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Physics Procedia

Magnetic Josephson Junction Technology for Digital and Memory Applications

V. V. Ryazanov, V. V. Bol’ginov, Danila S. Sobanin, I. V. Vernik, Sergey K. Tolpygo, Alan M. Kadin, Oleg A. Mukhanov

Published Jan 1, 2012• 120 citations
Publisher

Abstract

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.

DOI
10.1016/j.phpro.2012.06.126

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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.
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.
We develop a fast magnetic Josephson junction (MJJ)—a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient single flux quantum (SFQ) circuits. We present experimental results for superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs with high characteristic voltage IcRn of >700 μV proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ IcRn product is only ∼30% lower than that of conventional junction co-produced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.
We discuss the capabilities of ferromagnetic (F) Josephson junctions (JJs) in a variety of layouts and configurations. The main goal is to demonstrate the potential of these hybrid JJs to disclose new physics and the possibility to integrate them in superconducting classical and quantum electronics for various applications. The feasible path towards the use of ferromagnetic Josephson junctions in quantum circuits starts from experiments demonstrating macroscopic quantum tunneling in NbN/GdN/NbN junctions with ferro-insulator barriers and with triplet components of the supercurrent, supported by a self-consistent electrodynamic characterization as a function of the barrier thickness. This has inspired further studies on tunnel ferromagnetic junctions with a different layout and promoted the first generation of ferromagnetic Al-based JJs, specifically Al/AlOx/Al/Py/Al. This layout takes advantage of the capability to integrate the ferromagnetic layer in the junction without affecting the quality of the superconducting electrodes and of the tunnel barrier. The high quality of the devices paves the way for the possible implementation of Al tunnel-ferromagnetic JJs in superconducting quantum circuits. These achievements have promoted the notion of a novel type of qubit incorporating ferromagnetic JJs. This qubit is based on a transmon design featuring a tunnel JJ in parallel with a ferromagnetic JJ inside a SQUID loop capacitively coupled to a superconducting readout resonator. The effect of an external RF field on the magnetic switching processes of ferromagnetic JJs has been also investigated.
Magnetic flux quantization in superconductors allows the implementation of fast and energy-efficient digital superconducting circuits. However, the information representation in magnetic flux severely limits their functional density presenting a long-standing problem. Here we introduce a concept of superconducting digital circuits that do not utilize magnetic flux and have no inductors. We argue that neither the use of geometric nor kinetic inductance is promising for the deep scaling of superconducting circuits. The key idea of our approach is the utilization of bistable Josephson structures allowing the presentation of information in Josephson energy. Since the proposed circuits functionally rely on Josephson junctions only, they can be called all-Josephson junction (all-JJ) circuits. We consider the design methodology of the all-JJ circuits utilizing recently developed 2ϕ2\phi2ϕ Josephson junctions for simplicity, though the bistability can be obtained with the other types of magnetic junctions like ϕ\phiϕ- or 0−π0-\pi0−π, or by using combinations of conventional 0- and π\piπ-junctions. Besides some broadly used basic cells, we present an all-JJ 8-bit parallel adder confirming the applicability of the proposed approach to the design of complex logic circuits.
This review is devoted to a discussion of the prospects for solving the problem of a low degree of integration of the traditional elements for promising (due to the high performance and extremely low energy dissipation) superconducting digital electronics. We define three main directions on the path to compact multi-element Josephson electronic systems: (1) reduction of the Josephson junction to submicron size, (2) decrease of the area of standard logic cells, and (3) fabrication of a compact and rapid Josephson memory. We present the physical foundations of Josephson elements in order to show the fundamental constraints on establishing standard submicron tunnel contacts and compact logic cells/memory elements. This survey clearly demonstrates the essence of breakthrough technological solutions to create ultrasmall heterostructures with desired settings, reduce and optimize logic cells, and create memory unit cells based on Josephson junctions with magnetic layers.
Abstract Storage of a single magnetic flux quantum in a superconducting loop containing a Josephson junction represents a promising unit cell configuration for construction of a cryogenic memory of superconducting digital circuits. However, application of a DC bias current is required for operation of such a memory cell to maintain trapping of the flux quantum in the storage loop. In this work, we present a superconducting memory cell that uses a superconducting-magnetic π junction. The cell characteristics show flux quantum hysteresis centering at the zero-bias current. We develop a fabrication process that combines superconductor–ferromagnet–superconductor (SFS) junctions with superconductor–normal metal–superconductor (SNS) junctions. The critical current density of the SFS junctions shows a 0–π oscillation as a function of the ferromagnetic layer thickness. The formation of the π junction is confirmed further by the flux modulation curves of a superconducting quantum interference device made from SNS junctions with an additional SFS junction.