Josephson- transition super-conducting device

FIELD: physics, measurements.

SUBSTANCE: invention relates to cryogenic devices and can be used in measuring instruments, radio communication and data processing hardware operated at low temperatures. Josephson-transition superconducting device substrate has a weak-bond area formed as a thin-film FNF-structure connected with superconductor electrodes. Aforesaid thin-film structure is made up of F layers of ferromagnetic material with magnetisation directions lying in the plane of structure, a layer N of normal metal being arranged between aforesaid ferromagnetic metal F layers for them to turn said magnetisation directions relative each other. Superconducting electrodes are connected to opposite lateral sides of aforesaid FNF-structure.

EFFECT: higher-efficiency control over Josephson-transition critical current by external magnetic field due to provision of several independent channels for aforesaid critical current to pass through.

8 cl, 6 dwg

The invention relates to cryogenic devices and can be used in measuring equipment, radio and information systems operating at low temperatures.

Described a large number of designs of superconducting devices based on the Josephson effect (hereafter LDS), which are promising for use in various low-power superconducting devices (fast odnoslotovoy logic, a transmitting / receiving device, magnetic device). The Josephson effect occurs in the area of so-called "weak ties", which is formed in the region of contact between two superconductors through a non-superconducting material with any type conductivity. The main implementation issue is the selection of physical-chemical characteristics of the material layers that provide high values of the critical current I_cat a given distance L between the superconductors and the ability to control the magnitude and the sign of I_with.

Traditionally known LDS are formed on the dielectric substrate of the multilayer thin film structure including a superconductor, insulating, barrier and functional layers. Depending on the purpose and design is the choice of materials of the substrate and the active media.

So, described the SPD, bratvany on single-crystal dielectric substrate and having three layers: two layers of superconductor UVA ₂Cu₃About_7-x(YBCO), one of which is the bottom - placed directly on the substrate, separated by a barrier layer (US 6541789, Sato, et al., 01.04.2003). A weak link is formed on the end face of one of the superconductors.

Known SPD (JP 3190175, YUZURIHARA et al., 20.08.1991), which is a device with four leads, in which the current is set through one of the pairs of current leads, translates into a ferromagnetic phase existing within the device film of antiferromagnetic substances that are not in the area of the Josephson contact. The resulting magnetic moment creates a magnetic field, which leads to suppression of the critical current of a Josephson element, located between the other two leads of the device, and to generate its voltage pulse.

Also known SPD, designed to control the flow of electrons and having a multilayer structure of the superconductor - normal metal - superconductor" and not using dielectric barrier layers (US 6995390, Tsukui, 07.02.2006).

Known SPD intended for control of critical current five-layer dvuhjadernyh Josephson junctions, which are located inside the barriers material contains ferromagnetic film. Its purpose is to provide the Zeeman splitting of the resonance levels of the electrons in vnutri remoi area to control the magnitude of the critical current patterns by controlling the position of the split levels relative to the Fermi energy of the electrode voltage, applied to additional control contact patterns (US 6344659, Ivanov et al., 05.02.2002 - the closest equivalent).

The analysis of the prior art shows that the known device with Josephson junctions, including the closest analogue, typically provide for the current job on the thickness of the composite region of weak ties, i.e. in the direction perpendicular to the plane of the multilayer thin-film structure. Such devices have significant shortcomings in terms of management options current due to mutual field shielding layers of the same structure (for example, one ferromagnetic layer to the other), as well as small depths of penetration of the superconducting state in relation to the same in a normal metal.

Object of the invention is SPD, the design of which allows to eliminate the mentioned disadvantages, namely to ensure more effective management of the critical current of Josephson junctions by means of an external magnetic field through the organization of a number of independent channels of its course.

The problem is solved by the fact that superconducting Josephson device with the transition includes formed on the substrate region weak links in the form of a multilayer thin film structure associated with the electrodes of the superconductor. Thin-film structure consists of layers of ferromagnetic mA is Arial - normal metal - ferromagnetic material. The superconductor electrodes attached to opposite side faces of the thin-film structure, and the magnetization directions of the ferromagnetic layers of material lying in the plane of the thin-film structure, and the layers themselves are made with the possibility of a reversal of these directions of magnetization relative to each other.

The device can be characterized by the fact that the layers of ferromagnetic material have different values of the coercive field, as well as the fact that as a superconductor used niobium or an alloy based on it, and the fact that the quality of the superconductor used in the connection of rare-earth cuprates General formula ReBa₂Cu₃About_7-xwhere Re is a rare earth metal.

The device can be characterized, moreover, by the fact that as a ferromagnetic material is Ni, Co, Fe or metal alloys on their basis, and the fact that as a normal metal used item from a group of Cu, Au, Al, Pt.

The device can be characterized also by the fact that the thickness of the layers of ferromagnetic material and a normal metal are 10-100 nm.

The technical result of the invention is the possibility of independent changes of the directions of magnetization of the layers of the control value, the period of oscillation and the direction of current across the SPD through the organization of three independent channels of his passing. This is implemented through the structure of the SPD with the new configuration of the layers in the composite field S-(FNF)-S, where S, N, F - layers of superconductor and normal metal and ferromagnetic respectively. In this topology ensures that the job of the superconducting current in a direction parallel to FN-borders composite region weak links in S-(FNF)-S structure.

The invention is illustrated in the drawings, where:

figure 1 presents the patented design of the instrument;

in figure 2, 3 - dependence of the imaginary and real part of the wave vector q₃the magnitude of the exchange energy of one of the ferromagnetic layers, respectively;

figure 4 - dependence of the critical current on the distance between the superconducting electrodes;

figure 5 same as figure 4, for a fixed distance values;

figure 6 - the dependence of the normalized part of the critical current of the normalized values of the exchange energy.

Superconducting device (see figure 1) includes a substrate 1. On its surface is formed of a multilayer thin-film structure consisting of a first layer 2 of ferromagnetic material, a layer 3 of a normal metal and a second layer 4 of ferromagnetic material. Ferromagnetic film should be a mono-domain, the technology of making them known. Layers 2, 4 of ferromagnetic material should have different led the ranks of the coercive field, that allows you to expand the direction of magnetization in the layers relative to each other. This can be achieved, for example, manufacture of the layers 2 and 4 with slightly different thicknesses (˜30%) or the width of the film, as well as the choice of material of the substrate 1 or the normal metal layer 3.

Opposite side faces of the structure are connected to the electrodes 5 of the superconductor and the current leads 6. As a consequence, the superconducting current supplied through the current leads 6 to the electrodes 5, simultaneously flows through three independent channels FNF-patterns of length L formed by the layers 2, 3, 4. The side faces 7 of the structure connected to the electrodes 5 of the superconductor.

As part of FNF structure suitable for implementation of the patented device can be used the materials used in the technology of cryogenic materials and well-known specialists. For example, the substrate 1 can be used with any standard substrate (for example, silicon, sapphire, etc). As the ferromagnetic material layers 2, 4 - pure ferromagnetic Ni, Co, Fe or ferromagnetic alloys: Pt_xFe_1-xPt_xNi_1-xPt_xCo_1-xPd_xFe_1-xPd_xNi_1-xPd_xCo_1-x, Cu_xNi_1-x; as a layer 3 of a normal metal is Cu, Au, Al, Pt. As a material for SV is rprovides electrodes 5 - niobium, niobium nitride, or MgB₂and the connection based or high-temperature superconductors based on rare-earth cuprates General formula ReBa₂Cu₃O_7-xwhere Re is a rare earth metal, or other oxides (see, for example, US 6011981, Alvarez et al., 04.01.2000), technology for deposition of layers on the substrate are known. Estimates show that the typical thickness of the layers of ferromagnetic material and a normal metal for patented topology are 10-100 nm and are in the range of technologically feasible for thin-film electronics.

Superconducting device operates as follows. When applying a current through the current leads 6 to the electrodes 5 of the superconductor superconducting current simultaneously flows through three independent channels FNF-patterns of length L formed by the layers 2, 3, 4. When this occurs, the migration of injected through one of the side faces 7 of Cooper pairs in the opposite superconducting electrode 5, which provides a specified flow of superconducting current through the structure.

Rationale achieve a technical result, and requirements for the selection of the substrate materials of the layers forming the structure, and the physical principles underlying the invention are explained given numerical calculations, the results of which are shown in figure 2.

In figure 2, 3 depicts the dependence of the imaginary (figure 2) and valid (figure 3) parts of the wave vector q₃the magnitude of the exchange energy H₂one of the layers 2, 4 of ferromagnetic material at a constant value of the exchange energy of the other ferromagnetic H₁/πT_With=30. The dependence calculated in the framework of the equations of the microscopic theory of superconductivity for several values of z=(ξ_N/ξ_F)²=50, 150, 300, 600 ξ_N/ξ_N=4, ξ_N/ξ_F=10, T=0.5T_With. Here: T_With- critical temperature of the superconducting electrodes, ξ_Nand ξ_F- length of penetration of the superconducting state of superconductors in the normal and ferromagnetic materials, respectively.

Settings ξ_N=ξ_N(γ_N)^1/2and ξ_F=ξ_F(γ_F)^1/2characterize the coupling coefficient between F and N layers. (γ_F=γ_Ind_f/ξ_F; γ_N=γ_Bd_n/γξ_N; γ=ρ_Nξ_N/ρ_Fξ_F; γ_B=R_BA/ρ_Fξ_F, R_Band A - resistance and the area of the FN-borders ρ_Nand ρ_F- specific resistance F and N materials). It is seen that in the case of antiparallel orientation of the magnetizations F films is mq ₃strictly vanishes when H₂='N₁for all parameter values. The position of the second point on the axis of the H₂where Imq₃=0, depends on the parameter z and can be located both to the left and to the right of the values of H₂='N₁.

The result shows that the change in the direction of the magnetization of one ferromagnetic films in the opposite direction you can go from mode oscillating dependence of the critical current I_C(L) to the mode in which oscillation is missing completely. At a given distance L between the superconductors and the set temperature for this transition may be accompanied either by changing the sign of I_C(L)or increase to several times the value of I_C(L), or the simultaneous manifestation of these two effects. The existence of two values of H₂where is this in effect means that the implementation of the switch is achieved in three ways: either the full magnetization reversal of one of the films (H₂=-H₁)or partial magnetization reversal (H₂<-H₁), or additional magnetization in the opposite direction (H₂>-H₁).

Figure 4 presents the dependence of the magnitude component of the critical current structure of the distance L between the electrodes 5, calculated numerically with N₁π T_With=30, z=(ξ_N/ξ_F)²=300, ξ_N/ξ_N=4, ξ_N/ξ_F=10, T=T_Withthat γ_BF/γ_BN=1, and the values of N₂/πT_With=30, -10, -30, -78.4 (γ_BF=R_BFA_F/ρ_Fξ_F; γ_BF=R_BNA_N/ρ_Nξ_N; R_BF-R_BNand A_F-A_N- resistance and the area of SF and SN-faces, respectively). Figure 5 is given the dependence of the normalized part of the critical current I_C1designed for H₂/πT_C=30 for the same parameter values. It is seen that in the considered case, the components of the critical current I_C1and I_C2=I_C1are significantly less summand of I_C3and decrease with increasing L significantly faster. We also see that in practically interesting case of L>ξ_Ncontribution to the critical current I_C1and I_C2negligible, so that with exponential degree of accuracy I_With=I_C3. The critical current exponentially decreases with increasing L and experiences oscillations associated with the transition patterns from 0 to π state the length of the procedure (Imq₃)^-1≈ξ_N"(Imq₁)^-1, (Imq₂)^-1≈ξ_F. The oscillations disappear when H₂='N₁and H₂/πT_With=-(γ_F ²H₁/πT_C)^-1=-78.4.

Figure 6 presents the dependence of the normalized part of the module critical current I_C3γ_BNeR_BN/2πT| of the normalized values of the exchange energy H₂/πT_Withwhen H₁/πT_C=30, z=(ξ_N/ξ_F)=300, ξ_N/ξ_N=4, ξ_N/ξ_F=10, T=0.5T_Cthat γ /γ_BN=1, calculated for values of L/ξ_N=0.1, 2, 3, 4, with compression ratios on the chart 10, 1, 0.2, 0.01, respectively. Setting z=300 selected so that the period of oscillations of the critical current with H₂=N₁was minimal (see figure 2).

As follows from the graph, if L/ξ_N=0.1, the structure is always in the 0-state. Therefore, when switching from H₂=N₁in N₂=-H₁do not change the sign of I_C3but there is almost a threefold increase of the critical current. When H₂=N₁and L/ξ_N=1, 2 Josephson transition is π-state (see figure 4). In this case, the switching of the H₂=N₁in N₂='N₁leads to the transition from π- in the 0-state. With this switch I_cincreases by approximately 7-fold for L/ξ_N=1 and 3 times for L/ξ_N=2. Finally, in the case of L/ξ_N=4 if N₂=H₁the system is in the 0-state and the switching of the H₂=H₁in N₂='N₁leads to a threefold increase of the critical current is not accompanied by a change of its sign.

It is seen that the transition from 0 to π-state is possible if N₂ranges from 4πT_Cup to 15πT_C. When changing the sign of H₂with H₁on-N₁the critical current increases about 6 times. These facts is bisnovat operation amount, and the sign of the critical current by changing the direction of magnetization of one ferromagnetic film structure on the opposite. They also suggest that a change of sign of H₂accompanied with the increase of the critical current structure.

The data show that during the transition from the ferromagnetic configuration (H₂=H₁to antiferromagnetic geometry (H₂=-H₁) the critical current I_Withpatterns can be greatly increased, especially near the transition between "0" and "π" conditions. Away from the transition points, the gain can reach the same order, caused by a change in the characteristic length of decay of the critical current.

Thus, in Josephson structures with topology S-FNF-S may not only effective increase (compared to SFS topology) effective length of the decay of the critical current and the period of its oscillations to the length scale ξ_Nbut management as the magnitude and the sign of I_With.

1. Superconducting Josephson device with the transition, including formed on the substrate region weak links in the form of a multilayer thin film structure associated with the electrodes of the superconductor, characterized in that

thin-film structure consists of layers of ferromagnetic material is normally the first metal ferromagnetic material,

the superconductor electrodes attached to opposite side faces of the thin-film structure, and the

magnetization directions of the ferromagnetic layers of material lying in the plane of the thin-film structure, and the layers themselves are made with the possibility of a reversal of these directions of magnetization relative to each other.

2. The device according to claim 1, characterized in that the layers of ferromagnetic material have different values of the coercive field.

3. The device according to claim 1, characterized in that the quality of the superconductor used niobium or an alloy based on it.

4. The device according to claim 1, characterized in that the quality of the superconductor used in the connection of rare-earth cuprates General formula ReBa₂Cu₃O_7-x,where Re is a rare - earth metal.

5. The device according to claim 1, characterized in that the ferromagnetic material is Ni, Co, Fe or metal alloys based on them.

6. The device according to claim 1, characterized in that as the normal metal used item from a group of Cu, Au, Al, Pt.

7. The device according to claim 5, characterized in that the thickness of the layer of ferromagnetic material is 10-100 nm.

8. The device according to claim 6, characterized in that the thickness of the normal metal is 10-100 nm.