Current-induced Field Research Articles

Reduction of Screening Current-Induced Field in a REBCO Coil Wound by Solder-Square-Stacked Wire

REBa 2 Ca 3 O 7-x (REBCO, RE = rare earth) coated conductors are promising conductors for high magnetic field magnet applications. For REBCO magnets, the tape structures and their superconductive characteristic cause a persistent screening current flowing in the tapes. Due to the existence of the screening current induced field (SCIF), the overall magnetic field becomes unstable and shows a temporal drift. This problem limits the application of REBCO in high-precision magnets, such as MRI and NMR. In this paper, the soldered-stacked-square (3S) REBCO wire is proposed to reduce SCIF. To confirm the reduction effect, a solenoid coil and a benchmark coil were wound by the 3S wire and the commercial 5-mm-wide REBCO tape, respectively. The screening current distributions of 3S wire and 5-mm tape were analyzed. Then, the temporal drift of the overall magnetic field and SCIF hysteresis loop were both experimentally measured for comparison. The test results showed that the 3S wire can significantly reduce the negative effects of SCIF.

Generation of 25 T with an all-superconducting magnet system: field profile and field quality measurements of a layer-wound 4 T REBCO insert coil for a 21 T LTS magnet

High temperature superconductors, especially rare-earth barium copper oxide (REBa2Cu3O7−x or REBCO) tapes are an enabling technology for magnetic fields beyond 23.5 T, the limit of the existing technology based on the low temperature superconductors (LTS) niobium–titanium (NbTi) and niobium–tin (Nb3Sn). Due to their higher prices, REBCO tapes are in general only employed as insert coils in an LTS outsert. Due to the tape geometry, the effect of the screening currents is high in REBCO insert coils. The resulting screening current-induced field (SCIF) can distort the shape and magnitude as well as affect the temporal stability of the field after charging. Their influence has to be well understood and it can be a limiting factor in field quality critical applications such as nuclear magnetic resonance magnets. In this work, we present the test results of a layer-wound 4 T REBCO insert coil operated in the background of a 21 T LTS outsert reaching a combined field of 25 T. Using an array of 13 calibrated Hall sensors, we investigate the spatial and temporal effects of the SCIFs at different background fields. Series expansion is employed to separate the contributions of the insert and of the outsert. We present in this work the characterization of the effect of the SCIF on the shape of the field and on its temporal stability in background fields from 0 T up to 19 T. We compare the experimental results with existing simulation models for the validation of the model at high magnetic fields, which had never been done for layer-wound REBCO coils. Based on the experimental data, we demonstrate the necessity of SCIF-aware simulations for REBCO insert coils, regardless of the insert’s relative operating current. Before summarizing and concluding the work, we give a short overview of proposed methods for SCIF reduction in REBCO coils from the literature.

Current-induced switching of YIG/Pt bilayers with in-plane magnetization due to Oersted fields

We report on the switching of the in-plane magnetization of thin yttrium iron garnet (YIG)/Pt bilayers induced by an electrical current. The switching is either field-induced and assisted by a dc current, or current-induced and assisted by a static magnetic field. The reversal of magnetization occurs at a current density as low as 105 A/cm2 and magnetic fields of ∼40 μT, two orders of magnitude smaller than in ferromagnetic metals, and consistent with the weak uniaxial anisotropy of the YIG layers. We use the transverse component of the spin Hall magnetoresistance to sense the magnetic orientation of YIG while sweeping the current. Our measurements and simulations reveal that the current-induced effective field responsible for switching is due to the Oersted field generated by the current flowing in the Pt layer rather than by spin–orbit torques, and that the switching efficiency is influenced by pinning of the magnetic domains.

Experimental and Numerical Studies on a Method to Mitigate Screening Current-Induced Field for No-Insulation REBCO Coils.

We present experimental and numerical studies on a method to mitigate screening current-induced field (SCF) for NI REBCO coil. The SCF is the major field error to incorporate a REBCO insert for a high field LTS/HTS magnet. The field-shaking technique is going to be used to mitigate the SCF of 800-MHz REBCO insert magnet (H800) for MIT 1.3-GHz LTS/HTS NMR magnet (1.3 G). The field-shaking using 500-MHz LTS background magnet generates the SCF in H800, due to huge self and mutual inductances of them. In this paper, we tested the effect of the induced current in the NI REBCO coil on the field-shaking technique to mitigate the SCF. The amount of the induced current was decided by the NI REBCO coil status; the open- or closed-loop coil. We performed the three cases of experimental tests and analyzed them. From the test results, we may conclude that we need to limit the ramp rate of L500 during the field-shaking, to minimize the induced current in the HTS insert which consists of the NI REBCO coil.

Comparison between analytic and numerical approaches to calculate screening current induced field in HTS magnet

Comparison between analytic and numerical approaches to calculate screening current induced field in HTS magnet

Current-induced spin-orbit effective field modulations in synthetic antiferromagnetic structures

Abstract We demonstrate the modulation of spin-orbit torque (SOT) fields in synthetic antiferromagnetic (SAF) structures sandwiched between two heavy metals of opposite spin Hall angles. The SOTs fields that are measured by using harmonic Hall voltage technique, increase with net areal magnetization of the Pt/SAF/Ta structures. The result suggests an antiparallel orientation of the SOT fields in the two ferromagnetic layers. The antiparallel alignment of the SOT fields switches the magnetization of SAF structures with a current density as low as 2.3 × 1011 A/m2.

Evaluation of Near-Field Electromagnetic Shielding Effectiveness at Low Frequencies

Magnetic induction tomography (MIT) is a novel technology for flow measurement offering significant promise in the measurement of multiphase flows containing low-conductivity fluids such as saline water. Such measurements rely on optimal effective shielding to avoid external field interference and extraneous capacitive coupling that can lead to false readings and overestimations of the eddy current-induced fields. The performance of various attenuation materials in the low megahertz frequency spectrum is presented and compared with outcomes from a numerical computational method. The results demonstrate that the shielding mechanism that prevails at low frequencies is that of reflection. Consequently, hard shields such as metals show superior wave attenuation performance for MIT systems operating below 13 MHz. For higher frequencies, the absorption effect on the incident wave path within soft electromagnetic shields presents enhanced shielding properties. This paper also explores the limitations of traditional testing geometry for shielding effectiveness and proposes an alternative approach to near-field, free-space measurement for MIT sensors. The proposed semi-enclosed approach shows enhanced shielding effectiveness measurements compared with the traditional transversal barrier method. The proposed method was used to assess the electromagnetic shielding effectiveness of ferromagnetic and various metallic materials.

Numerical Simulation on Coupling Current for Multifilamentary HTS Wire

Since a high temperature superconducting (HTS) wire such as Bi-2223 (Bi2Sr2Ca2Cu3O) and REBCO((RE)Ba2Cu3O) tapes indicates good superconducting characteristics under high magnetic field, ultra-high field magnets wound HTS wire are applicable to a nuclear magnetic resonance (NMR) spectrometer and magnetic resonance imaging (MRI). The large and long-time-constant screening current is induced in the HTS wire, which is the tape shape and not twisted, and the magnetic field generated by screening current deteriorated the field quality such as temporal stability and spatial homogeneity. Because NMR and MRI requires highly accurate field on temporal stability and spatial homogeneity, it is necessary to investigate the influence of the screening current-induced field. In REBCO tape, the screening current can be reduced by dividing the superconductor layer. However, filaments are electrically connected because they are covered with copper due to strength and thermal stability. On the other hand, a Bi-2223 is wire which multiple superconducting filaments are covered with silver or a silver alloy, therefore, the screening current is smaller than that in a REBCO tape. However, in a Bi-2223 tape, a coupling current flows because of electrical bridge between the filaments. In this study, we discuss coupling current distribution from numerical simulation on the multifilamentary HTS coil which is given the local electrical contact between filaments. The screening-current field decreases with increasing the interval distance. In addition, the current distribution is different depending on the interval distance.

Investigation on Two Techniques for the Reduction of Screening Current-Induced Field Effects in REBCO HTS Coils

Magnetization currents are known to be the cause of field disturbances in high-temperature superconducting REBCO magnets. The tape-like shape of REBCO-coated conductors implies that a magnetic field orthogonal to their flat surface can induce large screening current loops, which in turn generate a screening current-induced field (SCIF). This SCIF results in detrimental effects on the overall field generated by the magnet, lowering its amplitude and disturbing both the field quality and time stability. This paper focuses on the experimental investigation of two techniques for suppressing the detrimental SCIF effects. First, vortex shaking uses an auxiliary ac magnetic field of small amplitude to accelerate the relaxation of the current density in the superconductor, thus, erasing the SCIF in a short period of time. The second technique, the current overshoot, consists in charging the magnet to its nominal operating point with an initial overshoot, which creates a favorable initial current density distribution mitigating the time drift observed on the field. Those techniques were tested on small REBCO single pancake coils at 77 K with different kinds of insulation: full Kapton insulation and metallic cowinding.

Current polarity-dependent manipulation of antiferromagnetic domains.

Antiferromagnets have several favourable properties as active elements in spintronic devices, including ultra-fast dynamics, zero stray fields and insensitivity to external magnetic fields 1 . Tetragonal CuMnAs is a testbed system in which the antiferromagnetic order parameter can be switched reversibly at ambient conditions using electrical currents 2 . In previous experiments, orthogonal in-plane current pulses were used to induce 90° rotations of antiferromagnetic domains and demonstrate the operation of all-electrical memory bits in a multi-terminal geometry 3 . Here, we demonstrate that antiferromagnetic domain walls can be manipulated to realize stable and reproducible domain changes using only two electrical contacts. This is achieved by using the polarity of the current to switch the sign of the current-induced effective field acting on the antiferromagnetic sublattices. The resulting reversible domain and domain wall reconfigurations are imaged using X-ray magnetic linear dichroism microscopy, and can also be detected electrically. Switching by domain-wall motion can occur at much lower current densities than those needed for coherent domain switching.

In situ Kerr and harmonic measurement in determining current-induced effective fields in MgO/CoFeB/Ta

A combination of the harmonic measurement and in situ Kerr imaging was used to experimentally determine the spin–orbit (SO) effective fields in a MgO/CoFeB/Ta structure. Here, we evaluate the SO effective fields through an analytical energy approach by transforming the anomalous Hall effect and planar Hall effect (PHE) voltage into a field dependency while imaging the magnetisation behaviour by differential Kerr microscopy. The analytical fitting to the measurement data indicates the significant coexistence of both a transverse field, , and longitudinal field, , in the longitudinal (HL = –12 Oe, HT = 8 Oe per 106 A cm−2) and transverse (HL = –12 Oe, HT = –17 Oe per 106 A cm−2) measurement schemes, respectively, due to the PHE. Additionally, dendritic-like domains, indicating the influence of the interfacial Dzyaloshinskii–Moriya interaction (DMI) at the CoFeB/Ta interface, were observed by in situ Kerr imaging. Micromagnetic simulations confirm the dendritic domain formation and edge tilting of the magnetisation, as being due to the DMI.

A Field-Shaking System to Reduce the Screening Current-Induced Field in the 800-MHz HTS Insert of the MIT 1.3-GHz LTS/HTS NMR Magnet: A Small-Model Study.

In this paper, we present experimental results, of a small-model study, from which we plan to develop and apply a full-scale field-shaking system to reduce the screening current-induced field (SCF) in the 800-MHz HTS Insert (H800) of the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) currently under construction-the H800 is composed of 3 nested coils, each a stack of no-insulation (NI) REBCO double-pancakes. In 1.3G, H800 is the chief source of a large error field generated by its own SCF. To study the effectiveness of the field-shaking technique, we used two NI REBCO double-pancakes, one from Coil 2 (HCoil2) and one from Coil 3 (HCoil3) of the 3 H800 coils, and placed them in the bore of a 5-T/300-mm room-temperature bore low-temperature superconducting (LTS) background magnet. The background magnet is used not only to induce the SCF in the double-pancakes but also to reduce it by the field-shaking technique. For each run, we induced the SCF in the double-pancakes at an axial location where the external radial field Br > 0, then for the field-shaking, moved them to another location where the external axial field Bz ≫ BR. Due to the geometry of H800 and L500, top double-pancakes of 3 H800 coils will experience the considerable radial magnetic field perpendicular to the REBCO tape surface. To examine the effect of the field-shaking on the SCF, we tested each NI REBCO DP in the absence or presence of a radial field. In this paper, we report 77-K experimental results and analysis of the effect and a few significant remarks of the field-shaking.

Human in-vivo brain magnetic resonance current density imaging (MRCDI)

Magnetic resonance current density imaging (MRCDI) and MR electrical impedance tomography (MREIT) are two emerging modalities, which combine weak time-varying currents injected via surface electrodes with magnetic resonance imaging (MRI) to acquire information about the current flow and ohmic conductivity distribution at high spatial resolution. The injected current flow creates a magnetic field in the head, and the component of the induced magnetic field ΔBz,c parallel to the main scanner field causes small shifts in the precession frequency of the magnetization. The measured MRI signal is modulated by these shifts, allowing to determine ΔBz,c for the reconstruction of the current flow and ohmic conductivity.Here, we demonstrate reliable ΔBz,c measurements in-vivo in the human brain based on multi-echo spin echo (MESE) and steady-state free precession free induction decay (SSFP-FID) sequences. In a series of experiments, we optimize their robustness for in-vivo measurements while maintaining a good sensitivity to the current-induced fields. We validate both methods by assessing the linearity of the measured ΔBz,c with respect to the current strength. For the more efficient SSFP-FID measurements, we demonstrate a strong influence of magnetic stray fields on the ΔBz,c images, caused by non-ideal paths of the electrode cables, and validate a correction method. Finally, we perform measurements with two different current injection profiles in five subjects. We demonstrate reliable recordings of ΔBz,c fields as weak as 1 nT, caused by currents of 1 mA strength. Comparison of the ΔBz,c measurements with simulated ΔBz,c images based on FEM calculations and individualized head models reveals significant linear correlations in all subjects, but only for the stray field-corrected data. As final step, we reconstruct current density distributions from the measured and simulated ΔBz,c data. Reconstructions from non-corrected ΔBz,c measurements systematically overestimate the current densities. Comparing the current densities reconstructed from corrected ΔBz,c measurements and from simulated ΔBz,c images reveals an average coefficient of determination R2 of 71%. In addition, it shows that the simulations underestimated the current strength on average by 24%.Our results open up the possibility of using MRI to systematically validate and optimize numerical field simulations that play an important role in several neuroscience applications, such as transcranial brain stimulation, and electro- and magnetoencephalography.

Size and temperature dependence of M-H loop for Pt/CoFe/IrMn heterojunction

M-H loops of current-driven spin-orbit torque (SOT) devices with NM/FM/AFM structure have been simulated at different temperature from 50-400 K by Hybrid Monte-Carlo micromagnetics in nanodots and large-sized films. The model includes the polycrystalline microstructure. The effects of anisotropy defects size on M-H loops of dots are discussed, which coincide with experimental scaling tendency, where a lower Kmin at the defect center strongly influences the switching coercivity. Both the defect size and the temperature increase reduces the perpendicular coercivity Hc of nanodots. If the equivalent current-induced switching field is set as 100 mT, the critical switchable diameter of nanodots is around 50 nm with defect damage region of Kmin<0.65K, whereas in nearly perfectly deposited films (Kmin→K or insensitive to dot areas change), the critical switchable diameter will increase to around 100 nm [A. Kurenkov, Appl. Phys. Lett., 110, 9, (2017)].

Antiferromagnetic resonance excited by oscillating electric currents

In antiferromagnetic materials the order parameter exhibits resonant modes at frequencies that can be in the terahertz range, making them interesting components for spintronic devices. Here, it is shown that antiferromagnetic resonance can be excited using the inverse spin-Hall effect in a system consisting of an antiferromagnetic insulator coupled to a normal-metal waveguide. The time-dependent interplay between spin-torque, ac spin-accumulation and magnetic degrees of freedom is studied. It is found that the dynamics of the antiferromagnet effects the frequency-dependent conductivity of the normal metal. Further, it is shown that in antiferromagnetic insulators, the resonant excitation by ac spin-currents can be orders of magnitude more efficient than excitation by the current-induced Oersted field.

Magnetic properties and evidence of current-induced perpendicular field in epitaxial ferrimagnetic Mn4N (002) film mixed with (111) phase

The magnetic properties and anomalous Hall effect of the Mn4N (002) film mixed with the (111) phase were investigated. The lessened magnetization of hysteresis loops at a low field and the peak at about 50 K of the temperature-dependent magnetization curves display that magnetocrystalline anisotropies play a significant role in the magnetic properties of the film. Moreover, the centres of the anomalous Hall loops are shifted to the left for positive currents and show the opposite shift for negative currents, which may indicate the presence of current-induced effective fields. It is proposed that the (111) phase in the film could not only supply the magnetization m derivation from the c aixs but also afford asymmetric interfaces to induce effective fields. The effective perpendicular field arises from the perpendicular components of the effective fields.

Spin-orbit torque induced magnetization switching in Co/Pt multilayers

Spin-orbit torque (SOT)-induced magnetization switching in Co/Pt multilayer structures with a Pt buffer layer is studied aiming to realize SOT-magnetic random access memory (MRAM) devices with high thermal stability. Current-induced magnetization switching and effective fields are measured using Hall-bar devices. The switching efficiency, defined as a ratio of the areal anisotropy energy density to switching current density, increases with increasing the number of Co/Pt stacks. This trend is in accordance with the stacking number dependence of effective fields per unit current density. The effective spin-Hall angle of the Pt buffer layer for the sample with multiple Co/Pt stacks is significantly larger than that of Pt previously reported, suggesting a generation of SOT in Co/Pt multilayers. These results indicate that Co/Pt multilayers are promising for SOT-MRAM devices possessing high thermal stability and small switching current.

Determination of eddy current response with magnetic measurements.

Accurate mutual inductances between magnetic diagnostics and poloidal field coils are an essential requirement for determining the poloidal flux for plasma equilibrium reconstruction. The mutual inductance calibration of the flux loops and magnetic probes requires time-varying coil currents, which also simultaneously drive eddy currents in electrically conducting structures. The eddy current-induced field appearing in the magnetic measurements can substantially increase the calibration error in the model if the eddy currents are neglected. In this paper, an expression of the magnetic diagnostic response to the coil currents is used to calibrate the mutual inductances, estimate the conductor time constant, and predict the eddy currents response. It is found that the eddy current effects in magnetic signals can be well-explained by the eddy current response determination. A set of experiments using a specially shaped saddle coil diagnostic are conducted to measure the SUNIST-like eddy current response and to examine the accuracy of this method. In shots that include plasmas, this approach can more accurately determine the plasma-related response in the magnetic signals by eliminating the field due to the eddy currents produced by the external field.

Ratchet effects in superconducting ring-shaped devices

The vortex ratchet effects in superconducting ring-shaped NbN devices are experimentally investigated. We have studied three types of samples: a narrowing eccentric circular loop, standard circular loop, and widening eccentric circular loop. Using the time-dependent Ginzburg–Landau simulations, we demonstrate that the sample edges significantly influence the dynamics of the vortices entering the samples. Through transport measurements with dc and ac currents, strong rectified voltage responses were observed for all the samples over a wide range of temperatures, critical currents, and magnetic fields. Moreover, there are some differences in the detailed ratchet behaviors among them. Furthermore, we also observed the ratchet effects in our samples caused by the current-induced field, which deserves further investigations.

A Study on Mitigation of Screening Current Induced Field with a 3-T 100-mm Conduction-Cooled Metallic Cladding REBCO Magnet

A 3-T 100-mm winding bore conduction-cooled REBCO magnet has been designed, constructed, and tested. It consists of a stack of double pancake coils wound with metallic cladding (MC) REBCO tapes, for which 1–2 $\mu {\text{m}}$ stainless steel was clad in a hermetic way around the tapes. This approach, first proposed by the SuNAM Company Ltd. in 2015, is expected to substantially reduce the charging time delay of a no-insulation magnet, which in turn will be effective in the manipulation of screening currents in such a way as to improve the field homogeneity of magnets, particularly for nuclear magnetic resonance or MRI. This paper presents test results for our approach on the current sweep cycling, which is used to reduce the screening current-induced fields in a newly constructed 3-T 100-mm MC all-REBCO magnet