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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SUPERSPIN (Superconducting Spintronics for Highly Energery Efficient Cryogenic Memory Applications)

Teaser

Summary

The dissipation of heat in traditional silicon (CMOS) based electronics is a major source of inefficiency and environmental impact. Superconductors are, by nature, dissipationless. Computing via logic circuits based on Josephson junctions is also faster, but the largest remaining problem is the lagging development of low-temperature memory. To achieve the promised efficiency increases of these computers requires a new type of low-temperature memory architecture. Traditionally considered competing phenomena, when artificially juxtaposed a wealth of physics at the interface between superconductors and ferromagnets emerges. Spin-triplet Cooper pairs are capable of surviving inside a ferromagnet over much longer distances than the regular (spin-singlet, anti-parallel) pairs. This new type of Cooper pair is the building block for super-spintronics; leading to a dissipationless spin-current combined with spintronic devices. Europe risks being left behind by large US research efforts such as the IARPA C3 programme. SUPERSPIN will take advantage of spin-polarised Cooper pairs for the promising application of cryogenic memory, where information can be stored by either the state of the system (superconducting or normal), or in the phase difference between superconductors across a Josephson junction. The outgoing host Prof. Birge is the world leading expert in ferromagnetic Josephson junction devices for cryogenic memory application. The fellow will be fully integrated in his IARPA C3 funded laboratories and through the SUPERSPIN programme, of exploring candidate materials systems and developing prototypical devices, will acquire all the skills and knowledge necessary to develop these exciting advances to application the E.U. during the return phase of the project.

Through SUPERSPIN, the fellow will broaden his scientific background, develop complementary knowledge in new areas, bring new knowledge from the TC host to the E.U. and increase his chances of success in academia.

Work performed

In the first reporting period of this project Satchell worked in the group of Prof. Norman Birge at Michigan State University (MSU). The Birge group are world leaders in ferromagnetic Josephson junctions for cryogenic memory and spin-triplet superconductivity. At the beginning stage of this project Satchell integrated into the group, learning the complicated nanolithography processing employed to produce Josephson junction devices. During his time at MSU, the fellow successfully produced over 100 such devices used in several research tasks.

The first task for the fellow at MSU was to create ferromagnetic Josephson junctions with spin-orbit coupling. It has been predicted that the spin-orbit coupling will generate the spin-triplet supercurrent required for the type of devices under study and that the spin-orbit coupling can be used to create a Josephson junction with arbitrary ground state phase (so called phi-0 junctions), however the prediction had not yet been experimentally realised in a Josephson junction device. This work led to two publications where the fellow was first author [1,3].

For the development of practical devices, one consideration was the surface roughness of the Nb superconducting electrode. Roughness of the Nb layer can degrade the switching of the subsequent ferromagnetic layers in the Josephson junctions and thus reduces the performance of devices. The method employed in the Birge group to minimize the surface roughness was to grow the Nb as a superlattice of [Nb/Al] or [Nb/Au] where the superlattice had a much lower roughness than a single layer of Nb with the same total thickness. Neutron scattering was used to probe the London penetration depth in each materials system. It was found that the London penetration depth was longer in the superlattices (120 â€“ 180 nm) vs the single layer Nb (90 nm). This work is in preparation for publication.

Journal articles associated with the SUPERSPIN project in the reporting period (chronological):

[1] N. Satchell and N. O. Birge â€œSupercurrent in ferromagnetic Josephson junctions with heavy metal interlayersâ€ Phys. Rev. B 97, 214509 (2018)
[2] N. Satchell â€œControlled superconducting vortex creation raises hope for a dissipationless memory deviceâ€ Supercond. Sci. Technol. 32, 020501 (2019) (Invited Viewpoint)
[3] N. Satchell, R. Loloee and N. O. Birge â€œSupercurrent in ferromagnetic Josephson junctions with heavy-metal interlayers. II. Canted magnetizationâ€ Phys. Rev. B 99, 174519 (2019)

Final results

So far, only very few experimental studies have explored the superconductor-ferromagnet proximity effect with spin-orbit coupling, and none of those employed Josephson junctions as the direct probe of the proximity effect. Given the many theoretical predictions focusing on that topic, it was ripe for study. Progress on this project has already achieved multiple journal publications indicating the interest in the spin-orbit Josephson junction experiment which moved the understanding of the proximity effect beyond the state of the art.

For the incoming stage of the fellowship the most important aim is for the fellow to implement the fabrication of Josephson junction devices in the Leeds laboratories. The Burnell group in Leeds has expertise in growing high quality perpendicularly magnetised films which are capable of supporting topological magnetic texture such as skyrmions. The first expected result is a proof of concept experiment designed to both establish the lithographic processing and measurement of Josephson junctions at Leeds. The candidate system to study initially is a Josephson junction containing two perpendicular magnetic layers with different anisotropies which form a perpendicular pseudo spin-valve device. This simple memory device can hold information based on the independent switching of the two ferromagnetic layers. Once set up stage in Leeds is complete, the fellow will take advantage of mature research projects within the Leeds spintronics group and create SUPERSPIN devices from new materials in addition to training others at Leeds in the processing and measurement of Josephson junctions.

The results already achieved in the SUPERSPIN project are of significant interest to researchers working in the fields of superconductor-ferromagnet proximity effect, superconductivity and spin-orbit coupling, and cryogenic memory. In addition to advancing the fundamental physics underpinning these phenomena, it is possible that the results achieved through SUPERSPIN will influence the design of practical memory devices for cryogenic memory application.