Critical Current Decreases Research Articles

Modulation of switching current density in T-type magnetic structure through magnetic anisotropy

We investigate magnetic field driven domain wall motion and spin-orbit torque (SOT) induced perpendicular magnetization switching of the ferrimagnetic layer in a T-type magnetic heterojunction with the structure of Pt/Co/Ta/Co81Tb19/Ta, where the bottom Co layer has in-plane magnetic anisotropy and the top Co81Tb19 layer has perpendicular magnetic anisotropy. It is found that the magnetic field driven domain wall motion and the depinning field of the Co81Tb19 layer can be effectively tuned by the thickness of the Co layer (tCo). Meanwhile, SOT-induced perpendicular magnetization switching of the Co81Tb19 layer is observed, and the critical switching current density first decreases and then increases with increasing the thickness of the Co layer. A lower critical switching current density was obtained at the optimized thickness (tCo = 1.2 nm), resulting in a 37% reduction in critical switching current density. Meanwhile, the research reveals a strong correlation between critical switching current density and magnetic anisotropy field, indicating that tuning the magnetic anisotropy is an efficient way to modulate the switching current density in a T-type magnetic structure. These results provide a new perspective on SOT manipulation and will be useful for the design of SOT-based devices.

Analysis of Critical Current Dependence on Specimen Length and Crack Size Distribution in Cracked Superconductor.

In order to describe the dependence of critical current on specimen length and crack size distribution in the superconducting tape with cracks of different sizes, a Monte Carlo simulation and a model analysis were carried out, employing the model specimens of various lengths constituted of multiple short sections with a crack per each. The model analysis was carried out to evaluate the effects of the two factors on the critical current of a specimen. Factor 1 is the size of the largest crack in a specimen, and Factor 2 is the difference in crack size among all sections at the critical voltage of critical current. Factors 1 and 2 were monitored by the smallest ligament parameter among all sections constituting the specimen and by the number of sections equivalent to the section containing the largest crack at the critical voltage of the critical current of the specimen, respectively. The research using the monitoring method revealed quantitatively that the critical current-reducing effect with increasing specimen length is caused by the increase in the size of the largest crack (Factor 1), and also, the critical current-raising effect is caused by the increase in the difference of crack size (Factor 2). As the effect of Factor 1 is larger than that of Factor 2, the critical current decreases with increasing specimen length. With the present approach, the critical current reducing and raising effects under various crack size distributions were evaluated quantitatively as a function of specimen length, and the specimen length-dependence of critical current obtained by the Monte Carlo simulation was described well.

Critical current of a layered high-temperature superconductor with artificial pinning centers: the Monte Carlo simulation

Current–voltage characteristics (EJ curves) and magnetic field dependences of the critical current have been calculated for a superconductor with artificial pinning in the form of submicron-sized holes and tilted radiation defects. Calculations have been performed within the framework of the three-dimensional model of a layered HTS by means of the Monte Carlo method. S-shaped features of the EJ curves have been observed for a sample with a rectangular lattice of holes. Such features have not occurred in calculations for HTSs with non-magnetic pinning centers before, but they have been observed in experimental studies. In this paper, the features occurred in magnetic fields close to 290 Gs (which is the lower critical field for the Bi2Sr2CaCu2O8-δ superconductor at 1 K) and they were sensitive to the magnitude of the external magnetic field. In addition, the features were more prominent at temperatures below 30 K and in samples with weak intrinsic pinning, and they were connected with matching-like effects in the vortex system (i. e. a certain number of vortices being pinned on each hole, screening new vortices from entering the sample). For samples with tilted radiation defects, decreasing field dependences of the critical current have been obtained, showing weak maxima near the lower critical field of the superconductor. Calculations have shown that, at a fixed value of the external field, the critical current decreases with the increasing tilt angle of the defects.

Ion-irradiation-induced field-free magnetization switching in synthetic antiferromagnets by spin–orbit torque

The perpendicularly magnetized synthetic antiferromagnets (SAFs) are expected to be ideal materials for magnetic random-access memory (MRAM) due to the low cross-talk noise and the high recording density. Normally, an in-plane magnetic field is required to break the inversion symmetry for deterministic magnetization switching in perpendicular SAFs by SOTs, which is undesirable for applications. In this work, the ion-irradiation method was adopted to engineer the interface of a perpendicularly magnetized Pt/Co/Pt/Ru/Pt/Co/Ta SAF to achieve the filed-free switching of the magnetization via the in-plane magnetization component of top-Co layer, which provides an in-plane effective field to bottom-Co layer and breaks the symmetry. Furthermore, with the increase of ion fluence, the critical switching current density decreases, which is consistent with the increase of spin Hall angle induced by ion-irradiation measured by the harmonic Hall voltage method. These results indicate that ion irradiation is a promising approach for realizing field-free magnetization switching in SAFs driven by SOTs and has potential application in SOT-based spintronic devices.

Contribution of Processes in SN Electrodes to the Transport Properties of SN-N-NS Josephson Junctions.

In this paper, we present a theoretical study of electronic transport in planar Josephson Superconductor-Normal Metal-Superconductor (SN-N-NS) bridges with arbitrary transparency of the SN interfaces. We formulate and solve the two-dimensional problem of finding the spatial distribution of the supercurrent in the SN electrodes. This allows us to determine the scale of the weak coupling region in the SN-N-NS bridges, i.e., to describe this structure as a serial connection between the Josephson contact and the linear inductance of the current-carrying electrodes. We show that the presence of a two-dimensional spatial current distribution in the SN electrodes leads to a modification of the current-phase relation and the critical current magnitude of the bridges. In particular, the critical current decreases as the overlap area of the SN parts of the electrodes decreases. We show that this is accompanied by a transformation of the SN-N-NS structure from an SNS-type weak link to a double-barrier SINIS contact. In addition, we find the range of interface transparency in order to optimise device performance. The features we have discovered should have a significant impact on the operation of small-scale superconducting electronic devices, and should be taken into account in their design.

Investigation of Field-Free Switching of 2-D Material-Based Spin–Orbit Torque Magnetic Tunnel Junction

Spin Hall-assisted magnetization switching in a three-terminal magnetic tunnel junction (MTJ) has attracted much attention due to its high-speed magnetization switching, which enables low-power memory and logic applications. In this study, 2-D materials with high spin–orbit coupling (SOC) effects and high charge-to-spin conversion efficiency are used to assess the performance characteristics of spin–orbit torque MTJ (SOT-MTJ). External field-free switching in SOT-MTJ is accomplished by employing a standard MTJ structure with a bias magnetic layer on top of the structure that projects a dipolar magnetic field onto a free layer (FL). In addition, the Dzyaloshinskii–Moriya interaction (DMI), an asymmetric exchange interaction, is considered while evaluating the critical current density. We were able to demonstrate that the required critical current density decreases by 99.5%, while the switching speed increases by 86.67% in the proposed external field-free 2-D material-based SOT-MTJ. We have also shown the significance of DMI in the field-free switching of the magnetization without the requirement for additional mechanisms when the device shrank down to the subnanometer regime.

Effect of column-ordered pinning centers on the critical current density of superconducting tape

This study investigates the effects of pinning centers on the critical current density of superconducting tapes. The equation of vortices in the two-dimensional simulation cell was written using the equation of motion in a viscous medium. The vortex dynamics were studied by considering the interaction between the vortices and pinning centers and the effects of temperature fluctuations. A specific number of vortices was randomly distributed in the simulation cell to examine their dynamics. The equation of motion for vortices was solved by considering the periodic boundary conditions. Our investigations showed that the critical current density decreases by increasing magnetic flux. Compared to the triangular lattice, it was found that the critical current density of a column-ordered pinning centers lattice could be improved for fields higher than the first matching field.

Study of Quench Behavior of No-Insulation REBCO Pancake Considering Complex Critical Current Density Distribution

This study analyzes the quench behavior of No-insulation (NI) REBCO pancake. An quench analysis simulation model considering the complex critical current density distribution was developed and validated experimentally. The developed model includes an equivalent circuit model, a thermal model, a magnetic model, and a magnet critical current estimation model. The simulation model is based on a co-simulation using MATLAB and COMSOL, considering the balance of accuracy and computation time. The lumped parameter of the NI magnet was used to solve the governing equations of the equivalent circuit module and thermal module in MATLAB. To accurately calculate the non-uniform electromagnetic field distribution over the coil, a magnetic finite element method was used in COMSOL. A neural network was used to predict the temperature-field-angle dependent magnet critical current. Considering the numerical convergence at different over-current phases, adaptive simulation was used to reduce the total simulation time. The quench behavior was analyzed using the proposed model, and the results were verified experimentally. The simulation results showed that when the azimuthal current is greater than the magnet critical current, it will gradually saturate and approach the magnet critical current, and then decrease at a similar speed. The rate of decrease is equal to the rate of decrease of the magnet critical current. Through the analyses, the corresponding sudden-discharging behavior was simulated and analyzed. The proposed model can be used to perform the post-quench and sudden discharging analyses of pancake coil and has promising applications in multi-pancake post-quench studies.

Long-time stable and high-performance YBa2Cu3O7 nanoSQUIDs with more interfaces

We report on the fabrication and characterization of nanopatterned superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions in epitaxially grown multilayer YBa2Cu3O7 (YBCO)/SiTrO3 (STO) thin films. Nanopatterning is performed by Ga+ focused ion beam milling. The evolution of the electric transport and noise properties of the YBCO nanoSQUIDs over a time span of more than one year are recorded and analyzed. We find that the multilayer YBCO/STO nanoSQUIDs show stable and high performance over time. The critical current decreases within ∼40 days by 30%–50% and then remains almost constant without obvious decline trend for nanoSQUIDs with STO layer as the interface even more than 380 days, which demonstrates the superiority of STO as the capping layer. Moreover, we find that the multilayer nanoSQUIDs have about an order of magnitude smaller low-frequency excess flux noise (compared to similar single layer devices) with root-mean-square spectral density ∼5–6 µΦ0/Hz1/2 at 1 Hz. For one device, we show that the low-frequency excess noise does not degrade within three months.

Axial Compressive Stress Dependence of Critical Current of REBCO Double-Pancake Coil

This study investigated the axial compressive stress dependence of the critical current of Rare Earth Barium-Copper-Oxide (REBCO) double-pancake coils. We measured the critical currents of REBCO coils cooled by liquid nitrogen while various axial compressive stresses from 0 MPa to 130 MPa were applied. After each stress was applied, the critical currents of the coils were also measured without the stress application to examine if the REBCO coils were irreversibly deteriorated. The experimental results showed that the REBCO coils were deteriorated at more than 100 MPa and the critical currents of the coils decreased by about 30% by applying 120 MPa to the coils. Moreover, reversible reduction of the critical currents of the coils were observed while the stresses of less than 100 MPa were applied. The reversible critical current decrease occurred even at a small stress, and the critical currents of the coils were reversibly reduced by about 15% while the stress of 100 MPa was applied. This study quantitatively clarified the dependence of the critical current of the REBCO coil on the axial compressive stress and the limitation of the axial compressive stress to prevent the deterioration of the REBCO coil.

Flux periodic oscillations and phase-coherent transport in GeTe nanowire-based devices

Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. We report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations measured on GeTe nanowires with Au contacts, a phase-coherence length of about 280 nm at 0.5 K is determined. The distinct phase-coherence is confirmed by the observation of Aharonov–Bohm type oscillations for parallel magnetic fields. We interpret the occurrence of these magnetic flux-periodic oscillations by the formation of a tubular hole accumulation layer. For Nb/GeTe-nanowire/Nb Josephson junctions we obtained a critical current of 0.2 μA at 0.4 K. By applying a perpendicular magnetic field the critical current decreases monotonously with increasing field, whereas in a parallel field the critical current oscillates with a period of the magnetic flux quantum confirming the presence of a tubular hole channel.

Влияние облучения протонами и ионами высоких энергий на структуру и свойства композитных ВТСП-2 лент

The paper considers the effect of radiation defects caused by irradiation with protons (2.5 MeV), heavy ions 132Xe27+ (167, 80, 40 MeV), 86Kr17+(107 MeV), 40Ar8+(48 MeV), on the critical parameters of HTSC-2 tapes based on compounds YBa2Cu3O7 – x and GdBa2Cu3O7 – x. The results of calculations based on the model of the thermal peak of the ion track sizes are presented. The projective ranges of ions and protons in these samples are calculated. The radiation resistance of the studied samples to ion and proton radiation of the indicated energies is determined. The performed studies made it possible to detect, at low fluences of irradiation with heavy ions, an increase in the critical current (Ic), an improvement in the adhesion between the superconducting layer and the substrate, and a decrease in internal stresses in the HTSC layer. At higher values of fluences, the critical current and critical temperature decrease. It is important that the decrease in Ic begins at lower fluences than Tc.

GdBCO (123) HTS 2G tapes superconducting characteristics investigation under the pulsed electron beam exposure impact

The 2nd generation composite HTS tapes samples based on the GdBCO (123) compound were irradiated in a pulsed electron accelerator (Terek-2 facility, IOF RAS) through a tantalum target to determine the thermal and shock loads effect at the tantalum-HTS boundary on the HTS tapes superconducting characteristics. Scanning Hall magnetometry was used to characterize the HTS samples after the electron irradiation exposure. The thermal modes and shock load in a composite superconductor under the irradiation influence are estimated depending on the magnitude of energy contribution to the target. At a silver surface temperature equal to the melting temperature, the critical current value drops by 87% of the initial value. At lower energy, a weaker critical current decrease was observed. The temperature effects and shock waves roles under the irradiation is discussed.

Modification of a Second-Generation HTS Tape Based on GdBCO(123) under Pulsed Electron Beam Irradiation

Samples of composite second-generation high-temperature superconducting (HTS) tapes based on the GdBCO (123) compound have been irradiated in a pulsed electron accelerator (system Terek-2, GPI RAS) through a tantalum target in order to determine how the thermal and shock loads arising on the tantalum-HTS interface affect the superconducting parameters of HTS tapes. Scanning Hall magnetometry was applied to characterize HTS samples subjected to electron irradiation. The thermal modes and shock load in a composite superconductor under irradiation are estimated as functions of the energy absorbed by the target. At a silver surface temperature equal to the melting point, the critical current decreases by 87% from the initial value. At lower irradiation energies, the decrease in the critical current is smaller. The role of the temperature effects and shock waves under this irradiation is discussed.

Modeling and Analysis of a Strongly Coupled Series–Parallel-Compensated Wireless Power Transfer System

In the wireless charging of high-power electric buses or low-power consumer electronics, the charging distance is normally very small, leading to a strong coupling. Current distortion occurs for a wireless power transfer (WPT) system with a strong coupling. This paper investigates the strongly coupled series–parallel (SP)-compensated WPT system, where the first harmonic approximation (FHA) is no longer valid. A mathematical model of an SP-compensated WPT system based on differential equations is built. There are two sinusoidal components with significantly different frequencies existing in the transmitter and receiver currents, which cause the current distortion. The proposed model and FHA are compared, and the proposed model is more accurate than FHA. The estimations of the current root-mean-square values from FHA tend to be smaller than those from the proposed model and the measured values. In addition, it is found that with the increasing rectifier dc current, the system transits from zero voltage switching to zero current switching. The critical rectifier dc current decreases with the increasing operating frequency. The experimental results verify the effectiveness of the proposed model. The proposed model promotes a better design of an SP-compensated strongly coupled WPT system.

Effect of monotonic and cyclic axial tensile stress on the performance of superconducting CORC® wires

High-current superconducting CORC® wires, wound from RE-Ba2Cu3O7−δ (REBCO) coated conductors, are being developed for use in high-field magnets that would allow operation at magnetic fields exceeding 20 T. The combination of high engineering current densities and high magnetic fields results in large Lorentz forces acting on the CORC® wire that could cause irreversible degradation to its performance. The effect of axial tensile stress on the critical current of CORC® wires containing annealed solid copper formers has been measured in liquid nitrogen to determine the irreversible stress limit at which irreversible degradation occurs. The results show no significant change in critical current before the irreversible stress limit is reached, after which the critical current decreases irreversibly with applied stress. The irreversible stress limit as high as 177 MPa depends on the yield strength of the former, the number of superconducting tapes wound into the CORC® wire and the angle at which the tapes are wound. Although the irreversible stress limit of CORC® wires is lower than a rudimentary rule of mixtures estimation would suggest, the irreversible strain limit, as high as 0.85%, exceeds that of single REBCO tapes. Both effects are likely the result of the helical fashion in which the REBCO tapes are wound into CORC® wires. The performance of CORC® wires was also measured as a function of axial tensile stress fatigue cycling in liquid nitrogen. No significant performance degradation was measured up to 100 000 cycles as long as the peak stress remained below the irreversible stress limit. Only once the peak stress was increased significantly above the irreversible stress limit would the critical current suddenly decrease with stress cycles. The results indicate that CORC® wires have matured into extremely robust high-current magnet conductors capable of withstanding high levels of axial tensile stress and strain. The irreversible stress limit of CORC® wires could be increased further by using stronger formers and winding the REBCO tapes at comparable angles, while the irreversible strain limit could potentially be increased by tailoring the winding angle of the REBCO tapes, making CORC® wires one of the strongest and most elastic high-current superconducting magnet conductors available.

Nonlinear Critical-Current Thermal Response of an Asymmetric Josephson Tunnel Junction

We theoretically investigate the critical current of a thermally-biased SIS Josephson junction formed by electrodes made by different BCS superconductors. The response of the device is analyzed as a function of the asymmetry parameter, $r=T_{c_1} /T_{c_2}$. We highlight the appearance of jumps in the critical current of an asymmetric junction, namely, when $r\neq1$. In fact, in such case at temperatures at which the BCS superconducting gaps coincide, the critical current suddenly increases or decreases. In particular, we thoroughly discuss the counterintuitively behaviour of the critical current, which increases by enhancing the temperature of one lead, instead of monotonically reducing. In this case, we found that the largest jump of the critical current is obtained for moderate asymmetries, $r\simeq3$. In view of these results, the discussed behavior can be speculatively proposed as a temperature-based threshold single-photon detector with photon-counting capabilities, which operates non-linearly in the non-dissipative channel.

Electrical, Structural, and Thermal Properties of Ferrite/Superconductor (Ni0.5Zn0.5Fe2O4)x/YBa2Cu3O7-δ) Nanocomposite Materials

The superconductor YBa2Cu3O7-δ (Y123) is prepared by the solid-state route, while the nanosized Ni0.5Zn0.5Fe2O4 (Ni-Zn ferrite) is synthesized via citric acid sol-gel technique. The composite material of the type (Ni0.5Zn0.5Fe2O4)x/YBa2Cu3O7-δ, where x = 0.00, 0.03, 0.10, and 0.50 wt%, is prepared by a modified solid-state method to study the effect of doping Ni-Zn nanosized ferrite on the superconducting, structure, and thermal properties of Y123. The prepared samples show Meissner effect which confirmed the presence of superconducting phase at liquid nitrogen temperature (77 K). The prepared samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion of X-ray (EDX), infra-red (IR) spectroscopy, thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), DC resistance by four-probe, and critical current density measurements. It was found that by increasing Ni0.5Zn0.5Fe2O4 nanoparticles in the Y123 compound, the unit cell volume of the orthorhombic phase decreases, but the transition temperature Tc and critical current density Jc decrease for low doping (x = 0.03 wt%) then increase (x = 0.10 and 0.50 wt%) for Ni0.5Zn0.5Fe2O4 doped in Y123. The transport properties of the (Ni0.5Zn0.5Fe2O4)x/YBa2Cu3O7-δ composite as electrical resistance R, Tc, and Jc are interpreted according to Bean’s critical state model and charge-vortex interaction combined with vortex pinning in Y123 high-Tc mixed-state type-II superconductor. The critical current density Jc decreased for low doping of nanosized Ni-Zn ferrite into Y123 with ratio of 0.03 wt% and increased by increasing doping ratio to 0.50 wt%. The physical properties are improved by the coexistence of the superconductor Y123 and nanosized Ni-Zn ferrites in nanocomposite material.

Effect of transverse compressive monotonic and cyclic loading on the performance of superconducting CORC® cables and wires

Superconducting CORC® cables and wires have become practical conductors for use in high-field magnets for fusion machines and particle accelerators by demonstrating their ability to carry very high currents in background magnetic fields of up to 20 T. The high mechanical stresses that develop on the CORC® conductor during such operation could result in permanent degradation of the conductor critical current. Transverse compressive stress is one of the predominant mechanical stresses when CORC® cables or wires are bundled into CICCs that allow fusion and detector magnets to operate at currents as high as 100 kA. The effect of transverse compressive load on the critical current of CORC® cables and wires has been investigated at 76 K to determine their irreversible load limit under monotonic loading and load cycling up to 100 000 cycles. The results show a clear effect of the CORC® conductor layout with respect to gap spacing between tapes, thickness of the copper plating surrounding the tapes and thickness of the former onto which the tapes are wound. CORC® cables and wires have demonstrated a remarkable resilience to transverse compressive load cycling where their critical current decreased by no more than a few percent after 100 000 load cycles at peak loads that resulted in an initial decrease in critical current of less than 5%. The results indicate that no significant degradation of CORC® cable and wire performance due to transverse compressive load is expected in large magnets after 100 000 load cycles, as long as the peak load on the conductor does not exceed the irreversible load limit defined at 95% retention in critical current. The irreversible load limit of CORC® conductors could be increased further by increasing the size or hardness of the former that makes up the conductor’s core.

Mechanical—electromagnetic property of stainless sheet laminated BSCCO–2223 wires

In order to elucidate the effect of applied tensile stress/strain on the critical current of DI-BSCCO-2223 wires, the critical current measurement and the tensile test were carried out at 77 K. Neutron diffraction measurements were also performed at 77 K to get direct information about the local strain exerted on the BSCCO superconducting filaments. It was reported that the critical current of BSCCO wires decreases linearly in a small range of tensile stress/strain and that the critical current returns reversibly on reducing the stress/strain. The gradual decrease of critical current was observed within the reversible region. When the tensile stress /strain increased beyond a characteristic value, the critical current decreased rapidly due to the brittle fracture of BSCCO filaments. It is proposed that the reversible stress/strain limit of critical current (Rrec and Arec) can be defined experimentally as the 99% Ic recovery stress (R99%rec) and strain (A99%rec). The reversible strain limit is proposed to consist mainly of three factors of pre-strain, thermal strain and tensile fracture strain of SC filaments themselves. The calculated relation between the Rrec and Arec could well explain quantitatively the observed relation between 99% Ic recovery stress and strain.