RU2554612C2 - High-frequency superconducting memory element - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: high-frequency superconducting memory element having planar, end face or bridge geometry consists of two superconducting electrodes and a region of weak interconnection, which includes magnetic layers with direct, tunnel or resonant conductivity and a superconducting layer between them. Difference from previously known Josephson SFS structures consists in the fact that at variation of direction of magnetisation of one of the magnetic layers in a superconducting film localised in the region of weak interconnection between magnetic layers a phase transition from a normal state to a superconducting state or from a superconducting state to a normal state occurs.

EFFECT: increase of variation of an amplitude of critical transition current under action of a low magnetic flux in comparison to previous geometries, which provides possibilities for miniaturisation of superconducting memory elements, as well as a possibility of rather high specific feature of a Josephson heterostructure, and as a result, rather high quick action of a memory element on its basis.

16 cl, 6 dwg

Description

The invention relates to cryogenic electronics, and in particular to elements of fast cryogenic memory, and can be used in measurement technology, radio engineering and information systems operating at low temperatures.

Today, devices based on the Josephson effect are characterized by very high speed: the characteristic frequency F _{c of} Josephson elements based on low-temperature superconductors lies in the range from tens to hundreds of gigahertz, and when using high-temperature superconductors, it can reach units or even tens of terahertz. High speed, high sensitivity, extremely low switching energy of Josephson elements E _J = 2 · 10 ^-15 VB · 10 ^-4 A ≈ 10 ^-18 J opens up great prospects for the development of analog and digital devices that can operate in higher frequency ranges of signals , provide higher information processing speeds compared to traditional semiconductor electronics. The traditionally known Josephson heterostructures (DGS) are a multilayer thin-film structure formed on a dielectric substrate, including superconductor layers, insulating, barrier, and functional layers. Depending on the purpose and design, the choice of substrate materials and the active media themselves is carried out.

Progress in this area is severely limited by the lack of high-speed, energy-efficient superconducting memory that is appropriate for speed and signal level. To date, the record demonstrated capacity of a random access superconductor memory device is only 4 kbps, which is a completely insufficient indicator for practical applications. Due to the lack of suitable superconducting memory in modern superconducting digital systems, hybrid circuits are used that contain both superconducting and semiconductor / magnetoresistive components (US 0244958, Bulzacchelli J., 01.10.2009). However, the use of this approach significantly reduces performance and allows you to solve only a limited range of tasks.

To enable integration of Josephson memory on magnetic Josephson contacts with ultrafast digital superconductor electronics circuits, the structures under study must satisfy two requirements:

1) allow the possibility of rapid magnetization reversal of the ferromagnetic layer included in the Josephson junction, a small magnetic field to ensure the operation "Record" and

2) have a short switching time of the Josephson junction (high characteristic frequency) for the implementation of the operation "Read".

There are various technical solutions aimed at creating elements and systems of Josephson memory. In a special group, it is necessary to highlight the solutions found and patented at the first stages of the development of fast single-quantum logic (BOC logic): in BOC schemes, the magnetic flux quantum is an information carrier, and, therefore, in the first superconducting memory devices, the superconducting quantum interferometer was an elementary cell ( Squid), and the logical “zero” / “unit” in the memory cell corresponded to the absence / presence of circular current in a sufficiently large inductance of the squid (JP 3194796, Suzuki, 12/22/1989; US 5260264, Kurosawa, 03/26/1990). Based on various squids, rather complex superconducting systems with memory were created, including:

- three-terminal cells for cryogenic computers (US 5365476, Mukhanov O.A., 02.26.1993);

- Josephson memory cells with several stable states (JP 11191294, Chan, 10.10.1997);

- superconducting memory elements with built-in error protection (US 5629889, Chandra P., 12/14/1995).

I would especially like to note the analogues of memory devices with random access to cells created by this approach (US 0255987, Nagasawa S., 11.16.2006). However, even the most sophisticated circuitry solutions could not compensate for the main drawback of a squid-based memory cell - its size is extremely large by the standards of modern electronics.

A device based on the Josephson junction (JP 3190175, Yuzurihara et al., 08/20/1991), which is a device with four current leads, in which the current set through one of the pairs of current leads, transfers the film of antiferromagnetic material inside the device to the ferromagnetic state, is known. not in the area of Josephson contact. The magnetic moment arising in this case creates a magnetic field, which leads to the suppression of the critical current of the Josephson element located between two other current leads of the device and to the generation of a voltage pulse on it.

From the beginning of the 2000s, work began on the development of compact superconducting memory elements in which applied fields and currents controlled the properties of current transport through a heterostructure with superconducting layers (US 6,233,171 B1, Youm and Beasley, 05/15/2001).

A current valve is described for controlling the flow of electrons, having a multilayer superconductor-normal metal-superconductor structure and not using dielectric barrier layers (US 6995390, Tsukui, 02/07/2006). Another invention describes a design that allows controlling the critical current of five-layer two-barrier Josephson junctions in which the material inside the barriers contains a ferromagnetic film that provides Zeeman splitting of the resonant levels of electrons in the intra-barrier region. This is necessary to control the critical current of the structure by controlling the position of the split levels relative to the Fermi energy of the electrodes by the voltage applied to the additional control contacts of the structure (US 6344659, Ivanov et al., 02/05/2002).

A device is known at the Josephson junction (US 20090233798 A1, Maeda, September 17, 2009), which consists of a superconducting layer placed on a substrate, on top of which a layer of ferromagnetic material (which can be electrically conductive) is applied, separated from the superconductor by an insulator barrier. However, all of these devices are characterized by a small scale of penetration of superconducting correlations into the magnetic region (or even the absence thereof) and there is no possibility of effective control of the critical current through a weak external magnetic field, which is a critical disadvantage for any promising prototype of the Josephson memory element.

A superconducting spin valve is known, including a superconducting film placed on a substrate and placed on it through an insulator layer to control the critical temperature Te of the superconducting film, consisting of two layers of ferromagnetic (F) materials separated by a layer of non-ferromagnetic material (US 6414870 B1, Johnson et al. July 2, 2002). To realize the valve, it is necessary to ensure large values of magnetic moments in magnitude in ferromagnetic films, such that the magnetic field created by the films can significantly change the characteristics of the superconducting film. Accordingly, to control the flowing current, it is necessary to create large magnetic fields of magnetization reversal, which is a significant drawback of this solution.

Recently Josephson junctions and spin gates have been known (RU 2343591, Karminskaya et al., 10.01.2009; RU 2439749, Karminskaya et al., 10.25.2010; RU 2442245, Karminskaya et al., 17.11.2010), in which a weakly bonded region is formed in the substrate in the form of a multilayer thin-film FNF structure, connected in a certain way with electrodes from superconductor S. The layers of the ferromagnetic material are configured to rotate the magnetization vectors relative to each other in the plane of the layered structure from the antiferromagnetic to the ferromagnetic state. In such magnetic Josephson junctions / spin gates, it is possible to more effectively control the critical current / critical temperature by means of an external magnetic field due to the organization of a number of independent current transfer channels, however, the characteristic voltage and, as a result, the characteristic Josephson frequency are too small for effective use as memory cells .

Known Josephson gate based on the structures of a ferromagnet-superconductor-insulator-ferromagnet-superconductor (F ₁ SF'SF ₂ ) using a weak ferromagnet PdFe as the F'-layer (US 20120302446 A3, Ryazanov et al., 01.26.2012). In such a gate, one can expect the appearance of a triplet component of the overcurrent weakly suppressed by the exchange magnetic field in the heterostructure, and the critical current of such a device can be controlled by changing the mutual orientation of the magnetization vectors in the layers F ₁ and F ₂ . The disadvantage of this structure will be a low characteristic voltage, as well as a small change in the critical current with a change in the mutual orientation of the magnetization vectors in the layers F ₁ and F ₂ .

The known heterostructure is a superconductor-insulator-ferromagnet-superconductor (SIFS) using soft magnetic materials (WO 2012103384, Ryazanov et al., 01/26/2011, closest analogue). Here, the application of relatively small fields allows one to noticeably change the value of the critical transition current. Memory cells based on such a magnetic Josephson SIFS junction (US 0184445 A1, Mukhanov et al., July 19, 2012), as well as memory systems with random access to cells (US 8270209, Herr et al., 18.06.2012) are proposed and patented. The use of an insulating layer increases the resistance of the Josephson junction R _N , however, the payoff for this is a decrease in the critical current I _C , and as a result, the characteristic frequency of the element ~ I _C R _N remains relatively small (1 ... 2 GHz). Small values of characteristic frequencies significantly limit the possibilities of using the patented SIFS structures as elements of fast Josephson memory. In addition, for a significant change in the critical current, it is necessary that during the “Recording” the effective magnetic flux through the weak coupling region of the Josephson structure is sufficiently large (close to one quantum of magnetic flux Φ ₀ = h / 2e, h is the Planck constant, e is elementary charge), which prevents the geometric dimensions of the SIFS junction from being reduced.

The patented invention is devoid of these drawbacks and allows switching between states with significantly different critical current values I _{C of the} Josephson heterostructure (DGS) with a high characteristic frequency.

A high-frequency superconducting memory element made in planar, end, or bridge geometry consists of two superconducting electrodes and a weak-coupling region, including magnetic layers with direct, tunneling, or resonant conductivity and a superconducting layer between them.

The difference from previously known Josephson SFS structures is that when the direction of magnetization of one of the magnetic layers in a superconducting film is changed, localized in the region of weak coupling between the magnetic layers, there is a significant restoration (suppression) of superconductivity in this film, which may be accompanied by and a phase transition from a normal state to a superconducting state or from a superconducting state to a normal state. This leads, respectively, to the formation or destruction, instead of one or two series-connected Josephson junctions, which is accompanied, respectively, by a sharp increase or decrease in the total critical current.

The device can be characterized in that in planar geometry the superconductor layer is deposited on the substrate, and the first magnetic layer, the intermediate superconductor layer, the second magnetic layer and the second superconductor layer are deposited on top of it in this sequence, while the areas of the said layers coincide.

The device can be characterized in that in the end geometry, the superconductor layer and the first magnetic layer are deposited on the substrate, and the intermediate superconductor layer, the second magnetic layer and the second superconductor layer are deposited on top of the layers in the indicated sequence, with the intermediate superconductor film, the second magnetic layer and the second a superconductor layer is deposited at the interface between the first layer of the superconductor and the first magnetic layer.

The device can be characterized in that in bridge geometry, the first magnetic layer is deposited on the substrate, and the intermediate layer of the superconductor and the second magnetic layer are deposited on top of it, while the areas of the said layers coincide. A superconductor layer is deposited on top of the second magnetic layer, in which a gap filled with an insulator is created.

The device can be characterized by the fact that in the bridge geometry under the first layer of magnetic material directly on the substrate there can be an auxiliary layer of an antiferromagnet.

The device can be characterized by the fact that in bridging geometry, a part of the second layer of magnetic material located directly below the superconducting electrodes is replaced by an insulator or normal metal.

The device can be characterized by the fact that niobium or an alloy based on it is used as a superconductor, an element from the group Cu, Au, Al, Pt is used as a normal metal, and a soft magnetic ferromagnet is used as the material of the magnetic layer, for example, Pd _x alloys Fe _1-x , Pd _x Ni _1-x , Pd _x Co _1-x , Pt _x Fe _1-x , Pt _x Ni _1-x , Pt _x Co _1-x with a ferromagnetic content of less than 10 atomic percent.

The device can also be characterized in that dielectric layers, for example, amorphous silicon, doped with either only magnetic atoms, Fe, Co, Ni, or atoms of magnetic and non-magnetic, Nb, W metals, are used as magnetic layers.

The device can also be characterized by the fact that layers of a magnetically soft ferromagnet and dielectric layers located directly below them are used as magnetic layers.

The device can be characterized in that the magnetization vectors in the magnetic layers are directed parallel to the plane of the layers and can be deployed by an external magnetic field.

The technical result of the invention consists in increasing the change in the amplitude of the critical transition current under the action of a small magnetic flux compared with previous geometries, which opens up opportunities for miniaturization of superconducting memory elements. An additional technical result of the invention is the ability to provide a sufficiently high characteristic frequency of the Josephson heterostructure, and, as a consequence, a sufficiently high speed of the memory element based on it.

The invention is illustrated by the following drawings.

Figure 1 shows the structure of a GVD with two magnetic and one intermediate superconducting layer between superconducting electrodes in a planar topology;

Figure 2 shows the structure of a DGS with two magnetic and one intermediate superconducting layer between the superconducting electrodes in the end topology;

Figure 3 shows the structure of a DGS made in the form of a bridge of variable thickness with two magnetic layers and one intermediate superconducting layer;

Figure 4 shows the structure of a DGS made in the form of a bridge of variable thickness with two magnetic layers and one intermediate superconducting layer, with the part of the second layer of magnetic material located directly below the superconducting electrodes being replaced by an insulator or normal metal;

Figure 5 shows the temperature dependence of the normalized values of the critical currents of patented structures for parallel and antiparallel magnetizations of the magnetic layers;

Figure 6 shows the normalized values of the critical currents of patented structures as a function of the thickness of the intermediate superconducting layer.

The Josephson heterostructure for the high-frequency superconducting memory element, made in planar geometry (see Fig. 1), is formed on the substrate 1 and contains a multilayer thin-film structure consisting of the first superconducting layer of the electrode S 21; a first layer 31 of magnetic material IF of thickness d _F1 , an intermediate layer 4 of superconducting material s of thickness d _s ; a second layer 32 of magnetic material IF of thickness d _F2 ; a second superconducting electrode S 22 with current leads 51 and 52 for connecting a weak coupling region to a current source.

The Josephson heterostructure for the high-frequency superconducting memory element, made in the end geometry (see Fig. 2), is formed on the substrate 1 and contains a multilayer thin-film structure consisting of the first superconducting layer of the electrode S 21; a first layer 31 of magnetic material IF; an intermediate layer 4 of superconducting material s; second layer 32 of magnetic material IF; a second superconducting electrode S 22 with current leads 51 and 52 for connecting a weak coupling region to a current source.

The Josephson heterostructure for a high-frequency superconducting memory element made in bridge geometry (see FIG. 3) is formed on a substrate 1 and contains a multilayer thin-film structure consisting of a first layer 31 of magnetic material IF of thickness d _F1 ; an intermediate layer 4 of superconducting material s of thickness d _s , second layer 32 of magnetic material IF of thickness d _F2 , and two 21 and 22 superconducting electrodes S with current leads 51 and 52 for connecting the weak coupling region to the current source.

The Josephson heterostructure for a high-frequency superconducting memory element made in bridge geometry (see Fig. 4) is formed on a substrate 1 and contains a multilayer thin-film structure consisting of a first layer 31 of ferromagnetic material IF of thickness d _F1 , an intermediate layer 4 of superconducting material s of thickness d _s , of the second layer 32 of ferromagnetic material IF with a thickness of d _F2 , and the part of the second layer of magnetic material directly below the superconducting electrodes is replaced by an insulator or trimeal metal 6; and two 21 and 22 superconducting electrodes S with current leads 51 and 52 for connecting a weak coupling region to a current source.

The thicknesses of the layers 31 and 32 of the ferromagnetic material (Pd _x Fe _1-x , Pd _x Ni _1-x , Pd _x Co _1-x , Pt _x Fe _1-x , Pt _x Ni _1-x , Pt _x Co _1-x ) and the intermediate superconducting layer 4 are 1-100 nm. As a material for the superconducting electrodes 21 and 22 and the intermediate layer 4, materials such as niobium, niobium nitride, vanadium, indium, tin, lead, and alloys based on them can be used.

Figure 4 shows the dependence of the normalized critical values of the critical currents of patented structures on temperature normalized to the critical temperature of the superconducting materials used. The solid line represents the temperature dependence of the normalized critical current for the case when the exchange energy in both layers of the ferromagnet (or magnetic insulator) is negligible. The dashed line is for the case when the exchange energy in both layers of a ferromagnet (or magnetic insulator) is equal in magnitude and different from zero, and the magnetization vectors lying in the plane of the magnetic layers are parallel. Dashed-dotted - for the case when the exchange energy in both layers of a ferromagnet (or magnetic insulator) is equal in magnitude and different from zero, and the magnetization vectors lying in the plane of the magnetic layers are antiparallel.

Figure 5 shows the normalized values of the critical currents of patented structures as a function of the thickness of the intermediate superconducting layer, normalized to the coherence length in the superconductor used. The solid line represents the temperature dependence of the normalized critical current for the case when the exchange energy in both layers of the ferromagnet (or magnetic insulator) is negligible. The dashed line is for the case when the exchange energy in both layers of a ferromagnet (or magnetic insulator) is equal in magnitude and different from zero, and the magnetization vectors lying in the plane of the magnetic layers are parallel. Dashed-dotted - for the case when the exchange energy in both layers of a ferromagnet (or magnetic insulator) is equal in magnitude and different from zero, and the magnetization vectors lying in the plane of the magnetic layers are antiparallel.

From the above data it can be seen that the invention allows to achieve a significant change in the critical current of the GVD when changing the mutual orientation of the magnetization vectors of the two magnetic layers used. This allows you to use the patented structure as an element of a high-frequency superconducting memory.

Technological applicability: for the implementation of the patented device, materials used in cryoelectronic technology and known to specialists can be used. As the substrate 1, any standard substrates (silicon, sapphire, etc.) or a layer of any antiferromagnetic material can be used. As magnetic layers 31 and 32, soft magnetic alloys based on ferromagnets Ni, Co, Fe: Pt _x Fe _1-x , Pt _x Ni _1-x , Pt _x Co _1-x , Pd _x Fe _1-x , Pd _x Ni _1-x , Pd _x Co _1-x , Cu _x Ni _1-x , Nd _x Ni _1-x ; dielectrics, for example, amorphous silicon doped with magnetic atoms, Fe, Co, Ni, can also be used as magnetic layers. As a material for superconducting electrodes 21 and 22 - niobium, niobium nitride, vanadium, indium, tin, lead or MgB ₂ and compounds based on it, or high-temperature superconductors based on rare-earth cuprates of the general formula ReBa ₂ Cu ₃ O _7-x ; where Re is a rare earth metal, or other oxides (see, for example, US 6011981, Alvarez et al., 04.01.2000), for which the technology of deposition of layers on substrates is known. Typical thicknesses of the layers of magnetic material and normal metal for the patented topology are 1-100 nm and are in the range technologically feasible for thin-film electronics.

Claims (16)

1. A high-frequency superconducting memory element, comprising a weakly bonded region formed on a substrate in the form of a thin-film layered structure containing layers of magnetic material and a layer of superconducting material between them, two electrodes of superconducting material and current leads for connecting the weak-bonding region to a current source, characterized in that in the field of weak coupling between the electrodes of the superconducting material there are two layers of magnetic material separated by a layer of superconducting material with the possibility of a phase transition from the superconducting state to the normal state or from the normal state to the superconducting state when the direction of magnetization of one of the magnetic layers changes. 2. The device according to claim 1, characterized in that in planar geometry the superconductor layer is deposited on the substrate, and the first magnetic layer, the intermediate superconductor layer, the second magnetic layer and the second superconductor layer are deposited on top of it in this sequence, while the areas of said layers coincide . 3. The device according to claim 1, characterized in that in the end geometry the superconductor layer and the first magnetic layer are deposited on the substrate, and the intermediate superconductor layer, the second magnetic layer and the second superconductor layer are deposited on top of the layers in the indicated sequence, wherein the intermediate superconductor film , a second magnetic layer and a second superconductor layer are deposited both at the interface of the first superconductor layer and the first magnetic layer. 4. The device according to claim 1, characterized in that in the bridge geometry, the first magnetic layer is deposited on the substrate, and the intermediate layer of the superconductor and the second magnetic layer are deposited on top of it, while the areas of the said layers coincide. A superconductor layer is deposited on top of the second magnetic layer, in which a gap filled with an insulator is created. 5. The device according to claim 3, characterized in that in the bridge geometry under the first layer of magnetic material directly on the substrate may be an auxiliary layer of an antiferromagnet. 6. The device according to claim 3, characterized in that in the bridge geometry, in addition to the second magnetic layer, an insulator or normal metal layer is introduced, located in the device region directly below the superconducting electrodes. 7. The device according to claim 1, characterized in that niobium or an alloy based on it is used as a superconductor. 8. The device according to claim 1, characterized in that the compound of rare-earth cuprates of the general formula ReBa ₂ Cu ₃ O _7-x , where Re is a rare-earth metal, is used as a superconductor. 9. The device according to claim 1, characterized in that a magnetically soft ferromagnet is used as the material of the magnetic layer, for example, Pd _x Fe _1-x , Pd _x Ni _1-x , Pd _x Co _1-x , Pt _x Fe ₁ alloys _-x , Pt _x Ni _1-x , Pt _x Co _1-x with the content of ferromagnetic metals less than 10 atomic percent. 10. The device according to claim 1, characterized in that as the magnetic layers used dielectric layers, for example amorphous silicon, doped with magnetic atoms, Fe, Co, Ni. 11. The device according to claim 6, characterized in that an element from the group Cu, Au, Al, Pt is used as a normal metal. 12. The device according to claim 1, characterized in that the layers of the magnetically soft ferromagnet and the dielectric layers immediately below them are used as magnetic layers. 13. The device according to claim 1, characterized in that the magnetization vectors in the magnetic layers are directed parallel to the plane of the layers and can be deployed by an external magnetic field. 14. The device according to any one of claims 1 to 3, characterized in that the layers of magnetic material have different values of the coercive field. 15. The device according to claim 1, characterized in that the thicknesses of the layers of magnetic material in the field of weak coupling are 1-10 nm. 16. The device according to claim 1, characterized in that the thickness of the superconducting material in the field of weak coupling is 10-100 nm.

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