured superconductors. Using Scanning Hall Probe Microscopy (SHPM), we have examined the combined influence of an applied homogeneous magnetic field and the local stray fields of Co/Pt magnetic dots on a lead (Pb) superconducting film. For the first time, we have showed experimentally the existence of vortex-antivortex molecules around magnetic dots, with vortices confined under the magnets (where the stray field is positive) and antivortices located on a shell around the magnet (where the stray field is negative). Additional homogeneous fields induce added (anti)vortices in the film (depending on the polarity), and we have studied the plethora of phenomena related to their interaction with pre-existing molecules in the film. We also concluded that it is actually the compensation between added vortices and existing antivortices that leads to the enhanced superconductivity in the film.

The magnet-vortex interaction can also be used for the controlled manipulation of vortices, popularly called 'fluxonics'. We have designed a superconducting logic element, where two energetically degenerate locations of vortices define logic states '0' and '1', and switching between them can be achieved by vortex interactions with strategically placed current loops (and their emergent magnetic fields). We have demonstrated that this latter switching can be performed on a nanosecond timescale in samples that are already experimentally realisable. We also demonstrated a fully functional majority gate, which is a basic logic gate in electronic circuits. All together, our findings form the full basis for 'Fluxonic cellular automata', the concept featured on the cover page of the Applied Physics Letters in November last year.

During this project, we also discovered that magnets themselves hold fascinating properties on the nanoscale. Their response to an applied magnetic field is absolutely non-trivial, and must be taken into account for the engineering of SC-FM hybrids. We performed a detailed study of a ferrimagnetic yttrium-iron-garnet (YIG) film in an applied magnetic field, and discovered a new, intrinsic type of magnetic domain pinning. As has been shown in numerous studies in the 60s, YIG films host alternating positive and negative labyrinth domains, whose width ratio changes in an applied field (e.g. positive domains expand in a positive field). We showed that in our films crystallisation of magnetic charges on the zig-zag fine structure of the domains leads to a strong interaction between the domain walls, disabling the expansion/shrinkage of the domains up to a very large applied field (app.1kOe!). At low fields, we were able to detect local nanoscale instabilities of the domains due to an applied field, with local displacements of domain walls of only 2-10 nm! This is the first direct experimental observation of nanoscale motion of domain walls in a magnetic structure of any kind.

The last part of our project is related to truly three-dimensional samples. Up to now, mostly planar, thin 2D superconductors have been studied in both theory and experiment. Using electrochemistry, we have fabricated 3D polyhedra of lead (Pb), with characteristically triangular facets, and tin (Sn), with mostly rectangular facets. The morphology of the investigated structures ranged from wires, through prisms, pentagonal pyramids, to truncated icosahedra and multipods, all on a micrometer scale. Besides the actual fabrication and manipulation of individual micro-samples onto micro Hall-probe arrays being a substantial experimental achievement, we actually performed magnetometry measurements on these structures, and obtained clear evidence of various new vortex states, some of which exhibit distinct symmetries. These findings are particularly important since both Pb and Sn are type-I superconductors, and in principle should not exhibit states comprising individual flux quanta, let alone their interaction and formation into symmetric configurations. We also mastered techniques for the pioneering fabrication of core-shell structures of different materials; e.g. the first measurements showed that the critical field of a tin rod can be enhanced by >20% if it is covered by a thin layer of Pb, which is obviously of considerable technological relevance.

In our opinion, the work performed in the past 2 years has by far exceeded the planned activities, and has established several clear directions for future research. The dissemination of our results in various scientific forums has shown that these latter directions will motivate similar research in a wide scientific community.