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Bulletin of the American Physical Society

Cryogenic Memories based on Spin-Singlet and Spin-Triplet Ferromagnetic Josephson Junctions

Eric C. Gingrich

Published Mar 15, 2016• 0 citations
OpenAlex

Abstract

The last several decades have seen an explosion in the use and size of computers for scientific applications. The US Department of Energy has set an ExaScale computing goal for high performance computing that is projected to be unattainable by current CMOS computing designs [1]. This has led to a renewed interest in superconducting computing as a means of beating these projections. One of the primary requirements of this thrust is the development of an efficient cryogenic memory. Estimates of power consumption of early Rapid Single Flux Quantum (RSFQ) memory designs are on the order of MW, far too steep for any real application [1]. Therefore, other memory concepts are required. S/F/S Josephson Junctions, a class of device in which two superconductors (S) are separated by one or more ferromagnetic layers (F) has shown promise as a memory element. Several different systems have been proposed utilizing either the spin-singlet or spin-triplet superconducting states [2]. This talk will discuss the concepts underpinning these devices, and the recent work done to demonstrate their feasibility. [1] Energy-Efficient Superconducting Computing Power Budgets and Requirements, D. Scott Holmes, Andrew L. Ripple, Marc A. Manheimer, IEEE Trans. Appl. Supercon., Vol 23, No. 3, June 2013 [2] B. M. Niedzielski et al., “Use of Pd-Fe and Ni-Fe-Nb as soft magnetic layers in ferromagnetic Josephson junctions for nonvolatile cryogenic memory,” IEEE Trans. Appl. Supercond., vol. 24, no. 4, Aug. 2014, Art. ID. 1800307.

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The last several decades have seen an explosion in the use and size of computers for scientific applications. The US Department of Energy has set an ExaScale computing goal for high performance computing that is projected to be unattainable by current CMOS computing designs [1]. This has led to a renewed interest in superconducting computing as a means of beating these projections. One of the primary requirements of this thrust is the development of an efficient cryogenic memory. Estimates of power consumption of early Rapid Single Flux Quantum (RSFQ) memory designs are on the order of MW, far too steep for any real application [1]. Therefore, other memory concepts are required. S/F/S Josephson Junctions, a class of device in which two superconductors (S) are separated by one or more ferromagnetic layers (F) has shown promise as a memory element. Several different systems have been proposed utilizing either the spin-singlet or spin-triplet superconducting states [2]. This talk will discuss the concepts underpinning these devices, and the recent work done to demonstrate their feasibility. [1] Energy-Efficient Superconducting Computing Power Budgets and Requirements, D. Scott Holmes, Andrew L. Ripple, Marc A. Manheimer, IEEE Trans. Appl. Supercon., Vol 23, No. 3, June 2013 [2] B. M. Niedzielski et al., “Use of Pd-Fe and Ni-Fe-Nb as soft magnetic layers in ferromagnetic Josephson junctions for nonvolatile cryogenic memory,” IEEE Trans. Appl. Supercond., vol. 24, no. 4, Aug. 2014, Art. ID. 1800307.
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 review the physical basis of and recent developments in ferromagnetic Josephson junctions with a focus on the π state, which is useful for applications to superconducting devices. Among various novel phenomena that emerge from the interaction between the superconductor and ferromagnet, the π state (junction) works as a phase shifter in superconducting devices, and several proposals and demonstrations of applications to quantum computing elements (qubits), classical superconducting logic circuits, and cryogenic memory have been presented. As a new research trend in superconducting spintronics, we also introduce recent interesting works on the long-range Josephson effect caused by spin-triplet Cooper pairs.
Magnetic Josephson junctions are the preferred candidate devices for designing fast and scalable cryogenic memory elements. This is especially the case for rapid single-flux quantum-based superconducting electronics, where the speed mismatch between logic and memory elements have remained a long-standing challenge. In this Letter, we demonstrate a simple tri-layer Josephson memory device using ferromagnetic insulating (FI) GdN-based S/FI/S vertical mesa-type junctions, with reliable nonvolatile memory operation without the need of a shunt resistor at 4.2 K. The characteristic frequency of our devices is approximately 90 GHz, corresponding to an IcRn product of 177 μV. We demonstrate a thorough study of the parameter spaces required for designing these devices and identify the scope for future improvements that can lead to further miniaturization and higher operating speed of these devices.
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.
The quantum behavior of Josephson junctions is often exploited to produce superconducting devices of outstanding performance. With respect to information technology applications, Josephson junctions can be used in circuits that perform logic operations in picoseconds, which may result in energy-efficient, high-performance cryogenic computers, provided that memory elements can be developed that can be switched between two stable states by integrated superconducting logic circuits. Here, we show that Josephson junctions based on pseudo-spin-valve barriers could enable such memory elements, by unambiguously differentiating the controlled changes in Josephson coupling from size-dependent remanent-field effects. These changes can be very large and even reversed in magnitude, which is strong evidence of the exchange-field effect on the Cooper pair phase. These devices are the first that demonstrate nonvolatile, size-independent switching of the Josephson coupling in magnitude or phase, and they may allow for the first scalable superconducting memory devices.
Cryogenic (cryo) memory blocks are envisioned to be crucial components for - (i) scalable quantum computing systems with> 1000 qubits [1], and (ii) superconducting (SC) single flux quantum (SFQ) systems [2]. Decades of research has led to a whole host of cryo-memory variants (SC, non-SC, and hybrid), and more are being actively explored. Despite having such massive interest in the search for cryo-storage solutions, the design possibilities for compatible sense amplifiers (SA) remain largely unexplored. The existing approaches for cryogenic sensing are mostly agnostic to the unique properties of the memory array [3]–[5]. For example, the recently proposed SC and twistronic memories promise seamless compatibility with the cryogenic processors in terms of speed, power, and temperature range [4]–[7]. However, they rely on current-driven read operation, and necessitate voltage-based sensing. Therefore, a current-based cryo-SA [3] cannot be used for these emerging cryo-memory variants. In this work, we propose a voltage sense amplifier (VSA) for cryogenic memories utilizing the Josephson junction field effect transistor (JJFET) [8] as the primary component, and the heater cryotron (hTron) [4], [9] as an auxiliary entity. We exploit their coupled interactions to sense ultra-low (< 1 mV) voltage difference (customary for many cryo-memory technologies [4]–[7]) between the binary memory states (0/1) at/below 4 Kelvin temperature.