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Superconducting Spintronics for Highly Energery Efficient Cryogenic Memory Applications

Periodic Reporting for period 2 - SUPERSPIN (Superconducting Spintronics for Highly Energery Efficient Cryogenic Memory Applications)

Reporting period: 2019-06-01 to 2020-05-31

The dissipation of heat in traditional silicon (CMOS) based electronics is a major source of inefficiency and environmental impact. Superconductors are, by nature, dissipationless. To achieve the promised efficiency increases of these computers requires a new type of low-temperature memory architecture. 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 conclusions of the action:

The scientific achievements of the project have studied in great detail the role of spin orbit coupling on a triplet state in a Josephson junction. The second scientific conclusion is the study of alternative base superconducting electrodes for Josephson devices, where the improved structural property of lower surface roughness greatly improves the junction’s characteristics. Finally, the incoming phase demonstrated PMA materials for cryogenic memory application. The scientific advances are significant advances beyond the state-of-the-art and have led to the publication of 10 peer reviewed journal articles.

Beyond the scientific achievements of the project, knowledge transfer has successfully allowed the fellow to implement new techniques in both host intuitions. The fellow has used his world leading knowledge of x-ray and neutron scattering to implement new experimental techniques in the Birge group at MSU. The MSU group itself is recognized as world leading on production of superconducting devices, and the fellow has successfully implemented the device fabrication procedure to the Leeds group.

The fellows career prospects have been greatly enhanced through participation in the fellowship and career development plan. In particular, the fellow has attended career workshops and had networking opportunities at major international conferences. It is the intention of the fellow to submit further research funding applications to continue his career in this field.
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. The first task for the fellow at MSU was to create ferromagnetic Josephson junctions with spin-orbit coupling. This work led to two publications where the fellow was first author [1,3].

For the development of devices, one consideration was the surface roughness of the Nb electrode. Roughness of the Nb layer reduces the performance of devices. To minimize the surface roughness, the fellow studied superlattices 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. This work is submitted for publication [9].

During the incoming phase of the project, the fellow established the lithography methodology for Josephson junction fabrication. The fellow developed a PMA spin-valve Josephson junction device, where two ferromagnetic layers (one Co and the other CoB) could be switched independently. The fellow demonstrated memory operation and this work is now published [6].

As a follow up to the spin-valve Josephson junction result, the fellow next chose to study in more detail the Pt/CoB/Pt part of the spin valve to show transition from a zero to pi ground state phase difference across the junction. Results of this study have been submitted for publication [10].

Journal articles associated with the SUPERSPIN project (chronological):
[1] N. Satchell and N.O. Birge Phys. Rev. B 97, 214509 (2018)
[2] N. Satchell Supercond. Sci. Technol. 32, 020501 (2019) (Invited Viewpoint)
[3] N. Satchell, R. Loloee and N.O. Birge Phys. Rev. B 99, 174519 (2019)
[4] R. Stewart, M.G. Flokstra, M. Rogers, N. Satchell, G. Burnell, D. Miller, H. Luetkens, T. Prokscha, A. Suter, E. Morenzoni, S.L. Lee Phys. Rev. B 100 (2), 020505 (2019)
[5] M. G. Flokstra, R. Stewart, N. Satchell, G. Burnell, H. Luetkens, T. Prokscha, A. Suter, E. Morenzoni, S. Langridge, and S. L. Lee App. Phys. Lett. 115, 072602 (2019)
[6] N. Satchell, P. M. Shepley, M. Algarni, M. Vaughan, E. Darwin, M. Ali, M. C. Rosamond, L. Chen, E. H. Linfield, B. J. Hickey, and G. Burnell App. Phys. Lett. 116, 022601 (2020)
[7] M. Vaughan, N. Satchell, M. Ali, C. J. Kinane, G. B. G. Stenning, S. Langridge, and G. Burnell Phys. Rev. Research 2, 013270 (2020)

Journal articles associated with the SUPERSPIN project to be published:
[8] N. Satchell, C. J. Kinane, J. F. K. Cooper, G. Stenning, T. R. Charlton, J. D. S. Witt, M. Batley, G. Burnell, P. J. Curran, S. J. Bending, and S. Langridge To appear in J. Vis. Exp.
[9] P. Quarterman, N. Satchell, B. J. Kirby, R. Loloee, G. Burnell, N. O. Birge, and J. A. Borchers Submitted to Phys. Rev. Materials arXiv:2001.09310
[10] N. Satchell, T. Mitchell, P. M. Shepley, E. Darwin, B. J. Hickey, and G. Burnell Submitted to Phys. Rev. Applied arXiv:2005.00384
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 was for the fellow to implement the fabrication of Josephson junction devices in the Leeds laboratories. This aim was quickly achieved. 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. Using the combination of the new fabrication methodology with the existing expertise at Leeds, we have reported two significant results. The first result was a proof of concept experiment designed to both establish the lithographic processing and measurement of Josephson junctions at Leeds. We studied a Josephson junction containing two perpendicular magnetic layers with different anisotropies which form a perpendicular pseudospin-valve device. This simple memory device can hold information based on the independent switching of the two ferromagnetic layers. The second project involved a more detailed study of one of the component ferromagnetic layers from the pseudospin-value, the amorphous alloy CoB. Using CoB, we have demonstrated pi-junctions, which have applications in superconducting electronics.

The results 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.
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