Cryogenic Memory Research Articles

Shunt-free cryogenic memory using ferromagnetic insulator-based Josephson junctions

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.

Electric field enhancement of the superconducting spin-valve effect via strain-transfer across a ferromagnetic/ferroelectric interface

In a ferromagnet/superconductor/ferromagnet (F/S/F) superconducting spin-valve (SSV), a change in the magnetization alignment of the two F layers modulates the critical temperature (Tc) of the S layer. The Tc-switching effect (the SSV effect) is based on the interplay between superconductivity and magnetism. The fast and large resistive switching associated with Tc-switching is suitable for nonvolatile cryogenic memory applications. However, the external magnetic field-based operation of SSVs is hindering their miniaturization, and therefore, electric field control of the SSV effect is desired. Here, we report epitaxial growth of a La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3 SSV on a piezo-electric [Pb(Mg0.33Nb0.67)O3]0.7-[PbTiO3]0.3 (001) substrate and demonstrate electric field control of the SSV effect. Electric field-induced strain-transfer from the piezo-electric substrate increases the magnetization and Tc of the SSV and leads to an enhancement of the magnitude of Tc-switching. The results are promising for the development of magnetic-field-free superconducting spintronic devices, in which the S/F interaction is not only sensitive to the magnetization alignment but also to an applied electric field.

Ferroelectric Hf0.5Zr0.5O2for Analog Memory and In‐Memory Computing Applications Down to Deep Cryogenic Temperatures

AbstractLow‐power nonvolatile memories operating down to deep cryogenic temperatures are important for a large spectrum of applications from high‐performance computing, electronics interfacing quantum computing hardware to space‐based electronics. Despite the potential of Hf0.5Zr0.5O2(HZO), thanks to its compatibility with complementary metal‐oxide‐semiconductor (CMOS) back‐end‐of‐line processing, only few studies of HZO‐based memory devices down to cryogenic operation temperatures exist. Here, analog ferroelectric memory stack fabrication with 10 nm HZO and their detailed characterization under wide range of pulse amplitudes and frequencies down to 4 K are reported. When operated at temperatures below 100 K, HZO devices can support high amplitude voltage pulses, yielding record highProf up to 75µC cm−2at ±7 Vp(14 Vpp) pulse amplitudes accompanied with frequency‐dependent memory window between 6 and 8 V. Devices show excellent endurance exceeding 109cycles of ±5 Vp(10 Vpp) andProf 30 µC cm−2without significant degradation of coercive voltages or loss of polarization at cryogenic temperatures. At least 20 reproducible analog states for temperatures below 100 K with almost ideal linearity of intermediate polarization states in both pulse directions is observed, demonstrating the high potential of analog cryogenic ferroelectric memories, essential for on‐line training in in‐memory‐computing architecture.

Demonstration of a Josephson vortex-based memory cell with microwave energy-efficient readout

The ongoing progress of superconducting logic systems with Josephson junctions as base elements requires the development of compatible cryogenic memory. Long enough junctions subject to magnetic field host quantum phase 2π-singularities—Josephson vortices. Here, we report the realization of the superconducting memory cell whose state is encoded by the number of present Josephson vortices. By integrating the junction into a coplanar resonator and by applying a microwave excitation well below the critical current, we are able to control the state of the system in an energy-efficient and non-destructive manner. The memory effect arises due to the presence of the natural edge barrier for Josephson vortices. The performance of the device is evaluated, and the routes for creating scalable cryogenic memories directly compatible with superconducting microwave technologies are discussed.

A Study of a Cryogenic CuAlMn Shape Memory Alloy

In extreme temperature environments, a newly emerging engineering application involves both the active and passive control of structures using cryogenic shape memory alloys, which are smart materials able to recover high deformation below the freezing point. With the objective of carrying out new advances in this area, the present work aims to investigate the Cu-7.5Al-13.5Mn (wt.%) shape memory alloy. Thermal, microstructural, and thermomechanical analyses of as-cast and hot-rolled specimens were performed, taking into account the effects of annealing and solubilization. It was observed that the phase transition occurs at temperatures below 120 K and changes according to the thermo-mechanical path. Specifically, hot-rolling lowers the phase transition temperature range with respect to the as-cast condition–from 34 K to 23 K for Mf, and from 89 K to 80 K for Af. Additionally, when the annealing temperature rises, the phase transformation temperature increases as well, and the alloy loses its cryogenic features when heat treated above 473 K. Finally, loss factors of 0.06 and 0.088, which were respectively found in dynamic and static settings, validate the material’s good damping response.

A nonvolatile memory element for integration with superconducting electronics

We demonstrate a nonvolatile cryogenic magnetic memory element needed to support emerging superconducting- and quantum-computing technologies. The central element is a switchable tri-layer thin film magnetic dot comprising two semiconducting ferromagnetic GdxSm1−xN layers separated by an exchange-blocking Al layer. The materials are explored for their tunable magnetic responses, the potential to engineer compensating magnetic moments in the anti-parallel tri-layers. The stability of the parallel and anti-parallel states and the reproducibility over repeated cycles are also demonstrated. We show that the tri-layer stacks can be formed into dots as small as 4 μm diameter, without affecting their magnetic behavior.

A review on future novel interconnect and polymeric materials for cryogenic memory packaging

A review on future novel interconnect and polymeric materials for cryogenic memory packaging

Word and bit line operation of a 1 × 1 μm2 superconducting vortex-based memory

The lack of dense random access memory is one of the main bottlenecks for the creation of a digital superconducting computer. In this work we study experimentally vortex-based superconducting memory cells. Three main results are obtained. First, we test scalability and demonstrate that the cells can be straightforwardly miniaturized to submicron sizes. Second, we emphasize the importance of conscious geometrical engineering. In the studied devices we introduce an asymmetric easy track for vortex motion and show that it enables a controllable manipulation of vortex states. Finally, we perform a detailed analysis of word and bit line operation of a 1 × 1 μm2 cell. High-endurance, non-volatile operation at zero magnetic field is reported. Remarkably, we observe that the combined word and bit line threshold current is significantly reduced compared to the bare word-line operation. This could greatly improve the selectivity of individual cell addressing in a multi-cell RAM. The achieved one square micron area is an important milestone and a significant step forward towards creation of a dense cryogenic memory.

Electrically controlled hybrid superconductor–ferromagnet cell for high density cryogenic memory

We report the fabrication and testing, at 4.2 K, of an S1IS2FS3 device, where S, F, and I denote a superconductor (Nb), a ferromagnetic material (Permalloy), and an insulator (AlOx), respectively. The F layer covers about one half of the top electrode of the S1IS2 Josephson junction and is positioned off-center. Electric current, Itr, along the S3 electrode can change the magnetization of the F layer in such a way that, for one direction of Itr, a magnetic flux penetrates the junction perpendicular to the layers, whereas for the opposite direction, the perpendicular magnetic flux can be removed. In the former state, the modulation pattern of the Josephson critical current, Ic, in the magnetic field, H, may acquire minimum near H = 0 and restores its usual shape with maximum in the second state. These states can be used for building a compact cryogenic memory compatible with single flux quantum electronics.

Cryogenic Storage Memory with High‐Speed, Low‐Power, and Long‐Retention Performance

AbstractCryogenic‐computing draws attention due to its variety of applications such as cloud‐computing, aerospace electronics, and quantum computing. Low temperature (e.g., 77 K) enables higher switching speed, improved reliability, and suppressed noise. Although cryogenic dynamic random‐access memory is studied, the cryogenic NAND flash is not explored intensively. Herein, a cryogenic storage memory based on the charge‐trap mechanism is reported. By removing the tunneling oxide from the conventional silicon/oxide/nitride/oxide/silicon (SONOS)‐type flash memory (therefore becoming silicon/oxide/nitride/silicon (SONS)), high‐speed and low‐power operation is aimed to be achieved while relieved from poor retention issue thanks to the cryogenic environment. The FinFET‐structured SONS memory device is demonstrated experimentally with gate length of 20–30 nm, which can achieve the retention issue (>10 years) with low voltage (≈6.5 V) and high speed (≈5 µs) operation at 77 K. To have a holistic system‐level evaluation, benchmark simulation of an interface between a host microprocessor and solid‐state‐drive is conducted, considering the refrigerator cooling cost and the heat loss via cables across two temperatures (300 and 77 K). The results show that the SONS‐type cryogenic storage system shows over 81% improvement in both latency and power, compared to the SONOS counterpart located at cryogenics.

Cryogenic memory technologies

The surging interest in quantum computing, space electronics, and superconducting circuits has led to new developments in cryogenic data storage technology. Quantum computers promise to far extend our processing capabilities and may allow solving currently intractable computational challenges. Even with the advent of the quantum computing era, ultra-fast and energy-efficient classical computing systems are still in high demand. One of the classical platforms that can achieve this dream combination is superconducting single flux quantum (SFQ) electronics. A major roadblock towards implementing scalable quantum computers and practical SFQ circuits is the lack of suitable and compatible cryogenic memory that can operate at 4 Kelvin (or lower) temperature. Cryogenic memory is also critically important in space-based applications. A multitude of device technologies have already been explored to find suitable candidates for cryogenic data storage. Here, we review the existing and emerging variants of cryogenic memory technologies. To ensure an organized discussion, we categorize the family of cryogenic memory platforms into three types: superconducting, non-superconducting, and hybrid. We scrutinize the challenges associated with these technologies and discuss their future prospects.

Electrical switching of a bistable moiré superconductor.

Electrical control of superconductivity is critical for nanoscale superconducting circuits including cryogenic memory elements1-4, superconducting field-effect transistors (FETs)5-7 and gate-tunable qubits8-10. Superconducting FETs operate through continuous tuning of carrier density, but no bistable superconducting FET, which could serve as a new type of cryogenic memory element, has been reported. Recently, gate hysteresis and resultant bistability in Bernal-stacked bilayer graphene aligned to its insulating hexagonal boron nitride gate dielectrics were discovered11,12. Here we report the observation of this same hysteresis in magic-angle twisted bilayer graphene (MATBG) with aligned boron nitride layers. This bistable behaviour coexists alongside the strongly correlated electron system of MATBG without disrupting its correlated insulator or superconducting states. This all-van der Waals platform enables configurable switching between different electronic states of this rich system. To illustrate this new approach, we demonstrate reproducible bistable switching between the superconducting, metallic and correlated insulator states of MATBG using gate voltage or electric displacement field. These experiments unlock the potential to broadly incorporate this new switchable moiré superconductor into highly tunable superconducting electronics.

Fowler–Nordheim Stress-Induced Degradation of Buried-Channel-Array Transistors in DRAM Cell for Cryogenic Memory Applications

The degradation of the fin-type buried-channel-array transistor (BCAT) in dynamic random access memory (DRAM) cell is investigated under Fowler–Nordheim stress at various temperatures, including 77 K. While the increase in the OFF current is dominated by the Shockley–Read–Hall junction leakage, the threshold voltage shift ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{T}$ </tex-math></inline-formula> ) comprises a positive component due to the interface trap generation and a negative component resulting from the electric field concentration-induced charge trapping into the gate insulator bulk, with respective activation energies of 0.22 and 0.13 eV. Furthermore, based on the consistency between the experimentally decomposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta } {V}_{T}$ </tex-math></inline-formula> components and the simulated ones, it is found that the anode hole injection dominates the degradations of the wordline-active operation and refresh time in cryogenic DRAM.

High-Quality Ferromagnetic Josephson Junctions Based on Aluminum Electrodes

Aluminum Josephson junctions are the building blocks for the realization of superconducting quantum bits. Attention has been also paid to hybrid ferromagnetic Josephson junctions, which allow switching between different magnetic states, making them interesting for applications such as cryogenic memories, single-photon detectors, and spintronics. In this paper, we report on the fabrication and characterization of high-quality ferromagnetic Josephson junctions based on aluminum technology. We employed an innovative fabrication process inspired by niobium-based technology, allowing us to obtain very high-quality hybrid aluminum Josephson junctions; thus, supporting the use of ferromagnetic Josephson junctions in advanced quantum circuits. The fabrication process is described in detail and the main DC transport properties at low temperatures (current-voltage characteristic, critical current as a function of the temperature, and the external magnetic field) are reported. Here, we illustrate in detail the fabrication process, as well as the main DC transport properties at low temperatures (current-voltage characteristic, critical current as a function of the temperature, and the external magnetic field).

Characterizing HfO2-Based Ferroelectric Tunnel Junction in Cryogenic Temperature

Since the discovery of ferroelectric properties in doped HfO2, various types of memory devices have emerged based on this novel material. Especially, the ferroelectric tunnel junction (FTJ) has gained considerable attention for selector-less crossbar array. Although various studies have been carried out for the HfO2-based FTJs, there are no investigations of them at a cryogenic temperature. While cryogenic memories are getting increasing attentions for diverse applications from high-performance computing to aerospace electronics, exploring the FTJ at cryogenic temperature is particularly interesting because it has been known for relatively poor retention characteristics and endurance at room temperature (RT). For the first time, we characterize the HfO2-based FTJ at cryogenic temperature down to 77 K. It was found that both the retention and endurance characteristics become greatly improved at 77 K. Furthermore, the read voltage was found to be critical for optimized performance in terms of ON-current, OFF-current, and their ratio at 77 K.

Nonvolatile memory cell using a superconducting-ferromagnetic π Josephson junction

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.

Sub-ns Switching and Cryogenic-Temperature Performance of Mo-Based Perpendicular Magnetic Tunnel Junctions

Mo-based perpendicular magnetic tunnel junctions (Mo-pMTJs) can outperform mainstream Ta-pMTJs in terms of perpendicular magnetic anisotropy (PMA) and thermal tolerance. However, studies on the ultrafast switching of Mo-pMTJ devices remain limited. In addition, although pMTJ devices have potential to function as cryogenic memory cells, there has been no report on the performance of Mo-pMTJs at low temperatures until now. In this Letter, Mo-pMTJs were prepared with strong PMA and patterned into nanoscale devices. Scanning transmission electron microscopy was employed to characterize the device lateral dimension and structure integrity. Systematic probability measurements were conducted under various pulse widths and current densities. On the ultrafast timescale down to sub-ns, the switching is confirmed to enter the precessional regime. The optimization of the switching energy is discussed. Moreover, we investigate the magneto-transport properties and switching of Mo-pMTJs at low temperatures down to 2 K. The feasibility of utilizing Mo-pMTJ devices in cryogenic memory is verified through this work.

Designing current-strain-assisted superconductor-ferromagnet multi-bit memories

Current superconducting memory devices lack the basic quality of high memory density for practical memories, mainly due to the size limitations of superconducting quantum interference devices. Here, we propose a superconductor–ferromagnet bilayer device with strain-pulse-assisted multi-bit ladder-type memory, by using strain-engineered ferromagnet domain structure to control carrier concentration in the superconductor, which is simulated by coupled Landau–Lifshitz–Gilbert and Ginzburg–Landau equations. Current- and strain-pulses are observed to deterministically control the resistivity of superconductor for one and two-bit device arrangements. The average carrier concentration of superconductor is observed to have multiple metastable states that can be controllably switched using current-pulse and strain-pulse to determine multiple resistivity states. These findings confirm the eligibility of superconductor–ferromagnet bilayers to be used as ladder-type multibit memories and open a new way for further theoretical and experimental investigations of the cryogenic memories.

Aluminum-ferromagnetic Josephson tunnel junctions for high quality magnetic switching devices

The competition between superconducting and ferromagnetic orderings in Josephson devices has promoted fundamental and applicative studies of high impact for superconducting digital technology, cryogenic memories, and spintronics, where the possibility of switching between different magnetic states is a crucial advantage. Here, we report on fabrication and characterization of very high quality tunnel ferromagnetic Josephson junctions (JJs) with aluminum electrodes, demonstrating hysteretic behavior of the magnetic field pattern preserving typical transport properties of Al junctions, underdamped behavior, and very low quasiparticle dissipation. The proposed technology paves the way for the possible implementation of Al tunnel-ferromagnetic JJs in superconducting quantum circuits, toward alternative approaches based on digital control of the Josephson device.

In Situ Observation of Thermoelastic Martensitic Transformation of Cu-Al-Mn Cryogenic Shape Memory Alloy with Compressive Stress.

The thermoelastic martensitic transformation and its reverse transformation of the Cu-Al-Mn cryogenic shape memory alloy, both with and without compressive stress, has been dynamically in situ observed. During the process of thermoelastic martensitic transformation, martensite nucleates and gradually grow up as they cool, and shrink to disappearance as they heat. The order of martensite disappearance is just opposite to that of their formation. Observations of the self-accommodation of martensite variants, which were carried out by using a low temperature metallographic in situ observation apparatus, showed that the variants could interact with each other. The results of in situ synchrotron radiation X-ray and metallographic observation also suggested there were some residual austenites, even if the temperature was below Mf, which means the martensitic transformation could not be 100% accomplished. The external compressive stress would promote the preferential formation of martensite with some orientation, and also hinder the formation of martensite with other nonequivalent directions. The possible mechanism of the martensitic reverse transformation is discussed.