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arXiv:2610.20268

Transport regimes in tunnel ferromagnetic Josephson junctions

R. Satariano, A. A. Golubov, E. Raja, R. Ferraiuolo, F. Calloni, H. G. Ahmad, A. Vettoliere, N. Poccia, L. Parlato, G. Ausanio, C. Granata, G. P. Pepe, M. Yu. Kupriyanov, F. Tafuri, D. Montemurro, D. Massarotti

Published Oct 14, 2026• 0 citations

Abstract

We present a systematic study of the temperature-dependent transport properties of Nb- and Al-based superconductor--insulator--thin superconductor--ferromagnet--superconductor (SIsFS) Josephson junctions. By varying the superconducting material and the thickness of the intermediate superconducting layer, we identify three distinct transport regimes: a tunnel-dominated SIS-like regime, a temperature-driven crossover, and a complex weak-link regime. A unified Usadel-based description connects these regimes through the self-consistent pair potential of the intermediate s-layer. While for Al-based devices an SIS-like transport regime is observed, a self-consistent calculation of the s/F/S proximity electrode, combined with perturbative tunnel coupling to the outer electrode, accounts for Nb devices with different thickness of the superconducting interlayer. The calculation places the 10-nm-interlayer sample in a proximity-supported, strongly suppressed s state, close to the SInFS limit in which the interlayer is effectively driven normal (\textit{n}). A reduced nonlinear long-junction description captures the same limiting physics. The evolution of I_cR_N(T) thereby provides a direct experimental signature of the three transport regimes. Beyond the fundamental interest, the results establish practical design guidelines: Nb junctions with sufficiently thick s interlayers are well suited for high-speed cryogenic memory elements and tunable superconducting parametric amplifiers, while Al junctions are natural candidates for coherent superconducting quantum circuits

DOI: 10.48550/arxiv.2610.20268

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The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor-ferromagnet-superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-$\ensuremath{\pi}$ transition in tunnel-ferromagnetic spin-filter JJs could also enhance the capabilities of SFS JJs as active elements. Here, we provide a self-consistent electrodynamic characterization of

\mathrm{Nb}\mathrm{N}

/

\mathrm{Gd}\mathrm{N}

/

\mathrm{Nb}\mathrm{N}

spin-filter JJs as a function of the barrier thickness, disentangling the high-frequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the

I

-

V

characteristics at 4.2 K and at 300 mK by using the tunnel-junction-microscopic model allows us to determine the subgap resistance

{R}_{\mathrm{sg}}

, the quality factor

\mathrm{Nb}\mathrm{N}

, and the junction capacitance

C

. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel-ferromagnetic JJs as active elements in quantum and classical circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.

We report experimental results on characteristics of SFIFS junctions and multiterminal SFIFSIS and SISFIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS (SFIS) junctions serve as injectors in the SFIFSIS (SISFIS) devices, which have transistorlike properties; for this reason, we call them superconducting-ferromagnetic transistors. We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic becomes linear. For three-terminal SFIFSIS devices, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For four-terminal SISFIS devices, we studied dependence of the transport current through the middle Nb electrode on injection current level through the SFIS junction. In both cases, we found that the output superconducting current (either Josephson or transport) can be efficiently modulated by the quasiparticle injection from SFIFS or SFIS input junction. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

Josephson elements are cornerstones of cryogenic classical and quantum superconducting technology, owing to their nonlinearity. Two important types of Josephson elements are often considered distinct: the tunnel junction (superconductor-insulator-superconductor, SIS) and the normal weak link (superconductor-normalsuperconductor, SNS) referring to any non-superconducting and non-insulating central region. SNS junctions and SIS junctions have appeared in related technological and basic science contexts over the last decade. In this perspective article, we review correspondences between SISIS junctions and SNS junctions in limiting regimes, in which a single, general energy-phase relationship describes the systems. We show how this insight helped to connect recent bodies of theoretical and experimental work in both systems. We conclude by describing a few important differences that also impact their use in applied contexts.