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 I c R n of >700 V proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ I c R n product is only ~30% lower than that of conventional junction coproduced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.High speed, low power superconducting Rapid Single Flux Quantum (RSFQ) digital circuits have already found their applications in Digital-RF systems impacting communications and signal intelligence applications [1,2]. Recently, a new energy-efficient generation of RSFQ circuits, eSFQ and ERSFQ logics, offered a way to overcome the low energy efficiency of conventional technologies for the next generation of supercomputers [3]. However, the practical applications of these superconducting digital technologies will inevitably be very limited without compatible in speed and signal levels, high-capacity, energy-efficient Random Access Memory (RAM). The largest superconducting RAM demonstrated to date, a 4 kbit RAM [4], is insufficient for practical applications and hardly compatible with SFQ-type circuits.The low density of superconducting memory is directly related to a relatively large size of memory cells based on SFQ storage loops coupled to address lines via transformers which are difficult to scale [5][6][7][8]. The required ac power posed an additional implementation problem for achieving larger capacity RAM integrated circuits [8]. In order to get around the low capacity of superconductor RAMs, hybrid superconductor-semiconductor schemes were pursued [9,10]. However, these approaches can address only limited applications and cannot satisfy the need for a fast, energy efficient memory in a close proximity to the digital circuits, preferably on the same chip. Alternatively, combining superconducting elements with ferromagnetic layers and dots was suggested to achieve higher density of superconducting memory [11,12]. However, these ideas did not go beyond initial concepts nor address compatibility with SFQ circuits.Recently, we have introduced a memory cell based on Magnetic Josephson Junction (MJJ), a Josephson switching device with ferromagnetic (F) layer(s). The MJJ critical current can change and retain its value by ferromagnet magnetization, so that a memory element size is defined by the scalable small MJJ device [13]. With achieving MJJ switching speed comparable to that of conventional JJs, both types of junctions can be integrated into a single circuit operating in an SFQ non-hysteretic switching regime, enabling a low power and high speed memory operation. Since such ...
We investigate a Magnetic Josephson Junction (MJJ) -a superconducting device with ferromagnetic barrier for a scalable high-density cryogenic memory compatible with energyefficient single flux quantum (SFQ) circuits. The superconductorinsulator-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 exhibit I c R n product 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.Index Terms-Energy efficient, memory devices, proximity effect, RAM, single flux quantum, superconducting logic elements.
Motivated by recent progress in development of cryogenic memory compatible with single flux quantum (SFQ) circuits we have performed a theoretical study of magnetic SIsFS Josephson junctions, where 'S' is a bulk superconductor, 's' is a thin superconducting film, 'F' is a metallic ferromagnet and 'I' is an insulator. We calculate the Josephson current as a function of s and F layers thickness, temperature and exchange energy of F film. We outline several modes of operation of these junctions and demonstrate their unique ability to have large product of a critical current I C and a normal-state resistance R N in the π state, comparable to that in SIS tunnel junctions commonly used in SFQ circuits. We develop a model describing switching of the Josephson critical current in these devices by external magnetic field. The results are in good agreement with the experimental data for Nb-Al/AlO x -Nb-Pd 0.99 Fe 0.01 -Nb junctions.
Digital superconductor electronics has been experiencing rapid maturation with the emergence of smaller-scale, lower-cost communications applications which became the major technology drivers. These applications are primarily in the area of wireless communications, radar, and surveillance as well as in imaging and sensor systems. In these areas, the fundamental advantages of superconductivity translate into system benefits through novel Digital-RF architectures with direct digitization of wide band, high frequency radio frequency (RF) signals. At the same time the availability of relatively small 4 K cryocoolers has lowered the foremost market barrier for cryogenically-cooled digital electronic systems. Recently, we have achieved a major breakthrough in the development, demonstration, and successful delivery of the cryocooled superconductor digital-RF receivers directly digitizing signals in a broad range from kilohertz to gigahertz. These essentially hybrid-technology systems combine a variety of superconductor and semiconductor technologies packaged with two-stage commercial cryocoolers: cryogenic Nb mixed-signal and digital circuits based on Rapid Single Flux Quantum (RSFQ) technology, room-temperature amplifiers, FPGA processing and control circuitry. The demonstrated cryocooled digital-RF systems are the world's first and fastest directly digitizing receivers operating with live satellite signals in X-band and performing signal acquisition in HF to L-band at ∼30 GHz clock frequencies.
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