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Triplet supercurrents and superconducting spintronics

Final Report Summary - SUPERSPIN (Triplet supercurrents and superconducting spintronics)

Within this project we have laid the foundations for an eventual technology of superconducting spin electronics (superspintronics). The starting point of the project was our observation that using an appropriate spin-mixing interface between a superconductor and a ferromagnet it is possible to convert the singlet Cooper pairs of electrons which are the basis of superconductivity in standard superconducting materials into triplet pairs consisting of parallel spin electrons. Since triplet pairs can, unlike singlet pairs, carry spin it is therefore possible to generate superconducting spin currents and so potentially combine the technologies of spin electronics (spintronics) and superconductivity).

Through this ERC-funded project we have established that superconducting spin currents formed can be established and controlled so that spin transport in the superconducting state could eventually be used to communicate with low energy loss between devices. We have created spin switch devices using several different novel materials systems – these devices show that exceptionally large changes in the superconducting properties can be achieved by varying the magnetic state of the structure. Such devices are clear candidates for superspintronic memories. We have also used such devices to demonstrate for the first time that the superconducting state can in turn be used to control the magnetic state – potentially providing a means of directly writing to such memories.
Although the initial focus of the project was on metallic device structures, the work has generated a lot of information about ferromagnetic insulators – particularly GdN, but several novel oxide materials have been identified as potential device components. GdN shows a complicated set of properties and much of our work has been spend trying to understand and control it. In particular we have shown that although thick films are conducting, the material becomes progressively more insulating as it is made thinner. This probably originates from carriers which can be depleted by electric fields which form at the interfaces.
In conjunction with superconductors, we have shown that GdN strongly modifies the properties of a superconductor in contact with it leading, not only to the some of the spin switch effects discussed above, but also to the generation of triplet states at the interface which we have measured directly via tunneling spectroscopy and through a novel dependence of the superconducting current on the superconducting phase difference. As well as optimizing the properties of tunnel junctions with GdN barrier we have also proved that Josephson junctions with GdN barriers exhibit true quantum coherence and so can potentially be applied to quantum electronic devices.
The project has demonstrated that superconductor / ferromagnet hybrid devices can be used to create all the ingredients required for a future superspintronic technology. The results gained from the ERC programme have formed the basis for a major device development programme grant from the UK Engineering and Physical Sciences Research Council (EPSRC - EP/N017242/1).