RU2598405C1 - Superconducting josephson device with composite magnetic layer - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: use: for creation of superconducting Josephson device. Essence of this invention consists in the fact that the superconducting Josephson device with composite magnetic layer based on thin-film structure has a planar geometry of thin films in the form of the heterostructure Sd-M-S (Sd - base film of cuprate superconductor, M - composite magnetic layer, S - upper superconducting electrode), formed on the substrate of NdGaO₃ crystal with orientation (110), epitaxial grown film of superconducting cuprate YBa₂Cu₃O_7-δ is used as the Sd base film, and sequentially deposited films of SrRuO₃ (SRO) strontium ruthenate with thickness d_SRO and optimally doped manganite La_0.7Sr_0.3MnO₃ (LSMO) of thickness d_LSMO are used as a composite of the M magnetic layer, AuNb superconducting thin-film two-layer is used as an upper electrode S, thickness of SRO and LSMO films are determined by number of laser ablation pulses, thus providing the calculated ratio of d_SRO and d_LSMO relative to corresponding lengths of coherence ξ_F in SRO and LSMO, thickness of the composite film d_M=d_SRO+d_LSMO can vary from units to tens of nanometres, Au thickness in AuNb upper electrode should provide superconducting proximity effect, and it makes the order of nanometres several units, wherein the thin-film topology of the device is made together with superconducting thin-film antenna of Sd and S films, placed at the same substrate, and planar size of L Sd-M-S structure (in the plane of the layers) varies from fractions to tens of micrometers.

EFFECT: possibility provision of creation of superconducting Josephson device.

1 cl, 4 dwg

Description

The invention relates to cryoelectronic devices and can be used in microwave devices in the microwave range, as well as in information systems operating at low temperatures.

Known Josephson end contact with electrically conductive (or semiconductor) layers forming a barrier layer between two superconducting layers (patent DE 4431340 (A1) 1996-03-07 "Josephson contact with multilayer barrier" - Schilling Meinhard). In such an end Josephson junction, the barrier consists of at least two layers consisting of metal and semiconductor materials with a perovskite crystal structure, in which (in the barrier layer) high conductivity is realized in at least one layer. In addition, in the Josephson end contact, at least one layer forming the barrier layer, in comparison with other layers, has a significantly longer superconducting coherence length. In this case, the barrier layer itself is formed from alternately connected layers so that the layer with low conductivity alternates with the layer with higher conductivity.

In another type of end Josephson end contact (patent DE 19634645 "Superconductive and non-superconductive layer sequence" R. Holczyk, U. Poppe, Ch. Jia). the sequence and composition of the layers may include a sequence of a superconducting material containing CuO ₂ planes and non-superconducting layers of the following materials:

(I) BaTbO ₃ ; (Ii) Ba _1-x Sr _x TbO ₃ (x = 0 1), (III) LaCu _1-x Tb _x O ₃ (x = 0 to 1), (Iv) RCu _1-x Tb _x O ₃ (R = Nd, Eu and Sm and x = 0 to 1), (V) Ba _1-x Sr _x MO ₃ (M = Tb, Pr or Ce and x = 0 to 1), (Vi) LaCu _1-x M _x O ₃ (M = Tb, Pr or Ce and x = 0 to 1), (Vii) RCu _1-x M _x O ₃ (R = Nd, Eu and Sm, M = Tb, Pr or Ce and x = 0 to 1), (Viii) R _1-y NyCu- _1x M _x O ₃ (R = La, Nd, Eu and Sm, N = Ba or Sr, M = Tb, Pr or Ce, x = 0 1 and y = 0 to 1), (Ix) R _2-y N _y Cu _1-x M _x O ₄ (R = La, Nd, Eu and Sm, N = Ba or Sr, M = Tb, Pr or Ce, x = 0 to 1 and y = 0 to one)

As an example of implementation, the said patent shows a 5-layer formed on a SrTiO ₃ substrate: Pt-YBa ₂ Cu ₃ O ₇ - BaTbO ₃ - YBa ₂ Cu ₃ O ₇ - BaTbO ₃ - YBa ₂ Cu ₃ O ₇ -SrTiO ₃ ( Pt - metallization). It is noted that the interface between YBa ₂ Cu ₃ O ₇ / BaTbO ₃ has a length of about 0.6 nm.

Known patent (US 2007281861 (A1) 2007-12-06 "Superconducting device and method of manufacturing the same" by Ishimaru Yoshihiro, Tarutani Yoshinobu, Tanabe Keiichi), which proposes (in addition to single-layer devices on transitions in a bicrystal configuration) a multilayer superconducting device, consisting of a substrate, a first superconducting film, an insulating layer (barrier) and a second superconducting film. In a multilayer superconducting structure, a barrier layer is formed on the lower (first) superconducting film.

A common drawback of this type of end multilayer devices is their geometry proper, which does not allow for the identity of the characteristics of a thin film in the region of the end bevel and on flat sections of the substrate [M. van Zalk, A. Brinkman, J. Aarts, and H. Hilgenkamp, Phys. Rev. B 82, 134513 (2010)].

A hybrid device with planar geometry is known - an oxide heterostructure (patent Bozovic I., WO 0239509-A2). The device includes a substrate and a structure monolithically formed on the substrate, containing one or more magnetic F-layers of metal oxides formed by molecular beam epitaxy (layer by layer). Note that to ensure the required quality of the S / F, F / F layer interfaces, it is necessary to use (as suggested in the mentioned patent) a unique molecular beam epitaxy unit, which cannot be replaced by other known methods for producing thin films.

Also known is a superconducting device with a Josephson junction (patent RU 2373610 (C1) - 2009-11-20), Karminskaya T.Yu., Kupriyanov M.Yu., Ryazanov V.V. (prototype of the proposed technical solution). The prototype device is a Josephson junction of length L, in which the weak bond is formed by a thin-film magnetically active three-layer FNF structure (F is a ferromagnetic material, N is a normal metal), and superconducting (S) electrodes are in contact with the opposite side faces of the layered structure. The layers S, F, N are characterized by the coherence length ξ _S , ξ _F , ξ _N, respectively, and the length L is selected in the interval from fractions to units ξ _F. In this case, the F layers are capable of reversing the magnetization vectors relative to each other in the plane of the layered structure, providing for the possibility of transition from the antiferromagnetic to the ferromagnetic state and ensuring the generation of the triplet type of superconducting pairing in the weak coupling region. The angle of rotation of the magnetization vectors should be in the range of values at which the maximum magnitude of the superconducting current module is achieved. The superconducting current is set in the direction parallel to the FN boundaries of the composite weak coupling region in the S- (FNF) -S structure.

The main disadvantage of the prototype is an exponential decrease in the critical current density j _C structure with an increase in its length L, which cannot exceed several coherence lengths ξ _F. According to [JWA Robinson, S. Piano, G. Burnell, C. Bell, and MG Blamire Phys. Rev. 76, 094522, (2007)] the ξ _F values for typical ferromagnetic materials Ni ₈₀ Fe ₂₀ - 0.46 nm, Ni - 1.2 nm, Co - 0.3 nm, Fe - 0.25 nm. To ensure the small length L S- (FNF) -S of the Josephson junction, difficult and expensive technological equipment is required that will provide submicron gaps with a controlled surface of the lower electrode. Moreover, the required length of the S / FNF and FNF / S interfaces is not specified in the prototype patent.

An analysis of the prior art shows that a common drawback of thin-film Josephson devices made in end geometry is the non-identical characteristics of the film in the region of the end bevel and on flat sections of the substrate. In the prototype device, this drawback is overcome due to the planar arrangement of the FNF layers of the structure, which is connected to the superconducting electrodes by side faces. For the practical implementation of such a device, an extremely short (of the order of nanometers) length L FNF structure is required, while it is necessary to ensure the required quality of the interface between the ends of the FNF structure and S-electrodes, provided that they are undeformed during the technological operations of manufacturing the device, which is problematic for the current level of technology, while the question of the integrability of the Josephson prototype device with the antenna remains open.

The aim of the invention is:

1) providing the ability to control the critical current density j _C by varying the thickness of the thin films of the composite magnetically active interlayer, which eliminates the disadvantage of the prototype - an exponential decrease in the critical current density j _C depending on the length L of a Josephson device with a magnetically active interlayer;

2) ensuring the integrability of a thin-film superconducting Josephson device with a superconductor antenna on a single chip.

This goal is achieved by the fact that the Sd-MS superconducting Josephson device (Sd is the YBCO base film, M is the magnetically composite interlayer, S is the upper superconducting electrode) is formed on one substrate (chip) by plasma-chemical and ion-beam etching, and its thin-film topology is integrable with a superconducting thin-film antenna. As a substrate, NdGaO ₃ with an orientation of (110) is used, for Sd, an epitaxially grown superconducting YBCO cuprate film is used, and sequentially deposited films of La _0.7 Sr _0.3 MnO ₃ , (LSMO) manganite and SrRuO ₃ strontium ruthenate are used as a composite magnetoactive layer M SRO). The direction of the magnetization vector of the LSMO film lies in the plane of the substrate, and in SRO the magnetization vector is directed at an angle of about 23 ° relative to the normal to the substrate, which ensures noncollinearity. The critical current density is determined by the sum and ratio of the thicknesses of the SRO and LSMO films (d _SRO and d _LSMO, respectively). The film thicknesses are determined by the number of laser ablation pulses, which, taking into account the coherence lengths ξ _F in SRO and LSMO, provides both the optimal ratio d _SRO / d _LSMO and the total thickness d _M = d _SRO + d _{LSMO of the} composite interlayer, varying from units to tens of nanometers . As the upper electrode S, a superconducting two-layer AuNb is used. The Au thickness in the upper AuNb electrode should provide a superconducting proximity effect. The planar size of the structure L (length and width) is from fractions to tens of micrometers and determines the amplitude of the critical current I _C = j _C L ² . When an external magnetic field is applied, the mutual spatial orientation of the magnetizations of the SRO and LSMO films in the M interlayer changes, providing not only the modulation of the critical current, but also modifying the current-phase dependence function.

Thin films of Sd and M are grown in situ in the same vacuum chamber at a high substrate heating temperature, and then the resulting ex situ structure is covered with a thin 20-30 nm Au layer after cooling to room temperature. The subsequent niobium layer for electrode S is applied by magnetron sputtering after applying an additional Au protective film (after etching to clean the surface of the gold layer deposited in situ in the previous step). The gold sublayer is used to preserve oxygen stoichiometry of the oxide layer and to prevent oxidation of the surface layer of niobium and to prevent the formation of oxides. The proximity effect between superconducting Nb and a thin layer of Au metal can manifest itself in a certain decrease in the critical temperature of the NbAu bilayer to a value of T _C = 8.5–9 K. The transparency of the interface I ₁ (between the cuprate superconductor S _d and magnetically active material M) and I ₂ (between the M layer and the upper S-electrode of the AuNb bilayer) are determined by the values of the characteristic resistance R _N A, where R _N is the resistance of the structure in the normal state, A = L ² is the area of the contact region.

The invention is illustrated in the drawings:

FIG. 1. Heterostructure of a superconducting Josephson device with a composite magnetically active interlayer: (a) schematic representation and functional model. 1 - substrate, 2 - thin YBCO film, 3 and 4 films of a composite magnetically active interlayer, 5 - Nb film. The current-voltage characteristics are measured by connecting the current source “A” and the voltmeter “V” through the contact pads made of gold on electrodes 2 and 5. In the functional model S, there is a superconducting two-layer NbAu, Sd is a superconductor YBCO, M is a magnetoactive interlayer, which consists of two ferromagnetic layers, I ₁ and I ₂ are the barrier regions formed at the interfaces, (b) an enlarged image (photograph) of a fragment of a heterostructure with an antenna formed on electrodes 2 and 5.

FIG. 2. A family of current-voltage characteristics at T = 4.2 K and a microwave frequency of exposure to 41 GHz. The parameters of a Josephson device with a magnetically composite interlayer: d _SRO = 6 nm, d _LSMO = 5.5 nm, L = 10 μm. Parameter α is the introduced attenuation into the path of the external electrodynamic system.

FIG. 3. Normalized dependences of the critical current (circles), the first Shapiro stage (triangles) and the half-integer Shapiro stage (rhombs) on the normalized amplitude of microwave radiation a = I _RF / I _C. The lines show the theoretical dependences calculated by a modified resistive model that takes into account the influence of the second harmonic of the current-phase dependence of the superconducting current.

FIG. 4. Fourier analysis of the periods of critical current modulation by an external magnetic field (ΔH _FFT ) and the corresponding Fourier amplitudes of the components depending on the 1 / L parameter for the magnetic field dependences of the critical current of three Josephson heterostructures located on the same chip.

A superconducting Josephson device with a composite magnetically active interlayer (Fig. 1) can be formed on the basis of epitaxially grown films of oxide materials. To do this, it is proposed to use the known methods of laser ablation, magnetron sputtering and cathodic sputtering at high oxygen pressures. To obtain the base films of the cuprate superconductor YBa ₂ Cu ₃ O _7-δ (YBCO), an NdGaO ₃ (NGO) substrate with (110) orientations is applicable. This approach allows one to control the properties of epitaxially formed multilayer structures and to obtain base films of cuprate superconductors with a critical temperature of T _C = 88-89 K. To obtain magnetic films of interlayer M, thin films of optimally doped La _0.7 Sr _0.3 MnO ₃ (LSMO) manganite can be used at temperatures T <100 K, possessing the properties of ferromagnets. A similar approach is applicable for the deposition of films of other oxide materials, in particular, SrRuO ₃ ruthenate (SRO), as well as the SRO / LSMO composite layer of ruthenate-manganite. On the x-ray spectra of the experimental samples, peaks of three materials of the YBCO, LSMO, and SRO heterostructure were observed, which indicates the epitaxial growth of the films in the heterostructure and the absence of mixing of materials at the interfaces The topology of the Josephson device can be formed by well-known methods of photolithography, plasma-chemical and ion-beam etching. Thin films in multilayer structures with a controlled thickness from units to tens of nanometers can be epitaxially grown without breaking the vacuum at a high substrate heating temperature. To heat and maintain the substrate at high temperature, a heater based on the ThermoCoax element can be used, consisting of a central NiCr core, a stainless steel coaxial sheath and an insulating layer of powdered MgO. Using a silver paste that provides good thermal contact, an insulated Cu / NiCr thermocouple is glued under the heater cover. To achieve good thermal contact, the substrate is also glued to the heater lid with silver paste.

Before deposition of Au / Nb bilayers, the surface is treated by methods of radio frequency etching or ion beam etching in an Ar atmosphere, depending on the type of spraying system used. The deposition of Au and Nb films is carried out by the methods of radio frequency sputtering and radio frequency magnetron sputtering of Au and Nb targets, respectively. The Au film thickness was 5-30 nm, Nb 200 nm. To prevent the chemical interaction of Nb with the photoresist during prolonged ion-beam etching of the YBCO film, an additional 30 nm thick Au protective layer is deposited on the Nb film surface.

Photolithography is used to form the topology of a superconducting Josephson device with a composite magnetoactive interlayer. The Au / Nb / Au / M / YBCO multilayer is covered with a layer of Shipley-1813 photoresist about 1 μm thick, which after exposure and development processes remained in the transition region, forming a mask, through which the Au / Nb / Au / MYBCO multilayer was then etched. Plasma-chemical etching in a mixture of CF ₄ and O ₂ is used for Nb etching. The removal of Au, YBCO films and the manganite interlayer is carried out by ion beam etching with a low ion energy of Ar + 250 eV and an ion current density of 0.2 mA / cm ² , which reduces the effect of ion bombardment on the surface layer of the manganite interlayer and the YBCO film. The multilayer structure formed by this method should then be isolated from the ends with an insulating layer of SiO ₂ , which allows localizing the current flow region and avoiding spurious contacts at the ends of the YBCO film. From the transition regions and contact pads, the SiO ₂ film is removed by explosive photolithography. At the last stage, additional Nb and Au layers are sprayed and a pattern is formed in them, which makes it possible to conduct 4-point electrical measurements at T <T _C. Figure 1b shows an enlarged image (photograph) of a fragment of a central hetero structure with an antenna on a 5 × 5 mm ² substrate, on which there are five Nb / Au / M / YBCO structures with sizes from 10 × 10 to 50 × 50 μm ² .

, and Z.

Phys. Rev. B, 84, 064511 (2011) и A.S. Mel′nikov, A.V. Samokhvalov, S.M. Kuznetsova et al., Phys. Rev Lett., 109, 237006 (2012)] предсказывают значительное (на несколько порядков) увеличение второй гармоники ток-фазовой зависимости сверхпроводящего тока для случая несимметричной прослойки (d₁≠d₂) при изменении угла между направлениями намагниченностей пленок прослойки. При измерении динамики изменения ступенек Шапиро было обнаружено отклонение от синусоидальности ток-фазовой зависимости. На ВАХ прибора с L=10 мкм и I_C=88 мкА и нормальным сопротивлением R_N=0,16 Ом под воздействием монохроматического СВЧ-излучения на частоте

ГГц наряду с целочисленными наблюдаются дробные ступени Шапиро (Фигуры 2 и 3). Для критической частоты

ГГц отношение

хорошо соответствует условию высокочастотного предела, что в эксперименте подтверждается величиной максимума первой ступени Шапиро I₁=94 мкА и соответственно отношением I₁/I_C=1.1. При этом максимальная высота полуцелой ступени Шапиро была I_1/2=15 мкА, что в рамках модифицированной резистивной модели [P. Komissinskiy, G.A. Ovsyannikov, K.Y. Constantinian et al., Phys Rev B, 78, 024501 (2008)] с учетом несинусоидальной ток-фазовой зависимости указывает на существенный вклад от второй гармоники. Модификация ток-фазовой зависимости под воздействием внешнего магнитного поля показана на Фигуре 4, демонстрирующей результат Фурье-анализа периодов модуляции ΔH_FFT критического тока I_C внешним магнитным полем Н и соответствующие амплитуды Фурье-компонент в зависимости от параметра 1/L для магнитнополевых зависимостей критического тока I_C(Н) для трех сверхпроводниковых джозефсоновских приборов с композитной магнитоактивной прослойкой, расположенных на одном чипе.Measurements of the dependences of the critical current I _C on the magnetic field showed that on the dependences I _C (H) the critical current increases when a weak magnetic field is applied H = 5-15 Oe. The specific value of the magnetic field at which the maximum of the critical current is observed depends on the parameters Nb / Au / M / YBCO and magnetic field directions. Theoretical calculations [L. Trifunovic, Z.

, and Z.

Phys. Rev. B, 84, 064511 (2011) and AS Mel′nikov, AV Samokhvalov, SM Kuznetsova et al., Phys. Rev Lett., 109, 237006 (2012)] predict a significant (by several orders of magnitude) increase in the second harmonic of the current-phase dependence of the superconducting current for the case of an asymmetric interlayer (d ₁ ≠ d ₂ ) with a change in the angle between the directions of magnetization of the interlayer films. When measuring the dynamics of the Shapiro steps, a deviation from the sinusoidality of the current-phase dependence was found. On the I – V characteristic of a device with L = 10 μm and I _C = 88 μA and normal resistance R _N = 0.16 Ohm under the influence of monochromatic microwave radiation at a frequency

GHz along with integer Shapiro fractional steps are observed (Figures 2 and 3). For critical frequency

GHz ratio

well corresponds to the condition of the high-frequency limit, which is experimentally confirmed by the maximum value of the first Shapiro stage I ₁ = 94 μA and, accordingly, the ratio I ₁ / I _C = 1.1. Moreover, the maximum height of the half-integral Shapiro step was I _1/2 = 15 μA, which is within the framework of the modified resistive model [P. Komissinskiy, GA Ovsyannikov, KY Constantinian et al., Phys Rev B, 78, 024501 (2008)], taking into account the non-sinusoidal current-phase dependence, indicates a significant contribution from the second harmonic. Modification of the current-phase dependence under the influence of an external magnetic field is shown in Figure 4, showing the result of a Fourier analysis of the modulation periods ΔH _{FFT of the} critical current I _{C with} an external magnetic field H and the corresponding amplitudes of the Fourier component depending on parameter 1 / L for the magnetic field dependences of the critical current I _C (Н) for three superconducting Josephson devices with a composite magnetoactive interlayer located on the same chip.

Thus, the technical result of the proposed superconducting Josephson device with a multilayer magnetically active interlayer is as follows: a) the design of the device provides the ability to control the density of the superconducting current by choosing the thickness of the ferromagnetic films of the composite magnetically active interlayer, b) the device is characterized by a non-sinusoidal current-phase dependence of the superconducting current modulated external magnetic field and microwave signal, c) the manufacture of the device is based on known and available methods of deposition of thin films with a controlled thickness of each layer; d) the formation of the device in planar topology allows its integration into a planar broadband thin-film antenna on a single chip.

Claims (1)

A superconductor Josephson device with a magneto-composite composite based on a thin-film structure, characterized in that it has a planar geometry of thin films in the form of an Sd-MS heterostructure (Sd is the base film of a cuprate superconductor, M is a composite magnetically active interlayer, S is the upper superconducting electrode) on an NdGaO ₃ crystal substrate with a (110) orientation, an epitaxially grown superconducting cuprate YBa ₂ Cu ₃ O _7-δ film is used as the base Sd film, as a composite the gnaroactive layer M, sequentially deposited films of strontium ruthenate SrRuO ₃ (SRO) of thickness d _SRO and optimally doped La _0.7 Sr _0.3 MnO ₃ (LSMO) manganite of thickness d _{LSMO are used} , and the superconducting thin-film two-layer AuNb, SRO and LSMO thicknesses are used as the upper electrode S films are determined by the number of laser ablation pulses, providing a calculated ratio of d _SRO and d _LSMO relative to the corresponding coherence lengths ξ _F in SRO and LSMO, the thickness of the composite film d _M = d _SRO + d _LSMO can vary from units to tens of nanometers, thicknesses Au in the upper electrode AuNb should provide a superconducting proximity effect and is of the order of several units of nanometers, while the thin-film topology of the device is formed together with the superconducting thin-film antenna from Sd and S films located on the same substrate, and the planar size L Sd-MS structure (in the plane of the layers) varies from fractions to tens of micrometers.

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