Ferromagnetic Josephson switching device with high characteristic voltage

Abstract

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 ...

Integrating atomic layer deposition and ultra-high vacuum physical vapor deposition for in situ fabrication of tunnel junctions

“…With another sample transportation system attached to the second sputtering chamber, a sample can be transported between different chambers for fabrication of JJs, MTJs, and more complicated devices such as magnetic Josephson junctions. 27,28 This means the current design of the ALD-UHV sputtering system is very versatile and a cluster of UHV chambers may be integrated to this system for fabrication of multiple functional materials.…”

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

“…For standard SFS junctions, τ switch is much too long to be useful for memory applications. One can shorten τ switch somewhat by increasing R N via the introduction of an insulating barrier to make an SIFS junction 6,21 . Suppression of I c by the insulating layer can be mitigated by adding a thin auxiliary nearly-superconducting (s) layer, to form an SIsFS junction with very large values of I c R N 26-29 .…”

“…Numerous ideas have been presented in the literature regarding how SFS junctions might be used as practical memory devices [19][20][21][22][23][24][25] . The ferromagnetic (F) layer influences the properties of the junction both through the magnetic field and the exchange field it generates, and ideas have been presented using either of those mechanisms.…”

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

“…A single S/F/S Josephson junction with controllable critical current amplitude could function as a superconducting memory cell, but one must find a way to address such a memory cell when it is embedded in a large memory array, and the speed at which the junction switches into the voltage state after the 'read' current is applied is limited by the small I c R N product of the junction. (S/F/S Josephson junctions with larger I c R N product have been demonstrated 24 , but not with two ferromagnetic layers in a spin-valve configuration.) A solution to both problems is a memory cell based on a SQUID with a phase-controllable Josephson junction 11 , such as the one shown schematically in Fig.…”