Ultra-low Field Research Articles

Giant Electrostriction Enabled by Defect-Induced Critical Phenomena in Relaxor Ferroelectric Polymers

Polymers that generate large shape changes under electric stimulation are of great interest for many applications. Recently, it was shown that converting a small amount of chlorofluoroethylene (CFE) in relaxor ferroelectric poly(vinylidene fluoride–trifluoroethylene–CFE) (PVDF-TrFE-CFE) terpolymer into fluorinated alkyne (FA) creates P(VDF-TrFE-CFE-FA) tetrapolymers with giant electromechanical (EM) response at ultralow electric fields (<50 MV/m). We investigate the microscopic origin of this effect and show that converting the bulky CFE into small-size FA defects dramatically weakens the relaxor behavior. Importantly, P(VDF-TrFE-CFE-FA) tetrapolymers with near 2 mol % FA exhibit a diffused critical endpoint transition region at which the energy barriers for switching from nonpolar to polar molecular conformations become small. Consequently, a small change of the electric field induces a large electroactuation, which can enable novel applications. This work opens up a totally new approach to designing ferroelectric polymers that generate large responses at ultralow electric fields.

EMD based statistical analysis of nighttime pre-earthquake ULF electric field disturbances observed by CSES

To explore the correlation between earthquakes and spatial ultra-low frequency electric field disturbances and to study the phenomenon of seismic ionospheric disturbances, this study uses 3 years of electric field ULF band data from 2019 to 2021 observed by the electric field detector carried by the CSES to identify anomalous disturbances using the anomaly automatic detection algorithm based on empirical mode decomposition for the 2,329 seismic events of magnitude not less than 5.0 and the electric field ULF disturbances in this period are analyzed by Superposed Epoch Analysis, and the statistical results are compared and analyzed in depth by earthquake location and different magnitudes in terms of both spatial and temporal scales and spatial distribution. The results show that: 1) There is a correlation between earthquakes of magnitude not less than 5.0 and ultra-low frequency disturbances in the electric field. The abnormal disturbance mainly occurred 11 days before the earthquake, 2 days before the earthquake to the day of the earthquake, and the location of the earthquake is within 200 km from the epicenter. 2) Sea earthquakes can observe more pre-seismic anomalous electromagnetic disturbances than land earthquakes. 3) In terms of earthquake magnitude, the larger the magnitude, the earlier the pre-earthquake anomalous disturbances appear and the wider the range of anomalies. This study provides an effective way to explain seismic ionospheric phenomena, and also provides a reference for the application of electromagnetic monitoring satellites in earthquake prediction and early warning as well as disaster prevention and mitigation.

Superior CoO/SiC nanowire field emitters with substantially increased stable emission sites: ultralow turn-on field, high current density and high stability

We explored high-performance field emitters with numerous CoO nanoparticles surrounding SiC nanowires for increasing emission sites, which exhibited ultralow turn-on field (0.40 V μm−1), high current density (18.6 mA cm−2) and high emission stability.

Three-dimensional branched SiC nanowire field emitters with single-crystal integrated structures and increased emission sites: ultralow turn-on field and high stability

High-performance single-crystal integrated three-dimensional nanowire field emitters with high-density SiC dendrites surrounding SiC backbone nanowires were explored.

High-Sensitivity and High-Resolution RI Sensor With Ultrawide Measurement Range Based on NCF With Large Offset Splicing and MPF Interrogation

A high-sensitivity and high-resolution refractive index (RI) sensor with ultrawide measurement range based on no core fiber (NCF) with large offset splicing and microwave photonic filter (MPF) interrogation is demonstrated theoretically and experimentally for the first time. In the sensor, an inline Mach–Zehnder interferometer (MZI) based on NCF with large offset splicing is used as the sensing head, and a single-passband MPF is designed to interrogate the sensing signal. Compared with the traditional interrogation in optical domain, the MPF interrogation converts the periodic interference pattern in optical spectrum into the nonperiodic bandpass frequency response in radio frequency (RF) domain, so the measurement range is unlimited in principle. Owing to the large offset splicing in the inline MZI structure, the sensitivity of the sensor as high as −1.1054 GHz/RIU is obtained in the experiment. More importantly, MPF interrogation improves the resolution by an order of magnitude to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9\times10$ </tex-math></inline-formula> −7 RIU. Meanwhile, the sensitivity and the resolution of the sensor are easily adjusted by changing the length of dispersive media. The high sensitivity and high resolution of the sensor have potential applications in ultralow concentration biochemical detection field.

Ultra-Low Geomagnetic Field Intensity in the Mesoproterozoic Based on the Dike Complex Rocks of the Bunger Oasis (1133 Ma, East Antarctica)

Ultra-Low Geomagnetic Field Intensity in the Mesoproterozoic Based on the Dike Complex Rocks of the Bunger Oasis (1133 Ma, East Antarctica)

In vivo T1 mapping of neonatal brain tissue at 64 mT.

Ultralow-field (ULF) point-of-care MRI systems allow image acquisition without interrupting medical provision, with neonatal clinical care being an important potential application. The ability to measure neonatal brain tissue T1 is a key enabling technology for subsequent structural image contrast optimization, as well as being a potential biomarker for brain development. Here we describe an optimized strategy for neonatal T1 mapping at ULF. Examinations were performed on a 64-mT portable MRI system. A phantom validation experiment was performed, and a total of 33 in vivo exams were acquired from 28 neonates with postmenstrual age ranging from 31+4 to 49+0 weeks. Multiple inversion-recovery turbo spin-echo sequences were acquired with differing inversion and repetition times. An analysis pipeline incorporating inter-sequence motion correction generated proton density and T1 maps. Regions of interest were placed in the cerebral deep gray matter, frontal white matter, and cerebellum. Weighted linear regression was used to predict T1 as a function of postmenstrual age. Reduction of T1 with postmenstrual age is observed in all measured brain tissue; the change in T1 per week and 95% confidence intervals is given by dT1 =-21 ms/week [-25, -16] (cerebellum), dT1 =-14 ms/week [-18, -10] (deep gray matter), and dT1 =-35 ms/week [-45, -25] (white matter). Neonatal T1 values at ULF are shorter than those previously described at standard clinical field strengths, but longer than those of adults at ULF. T1 reduces with postmenstrual age and is therefore a candidate biomarker for perinatal brain development.

High-resolution ultra-low field magnetic resonance imaging with a high-sensitivity sensing coil.

We present high-resolution magnetic resonance imaging (MRI) at ultra-low field (ULF) with a proton Larmor frequency of around 120 kHz. The key element is a specially designed high-sensitivity sensing coil in the shape of a solenoid with a few millimeter gap between windings to decrease the proximity effect and, hence, increase the coil's quality ( ) factor and sensitivity. External noise is strongly suppressed by enclosing the sensing coil in a copper cylindrical shield, large enough not to negatively affect the coil's factor and sensitivity, measured to be 217 and 0.47 fT/Hz , respectively. To enhance small polarization of proton spins at ULF, a strong pulsed 0.1 T prepolarization field is applied, making the signal-to-noise ratio (SNR) of ULF MRI sufficient for high-quality imaging in a short time. We demonstrate ULF MRI of a copper sulfate solution phantom with a resolution of and SNR of 10. The acquisition time is 6.3 min without averaging. The sensing coil size in the current realization can accommodate imaging objects of 9 cm in size, sufficient for hand, and it can be further increased for human head imaging in the future. Since the in-plane resolution of is typical in anatomical medical imaging, this ULF MRI method can be an alternative low-cost, rapid, portable method for anatomical medical imaging of the human body or animals. This ULF MRI method can supplement other MRI methods, especially when such methods are restricted due to high cost, portability requirement, imaging artifacts, and other factors.

Polarizing insensitive nuclei at ultralow magnetic fields using parahydrogen: A facile route to optimize adiabatic magnetic field sweeps.

Parahydrogen induced polarization (PHIP) provides a powerful tool to enhance inherently weak nuclear magnetic resonance signals, particularly in biologically relevant compounds. The initial source of PHIP is the non-equilibrium spin order of parahydrogen, i.e., dihydrogen, where the two protons make up a singlet spin state. Conversion of this spin order into net magnetization of magnetic heteronuclei, e.g., 13C, provides one of the most efficient ways to exploit PHIP. We propose a facile route to increase the performance of PHIP transfer in experiments with adiabatic sweeps of the ultralow magnetic field. To date, this technique yields the highest efficiency of PHIP transfer, yet, it has been mostly utilized with linear field sweeps, which does not consider the underlying spin dynamics, resulting in sub-optimal polarization. This issue was previously addressed by using the "constant" adiabaticity method, which, however, requires extensive calculations for large spin systems. In this work, the field sweep is optimized by utilizing the field dependence of the average 13C polarization. Both the experimental detection and the numerical simulation of this dependence are straightforward, even for complex multi-spin systems. This work provides a comprehensive survey of PHIP transfer dynamics at ultralow fields for two molecular systems that are relevant for PHIP, namely, maleic acid and allyl pyruvate. The proposed optimization allowed us to increase the resulting 13C polarization in 13C-allyl pyruvate from 6.8% with a linear profile to 8.7% with an "optimal" profile. Such facile optimization routines are valuable for adiabatic experiments in complex spin systems undergoing rapid relaxation or chemical exchange.

Real-Time Compensation of Power Frequency Interference and Eddy Current Magnetic Field for SQUID-Based Ultralow Field Magnetic Resonance Imaging

In ultralow field magnetic resonance imaging (ULF MRI), extremely sensitive superconducting quantum interference devices (SQUIDs) and prepolarization field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathrm{B}}_{\mathrm{P}}$</tex-math></inline-formula> ) are used to enhance the signal-to-noise ratio (SNR). By placing the ULF MRI system in a conductively shielded room (CSR), external electromagnetic interference at frequencies in the kHz range, which is the typical ULF MRI imaging frequency range, can be effectively suppressed. However, CSR has a limited shielding effect on electromagnetic interference below 1 kHz, which are mainly the power frequency interference and its harmonics. In addition, eddy current is induced by the quick turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathrm{B}}_{\mathrm{P}}$</tex-math></inline-formula> current in the metal plates of CSR, which in turn induces an exponentially decaying magnetic field <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EC</sub> in the imaging region. Power frequency interference and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EC</sub> dominate the voltage output of SQUID readout electronics and even exceed its output range. Thus, the gain of the SQUID readout electronics has to be decreased by reducing its feedback resistance. In this case, the analog-to-digital converter noise may start to contribute to the noise of ULF MRI. In this article, we describe a real-time compensation technique to suppress the large variation of the signal channel output due to power frequency interference and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EC</sub> . The experimental results demonstrate that this technique can significantly reduce the maximal variation range of the signal channel output by 79.2% and improve the SNR of ULF MRI images by 4.2 times. With this technique, we may use SQUID detectors with larger imaging areas and/or stronger <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mathrm{B}}_{\mathrm{P}}$</tex-math></inline-formula> fields to further improve SNR without the reduction of the gain.

Measurement of In-Field E-J Characteristics in Multi-Filamentary Bi-2223 Tapes at Ultra-Low Electric-Field Down to Around 10−13 V/m

We have developed a method to investigate in-field electric-field v.s. current-density (E-J) characteristics at ultra low electric field criterion around 10-13 V/m in a multi filamentary Bi-2223 tape at 5 K based on scanning Hall probe microscopy (SHPM). Bi-2223 is widely used in high field applications, e.g. a 30.5 T NMR magnet system is currently being developed in Japan. Ultra-low electric field is induced in such magnets operating in persistent current mode with extremely steady field. Namely, E-J characteristics need to be clarified around that criterion. E-J characteristics can be estimated from decay of magnetization in the whole sample, assuming the uniformity of J. But for multi filamentary Bi-2223 tape, magnetization current path should be clarified. In this study, we measured local magnetic field profile as a function of time for Bi-2223 tapes using SHPM. Additionally, we carried out flux annealing measurements to realize ultra-low electric field induced in the tape. As a result, two main messages we obtained. Experimental field profile or sheet current density obtained from the inverted Biot-Savart law indicated that filaments remain coupled up to high field, and even at ultra-low E criterion. Moreover, local E-J characteristics around 10-13 V/m are obtained at 5 K and 4.5 T of perpendicular magnetic field, which is comparable perpendicular field component in a Bi-2223 insert coil in the high field NMR magnet system. Ic properties at 10-12 V/m decrease to nearly 50%, in comparison to the transport Ic properties at 10-4 V/m.

A Novel 4H-SiC JBS-Integrated MOSFET With Self-Pinching Structure for Improved Short-Circuit Capability

Monolithic integration of junction barrier-controlled Schottky (JBS) diode with SiC MOSFET (termed JMOS) offers unique advantages. However, the short-circuit (SC) ruggedness issue stands in the way of the development of the conventional JMOS. The purpose of this numerical study is to investigate a new 4H-SiC JMOS with a self-pinching (SP) structure formed in the JFET region (termed SP-JMOS). The SP structure features that an N-type current spread layer is sandwiched between the P+ layer and the buried P-shield layer, forming a lateral JFET channel. In the forward conduction state, the lateral JFET channel self-pinches off and clamps the potential, thus limiting the saturation current of the device. In the blocking state, both the P+ layer and the buried P-shield layer collaboratively shield the Schottky contact and the SiC/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface from a high electric field for long-term reliable operation. Numerical simulation results show that the proposed SP-JMOS not only withstands a roughly <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.6\times $ </tex-math></inline-formula> longer SC withstanding time than that of the conventional JMOS, but also shows an ultralow oxide electric field in the SP-JMOS.

Magnetic shieldless ultra-low-field MRI with an optically pumped magnetometer

Among magnetic resonance imaging (MRI) techniques, ultra-low field (ULF) MRI has the potential to significantly lower the cost of implementation and maintenance, as well as the size of the scanning system. Due to the small amplitude of the signals produced by ULR-MRI, extremely sensitive magnetic sensors are required. Optically pumped magnetometers (OPMs) have been proposed for use in ULF-MRI as ultra-sensitive magnetic sensors capable of detecting very small signals. However, the cost of a ferromagnetic magnetic shield is often not affordable for many applications. By increasing the Larmor frequency, the influence of low-frequency magnetic noise can be mitigated, allowing OPM to be operated without the use of a magnetic shield chamber. This lowers the cost of the magnetic shield and further raises the signal strength, resulting in benefits such as non-prepolarization. We present a method for implementing the ULF-MRI using low-cost OPMs in this study. The Larmor frequency was adjusted to 300 kHz, and three-dimensional (3D) images of a phantom were acquired with a digital resolution of 3 × 3 × 3 mm3 using a static magnetic field of 7.05 mT without using a magnetic shield room or a prepolarization coil. Additionally, we corrected the frequency response to acquired images to consider the narrow bandwidth, and the SNR of 3D imaging was 18. The experimental results, we believe, establish a new guideline for higher-performance, lower-cost ULF-MRI that does not require expensive magnetic shielding.

Deuteron-Decoupled Singlet NMR in Low Magnetic Fields: Application to the Hyperpolarization of Succinic Acid.

The reaction of unsaturated substrates with hydrogen gas enriched in the para spin isomer leads to products with a high degree of nuclear singlet spin order. This leads to greatly enhanced NMR signals, with important potential applications such as magnetic resonance imaging (MRI) of metabolic processes. Although parahydrogen-induced polarization has the advantage of being cheap, compact, and mobile, especially when performed in ultralow magnetic fields, efficiency is lost when more than a few protons are involved. This strongly restricts the range of compatible substances. We show that these difficulties may be overcome by a combination of deuteration with the application of a sinusoidally modulated longitudinal field as a well as a transverse rotating magnetic field. We demonstrate a six-fold enhancement in the 13 C hyperpolarization of [1-13 C, 2,3-d2 ]-succinic acid, as compared with standard hyperpolarization methods, applied in the same ultralow field regime.

A frequency-swept, longitudinal detection EPR system for measuring short electron spin relaxation times at ultra-low fields

A frequency-swept longitudinal detection (LOD) EPR system is described for ultra-low field spectroscopy and relaxometry. With the capability of performing simultaneous transmit and receive with −80 dB isolation, this LOD-EPR can capture signals with decay constants in the nanosecond range and in theory even sub-nanosecond range, at fields close to the earth’s magnetic field. The theoretical principles underlying this LOD-EPR are based on a fictitious field that accounts for the Z-axis magnetization polarized by a radiofrequency field alone. The electron spin relaxation time is obtained directly from a previously derived equation that describes the relationship between the relaxation time and the spectral peak position. Herein, the first frequency-swept LOD-EPR system is described in detail, along with experimental measurements of the short relaxation time (∼30 ns) of the free radical, 2,2-diphenyl-1-picrylhydrazyl, at zero to low field.

Controllable volatile-to-nonvolatile memristive switching in single-crystal lead-free double perovskite with ultralow switching electric field

All-inorganic lead-free double perovskite offers a potential material platform for electronic or optoelectronic memory devices owing to its ionic migration-based electrical transport, high photosensitivity, low toxicity, and environmental stability. However, the commonly used polycrystalline perovskite films severely restrict device performance. Herein, we demonstrate a high-performance memristor based on single-crystal double perovskite Cs2AgBiBr6 with an ultralow switching electric field of 6.67 × 104 V m−1 and a high current on/off ratio of 107. Remarkably, the resistive switching of Cs2AgBiBr6 is found to be thickness-dependent, which evolves from volatile threshold to nonvolatile resistance switching with the crystal thickness from 100 to 800 nm. Elemental analysis reveals that the formation of conductive channels in Cs2AgBiBr6 is associated with the migration of Br vacancies with low activation energy. In addition, the formed conductive channels can be annihilated by light illumination with controlled wavelength and intensity, which leads to the realization of optoelectronic memories with separate electrical-writing and optical-erasing processes. Our findings provide deep insights into the ionic migration in single-crystal perovskite and pave the way for its future application in electronic and optoelectronic memory devices.

Early Cambrian renewal of the geodynamo and the origin of inner core structure

Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core.

New palaeodirections and palaeointensity data from extensive profiles through the Ediacaran section of the Volyn Basalt Province (NW Ukraine)

SUMMARY Changes in the geomagnetic field over geologic timescales can be used to study the evolution and processes of its sources in Earth's deep interior. As a stark contrast to the geomagnetic field behaviour of the recent past, the field in the late Neoproterozoic is defined by ambiguous polar wander paths, ultralow field strengths and a period of extreme reversal hyperactivity. Palaeointensity data from this time are still scarce and conclusions made from these data suffer from large uncertainties brought by a low sampling density. In this study, we present new palaeomagnetic and palaeointensity data from quarry outcrops and five deep drill cores, covering the longest and most complete profiles through the Ediacaran units in the Volyn Basalt Province (NW Ukraine) to date. Palaeodirectional analysis of thermal and alternating field demagnetization experiments reveals at least six reversals in the profile as well as four palaeopoles that agree with previously published poles. Whole-rock palaeointensity estimates are derived from a multimethod approach that utilizes thermal and microwave Thellier experiments performed using Coe and IZZI protocols as well as Wilson and double-heating Shaw experiments. These produce instantaneous site-mean palaeointensity estimates in the range of 2.1 ± 0.4 to 11.1 ± 3.5 µT and virtual dipole moments in the range of 0.31 ± 0.06 × 1022 to 1.67 ± 0.53 × 1022 A m2. Small-scale variations of palaeointensities throughout the profile not only show extremely weak field strengths around polarity changes but could also suggest that the field strength never fully recovered between reversals. These new results increase the density of intensity data coverage for Baltica in the Late Ediacaran and, in combination with previously published Ediacaran dipole moments, support an extremely weak field that seems to increase in strength towards the Ediacaran–Cambrian transition.

Challenges in Deploying Low-Field and Ultra-Low Field MRI in Research, Clinical Care, Population Screening, and Direct-to-Consumer Use (P15-7.001)

Challenges in Deploying Low-Field and Ultra-Low Field MRI in Research, Clinical Care, Population Screening, and Direct-to-Consumer Use (P15-7.001)

Utility and Feasibility of Portable Ultra-Low Field MRI in Patients with Progressive Multifocal Leukoencephalopathy (S38.009)

Utility and Feasibility of Portable Ultra-Low Field MRI in Patients with Progressive Multifocal Leukoencephalopathy (S38.009)