Project Overview
We will launch an exhaustive investigation of hybrid nanostructures incorporating superconducting (S) and ferromagnetic (F) metal components. We will investigate experimentally and theoretically fundamental electronic and magnetic phenomena occurring in superconductor and ferromagnet-based hybrids, dynamic and kinetic transport effects, the processes of spin filtering and spin injection, and the possibility of coherent manipulation of an individual electron spin. In the course of this work, we shall master the metal-ferromagnet interface technology and develop techniques for manufacturing micro- and submicron-size circuits. Understanding of the physics involved will be combined with searching novel guiding principles for the next generation magnetic memory and, in the long term, enhancing the material and device functionalities via combining the specific properties of ferromagnets and superconductors.
Electronic nanostructures display physical properties that are strikingly different from the properties of homogeneous bulk materials. In quantum dots and wires this is due to a fully coherent electron dynamics at the nano- and micrometer scale. Amazing observations involving ferromagnetic materials include magneto-interference phenomena like the non-local exchange coupling, the giant magneto-resistance, current-induced magnetization switching and spin-pumping. Magneto-electronics has lead to reprogrammable logics and non-volatile magnetic random access memories (MRAM) - the latter being expected on the market this year.
In ferromagnetic – normal metal (N) layered structure and S-N junctions, new phenomena are caused by the mutual influence and extended electron states shared by fundamentally different materials – the proximity effect. The exciting new feature of S-F hybrid structures consists of the competition between ferromagnetism and superconductivity, representing two antitheses in condensed matter physics, which is of abiding interest in fundamental and applied materials science. It occurs naturally only in few exotic materials (the Chevrel phase alloys and the Rutheno-Cuprates), but now we aim to produce it artificially in nano-structured composite materials, such as S-films with embedded magnetic nano-clusters, in S-F-S and F-S-F multi-layers. A few observations of non-local interface resistance in F-S structures [1-3] have already attracted attention and diverse interpretations, including the induced triplet-pairing models [4,5] and a Fulde-Ferrell-Larkin-Ovchinikov state in an F-S junction [6]. For nano- and submicron-size S-F and S-F-S multi-layers and superconducting films densely patterned with magnetic nano-clusters, even the nature of the ground state is an open issue, not to mention charge transport properties and magnetization dynamics.
The suppression of spin current in a spin-singlet superconductor results in a spin-filtering effect and in the spin accumulation at the S-F interface [7]. The latter may force a hybrid system into a strongly non-equilibrium state and cause injection of spin-polarized quasi-particles into the superconducting part of a microcircuit. It is worth mentioning that spin relaxation of quasi-particles in superconductors is very slow. The principle physical novelty of F-S structures is, therefore, the occurrence of effects related to the long-range spin and phase memory of electrons at low temperatures. The spin-injection into superconductors would generate long-range charge imbalance and non-locality in magnetic, electric and thermal transport properties [8], so that the information carried by spin polarization of a quasi-particle may be communicated to leads at a mesoscopic distance.
Aiming to understand the fundamental quantum physics governing the electronic transport and magnetic properties in novel hybrid systems, SFINX will focus efforts on fabrication, theoretical and experimental study of three types of S-F nanostructures: superconducting films with embedded magnetic nano-clusters, S-F multi-layers and submicron size circuits with S and F components. Particular attention will be paid to complex kinetics of such systems in strongly non-equilibrium conditions. The new understanding will be used for designing hybrid micro-circuitry (such as SQUIDs) and for predicting new modes of operation. The primary research objectives of this project consist in resolving the following issues:
(i) Revealing spin-depolarization processes caused by spin-orbit coupling at metallic surfaces and interfaces and by formation of non-collinear spin textures in F-F’ tunnel junctions and F-M-F’ tri-layers. This will be achieved via simultaneous experimental studies of the interface transport properties and development of adequate theoretical models for the ferromagnetic - normal metal and F-F’ interfaces. Insights will subsequently be applied to manufacturing and optimization of micro-circuits with ferromagnetic components.
(ii) To create and study coexistence of superconductivity and ferromagnetism in S-films with embedded magnetic nano-clusters. For this, we will have to grow such films, investigate their fundamental spectral, transport and magnetic properties using surface scanning tunneling microscopy and standard low-temperature methods, and compare their properties with theoretical results obtained by a self-consistent multi-scale modelling scheme to be newly developed.
(iii) To realize spin filtering and spin injection into superconductors, investigate implications of mesoscale range non-equilibrium transport across hybrid S-F nanostructures. This will be achieved via development of various configurations of hybrid structures and microcircuits, supplemented by studies of their current-voltage characteristics and a detection of quasi-particle interference effects in the non-equilibrium transport in hybrid micro-circuitry. Experimental studies will be supported by theoretical modelling.
(iv) Understanding the nature of the ground state and transport properties of nanometer-thin S-F heterojunctions, S-F-S, S-F-M-F’-S and S-F-I-F’-S multi-layers, subject to the coexistence of superconductivity and ferromagnetism (here ‘I’ stands for an insulating surface layer). These studies will include theoretical analysis of spin-polarized Andreev bound states in F-S hybrids, measurement of Josephson currents and critical temperatures in spin-valve structures with superconducting components.
SFINX aims at creating the long-term innovation basis for emerging novel nano-structured materials with principally new qualities needed for microelectronics. The project is a long-term innovation research plan which meets the content of the NMP Call Improving the knowledge in nanoscience: it develops know-how for fabrication of emergent nano-electronics areas, offers leading edge experimental facilities to study the related quantum phenomena and advances modern theoretical approach to the description of the processes involved. The properties of F-S hybrids will enable development of new magneto-resistance materials for applications as magnetic probes and sensors. Superior switching properties of magnetic memory elements as well as metallic spin transistors with useful current gain and other revolutionary devices will outweigh the disadvantage of the low temperatures needed to realize the superconducting state. Advancement of nano-fabrication and interface technologies during the realisation of this project will be another benefit for the EU science and industry.
To accomplish the above goals, we will form collaboration between 7 groups that have complementary expertise in techniques of making multilayer F-N and F-S structures and hybrid microcircuits, in experimental studies of electronic and magnetic properties of nanostructures, in theoretical studies of nanostructures and interfaces, and in superconductivity and magnetism. All these groups have proven their excellence in research in these areas, have incorporated the development and studies of ferromagnet – superconductor hybrid nanostructures into their long-term research strategies. These groups have already established links via the EU COST Action ‘Mesoscopic Electronics’.
In this co-operation, the Lyon and Leeds groups will provide the technological base for the MBE and sputtering ultra-thin film growth. Facilities are in place for the deposition of a wide variety of metallic materials, as well as magnetic tunnel junctions. This is supported by extensive instrumental base for characterisation of structural, magnetic and transport properties of films. The Lyon node has a great expertise in the synthesis and characterization of nano-structured films prepared from magnetic clusters. It was the first group to develop the low energy cluster beam deposition technique and to produce original nano-structured thin films from pre-formed clusters embedded in various co-deposited matrices. The Leeds node has an international reputation in the field of molecular beam epitaxy (MBE) growth and exploitation of magnetic ultra-thin films, nanostructures, and spin electronics. It is now entering the field of F-S hybrids.
The Grenoble and Jyvaskyla groups will carry out nano-processing of structures and fabrication of micro-circuits and will make state-of-the-art electric measurements, including low-temperature transport and local probes studies of novel structures. The Grenoble node has a great expertise in STM spectroscopy of mesoscopic structures at very low temperature and is presently developing a combined AFM/STM microscope, already with the best resolution ever demonstrated in STM spectroscopy experiments. At Grenoble, we also run a nano-fabrication room equipped with e-beam lithography, focused ion-beam lithography and etching. The Jyvaskyla node is also able to carry out high precision and low noise electric measurements. It has a facility to prepare microcircuits: scanning electron microscopes, ultra-high vacuum e-beam evaporators and reactive ion etching complex. Jyvaskyla team has recently developed a method of low energy plasma etching enabling progressive reduction of pre-fabricated microstructures, pushing the limit for of a line width down to ~ 20 nm keeping the line length > 50 mm. Team members have an access to the local accelerator ion source.
The Delft/Twente, Lancaster and Trondheim theory groups will develop self-consistent quantum multi-scale modelling approach to the description of proximity effect and non-equilibrium transport in F-S nano-hybrids and work on the in situ interpretation of the routine experimental data. All these groups have substantial publication records in theoretical nanoscience and mesoscopic physics. To analyze the short range interface properties (transmission amplitudes and phases for electrons passing across the F-N boundary), we will use both an empirical tight-binding model approach and the first-principles LMTO codes (developed by P.J. Kelly). The interface properties will subsequently be included via boundary conditions into semiclassical approaches such as Eilenberger’s and Usadel’s equations and quantum kinetic theory for the bulk of the ferromagnetic and superconducting layers to describe transport in the overall structure. The long-range properties of F-S hybrid systems determine the sub-gap current-voltage characteristics and the critical current in S-F-S sandwiches. This combination of semiclassical concepts in the bulk of layers and quantum coherent transport across the interfaces, which was successful in modeling transport in hybrid normal - ferromagnetic metal system, should provide us with a powerful tool to model S-F hybrids, and to predict/explain their properties.
The work on SFINX will provide an excellent opportunity to train young researchers in highly competitive new areas of science and technology and to develop skills which are in high demand on the overstretched EU job market. Each of the participating groups will involve PhD students and junior trainees into this work.
