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Physical Review Applied

Josephson junctions with ferromagnetic barriers for digital superconducting electronics: A review

Ana Mitrović, Mark F. Bocko

Published Jun 2, 2025• 11 citations
Publisher

Abstract

Josephson junctions with ferromagnetic barriers (FJJs) enable self-biased operation and inductorless logic, improving energy efficiency and scalability of superconducting digital circuits. This review summarizes the physics of FJJs, the materials and existing devices, junction dynamics, and provides an overview of circuit design schemes that incorporate FJJs, offering practical insights for future applications. The authors discuss challenges in the implementation of these approaches, and highlight directions in device and circuit design.

DOI
10.1103/physrevapplied.23.067001

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Josephson junctions with ferromagnetic barriers (FJJs) enable self-biased operation and inductorless logic, improving energy efficiency and scalability of superconducting digital circuits. This review summarizes the physics of FJJs, the materials and existing devices, junction dynamics, and provides an overview of circuit design schemes that incorporate FJJs, offering practical insights for future applications. The authors discuss challenges in the implementation of these approaches, and highlight directions in device and circuit design.
We investigate a Magnetic Josephson Junction (MJJ) - a superconducting device with ferromagnetic barrier for a scalable high-density cryogenic memory compatible with energy-efficient single flux quantum (SFQ) circuits. The superconductor-insulator-superconductor-ferromagnet-superconductor (SIS'FS) MJJs are analyzed both experimentally and theoretically. We found that the properties of SIS'FS junctions fall into two distinct classes based on the thickness of S'layer. We fabricate Nb-Al/AlOx-Nb-PdFe-Nb SIS'FS MJJs using a co-processing approach with a combination of HYPRES and ISSP fabrication processes. The resultant SIS'FS structure with thin superconducting S'-layer is substantially affected by the ferromagnetic layer as a whole. We fabricate these type of junctions to reach the device compatibility with conventional SIS junctions used for superconducting SFQ electronics to ensure a seamless integration of MJJ-based circuits and SIS JJ-based ultra-fast digital SFQ circuits. We report experimental results for MJJs, demonstrating their applicability for superconducting memory and digital circuits. These MJJs exhibitIcRnproduct only ~ 30% lower than that of conventional SIS junctions co-produced in the same fabrication. Analytical calculations for these SIS'FS structures are in a good agreement with the experiment. We discuss application of MJJ devices for memory and programmable logic circuits.
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The lack of dense, fast, energy efficient memory has been the main detractor for multiple superconducting digital projects in the past. Recently, fundamental physics research in superconductor-ferromagnet thin-film tunnel structures created a new opportunity to solve this long-standing problem. Superconductivity and ferromagnetism, two deeply antagonistic electronic properties, can co-exist in form of Magnetic Josephson Junctions (MJJs). The superconducting-ferromagnetic MJJs are electrically compatible with traditional superconductor-insulator-superconductor (SIS) Josephson junctions (JJs) used for digital energy-efficient single flux quantum (eSFQ/ERSFQ) circuits. Both MJJ and JJ circuits have similar fabrication process and can be integrated on a single chip. As a result, a combination of MJJs and JJs can be used to form addressable memory cells, energy-efficient memory periphery circuits and programmable logic elements. In this paper, we present the test results of superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs showing their applicability for superconducting spintronic memory and digital circuits.
Superconducting digital circuits based on Josephson junctions with amorphous niobium-silicon (a-NbSi) barriers have been designed, fabricated, and tested. Single-flux-quantum (SFQ) shift registers operated with ±30% bias margins, confirming junction reproducibility and uniformity. Static digital dividers operated up to 165 GHz for a single value of bias current, which was only marginally slower than circuits fabricated with externally shunted AlOx-barrier junctions having a comparable critical current density of 4.5 kA/cm2. In comparison, self-shunted a-NbSi junctions enabled a doubling in circuit density. This and their relatively thick 10 nm barriers could increase the yield of complex SFQ circuits.
In superconducting spintronics, ferromagnetic \pi Josephson junctions are attractive for qubits, cryogenic memory, and phase shifters in logic circuits, offering reduced cell size and superior coherence and scalability. This study demonstrates such junctions based on niobium nitride, which can be grown epitaxially on MgO and is compatible with existing hardware. Its results provide physical insight into superconducting spintronics, as well as an interesting component for applications in quantum information processing.
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