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IEEE Transactions on Magnetics

Abrikosov vortex memory based on a single vortex

Q. Li, Douglas K. Finnemore

Published Mar 1, 1991• 9 citations
Publisher

Abstract

An Abrikosov vortex memory device based on a cross strip superconducting-insulating-normal-superconducting (SINS) Josephson junction was developed to investigate the feasibility of using the motion of a single vortex to store information and perform logic. The write operation is performed by moving a single vortex from one pinning site to another using a current in one leg of the cross strip junction. The read operation is performed by measuring the Josephson voltage at a fixed bias Josephson current. For these PbBi-Al/sub 2/O/sub 3/-Al-Pb SINS junctions, the Josephson voltage depends on the vortex position because the vortex distorts the Fraunhofer diffraction pattern in a predictable way. The device is superior to earlier superconductor-normal-superconductor (SNS) devices in that the junction voltages are millivolts and can be read with rather simple electronics.

DOI
10.1109/20.133819

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An Abrikosov vortex memory device based on a cross strip superconducting-insulating-normal-superconducting (SINS) Josephson junction was developed to investigate the feasibility of using the motion of a single vortex to store information and perform logic. The write operation is performed by moving a single vortex from one pinning site to another using a current in one leg of the cross strip junction. The read operation is performed by measuring the Josephson voltage at a fixed bias Josephson current. For these PbBi-Al/sub 2/O/sub 3/-Al-Pb SINS junctions, the Josephson voltage depends on the vortex position because the vortex distorts the Fraunhofer diffraction pattern in a predictable way. The device is superior to earlier superconductor-normal-superconductor (SNS) devices in that the junction voltages are millivolts and can be read with rather simple electronics.
A new vortex transitional NDRO memory cell has been proposed for use as a high-speed cache memory. The memory cell consists of two superconducting loops with one Josephson junction and a two-junction interferometer gate. The superconducting loop stores single-flux quantum. The interferometer gate operates as a sense gate. The memory cell employs a vortex transition in the superconducting loop for the writing and reading of data. The sense gate current margin increases to ±38% since the coupling magnetic flux is amplified by the vortex transition for nominally designed cell parameters. The memory cell operates with a ±33% address signal current margin. This paper describes the memory cell design and the dynamic behavior of computer simulations.
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.
The possibility of a fast and precise Abrikosov vortex manipulation by a focused laser beam opens the way to create laser-driven Josephson junctions. We theoretically demonstrate that a vortex pinned in the vicinity of the Josephson junction generates an arbitrary ground state phase which can be equal not only to 0 or $\ensuremath{\pi}$ but to any desired ${\ensuremath{\varphi}}_{0}$ value in between. Such ${\ensuremath{\varphi}}_{0}$ junctions have many peculiar properties and may be effectively controlled by the optically driven Abrikosov vortex. Also we theoretically show that the Josephson junction with the embedded vortex can serve as an ultrafast memory cell operating at sub THz frequencies.
A proposal vortex transitional nondestructive read-out Josephson memory cell is successfully fabricated and tested. The memory cell consists of two superconducting loops in which a single flux quantum is stored and a two-junction interferometer gate as a sense gate. The memory cell employs vortex transitions in the superconducting loops for writing and reading data. The vortex transitional memory operation of the cell contributes to improving its sense discrimination and operating margin. The memory cell is activated by two control signals without timing control signals. Memory cell chips have been fabricated using a niobium planarization process. A±21% address signal current margin and a ±33% sense gate current margin have been obtained experimentally. Successful memory operations of a cell driven by two-junction interferometer gates has been demonstrated. The single flux quantum operations of this memory cell makes it an attractive basic element for a high-speed cache memory.
The design and experimental results for a novel type of Josephson memory cell are presented. The cell has a simpler construction than the conventional Henkels-type, using a one-junction SQUID (superconducting quantum interference device) capacitively coupled to a single junction. Binary data are stored as single flux quanta and are destructively read out by sensing a pulse caused by a vortex-mode transition. Computer simulations suggest that the read operation could be performed with a bias margin for the sense junction of +or-28% and that the rise time of the sense current in a 16-bit memory cell would be as fast as 30 ps, despite having to sense a vortex-mode transition. Results are presented for a memory cell fabricated with Nb-AlO/sub x/-Nb junctions.>
Nonhysteretic Josephson junctions with a superconductor-insulator-normal metal-insulator-superconductor (SINIS) structure fabricated using the reliable Nb-AlO/sub x/ technology have been used for the design and fabrication of a 1-V inherently stable Josephson voltage standard. The superconducting circuit can be driven at the same microwave frequencies as conventional SIS circuits. The microwave-induced 1-V step is suitable for calibration purposes. The successful operation of the 1-V circuit is due to an internal, phase-locked oscillation of all SINIS junctions which are embedded in a low-impedance strip-line. The microwave power required for exciting the coherent oscillation is very low and allows the integration of low power sources. Even operation without any external microwave source appears to be possible.