First Successful Attempt in 55 years: Physicists in Russia and Germany Confirm 1969 Experiment Results
A team of researchers, with the participation of physicists from HSE University, replicated the 1969 experiment on superconductivity and its properties. The scientists induced superconductivity by deliberately deteriorating the interfaces between the layers of superconductors and ferromagnets in the system, resulting in better performance of spin valves compared to the classical version, where the interfaces between the layers are ideal. This approach could lead to the development of more efficient devices for data storage and computing. The study findings have been published in the Beilstein Journal of Nanotechnology.
When an electric current passes through a metal wire, it encounters resistance. However, when certain materials are cooled to extremely low temperatures, their resistance vanishes, allowing electricity to flow without any loss. This property is known as superconductivity.
Since the 20th century, scientists have been working on developing a system in which superconductivity could be switched on and off at will. Eventually, a structure was selected in which a superconducting metal interfaced with two ferromagnets influencing its superconducting properties. The arrangement consists of one superconducting layer (S) and two ferromagnetic layers (F), configured as either F-S-F or F-F-S. The alignment of the magnets relative to each other influences the overall superconductivity of the system; by fixing the direction of one magnet and rotating the other, superconductivity can be switched on and off. This phenomenon is known as the superconducting spin valve effect.
It is believed that the maximum spin valve effect can be achieved when transitions between different layers (magnetic and non-magnetic) occur without barriers, defects, or impurities. However, in a 1969 experiment, scientists Deutscher and Meunier demonstrated that the system could function effectively even when dielectric layers—materials that do not conduct electric current but allow electron passage—were introduced. According to Deutscher and Meunier’s findings, a pronounced spin valve effect can be maintained even in a structure that includes dielectric layers. However, other research teams have been unable to replicate this result.
For the first time, a team from the RAS Kazan E. K. Zavoisky Physical-Technical Institute, the Leibniz Institute for Solid State and Materials Research in Dresden, the RAS L. D. Landau Institute for Theoretical Physics, and HSE University successfully replicated the experiment. They created a layered structure using lead as the superconductor and cobalt as the ferromagnet, and experimentally introduced dielectric layers between them. To achieve this, oxygen was added at one stage of the fabrication process to oxidise the interfaces. The resulting oxides were non-conductive to electric current.
The results revealed a pronounced superconducting spin valve effect in the structures obtained. The scientists attribute the result to the dual role of the oxide insulating layers. These layers both reduce the influence of the metallic ferromagnetic layer on the superconducting layer and preserve a proximity effect that enables switching between normal and superconducting states. However, it remains uncertain whether the insulating layers themselves are magnetic. Further research is needed to answer this question.
'From a naive perspective, introducing elements that do not conduct current seems like it would degrade the system. However, it turns out that this is not always the case. Dielectric layers actually enhance the system, while their absence can, conversely, undermine superconductivity,’ explains Yakov Fominov, Professor at the Faculty of Physics and Leading Research Fellow at the International Laboratory for Condensed Matter Physics of HSE University. 'Apparently, we are dealing with a type of superconductivity that is conditionally fragile and sensitive to external influences. When ferromagnets in the system attempt to completely suppress superconductivity, introducing insulating interface layers can restore it.'
The spin valve effect is used in reading information from hard drives, in sensors for measuring magnetic fields in compasses, and in various other devices. The scientists believe that further research on this approach can enhance the performance of superconducting spin valves.
The study was supported by the Russian Science Foundation (project 21-72-20153) and the HSE Basic Research Programme.
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