Offset Magnetic Field Research Articles

All-optical measurement of magnetic fields for quantum gas experiments

We present an all-optical method to measure and compensate for residual magnetic fields present in a cloud of ultracold atoms trapped in an optical dipole trap. Our approach leverages the increased loss from the trapped atomic sample through electromagnetically induced absorption. Modulating the excitation laser provides coherent sidebands, resulting in a Λ-type pump–probe scheme. Scanning an additional magnetic offset field leads to pairs of sub-natural linewidth resonances, whose positions encode the magnetic field in all three spatial directions. Our measurement scheme is readily implemented in typical quantum gas experiments and has no particular hardware requirements.

North–South Plasma Asymmetry Across Mercury's Near‐Tail Current Sheet

AbstractAmong nearly 300 near‐Mercury tail current sheet crossings performed by the MESSENGER spacecraft, we identified 37 traversals of an asymmetric current sheet, wherein the lobe densities on opposite sides differ by a factor of three or more. These asymmetric current sheet crossings primarily occur on the dawnside. A global magnetohydrodynamic (MHD) simulation was found to be in excellent agreement with the observations. The results suggest that the north–south density asymmetry is caused by solar wind entering via an upstream‐connected window in one hemisphere. Furthermore, the Parker spiral interplanetary magnetic field (IMF) controls the near‐tail density asymmetries, whereas Mercury's offset dipole magnetic field controls those in mid‐ or distant‐tail regions. We propose that hemispheric asymmetries in Mercury's magnetospheric convection occur under strong IMF conditions.

Critical Offset Magnetic PArticle SpectroScopy for rapid and highly sensitive medical point-of-care diagnostics

Magnetic nanoparticles (MNPs) have been adapted for many applications, e.g., bioassays for the detection of biomarkers such as antibodies, by controlled engineering of specific surface properties. Specific measurement of such binding states is of high interest but currently limited to highly sensitive techniques such as ELISA or flow cytometry, which are relatively inflexible, difficult to handle, expensive and time-consuming. Here we report a method named COMPASS (Critical-Offset-Magnetic-Particle-SpectroScopy), which is based on a critical offset magnetic field, enabling sensitive detection to minimal changes in mobility of MNP ensembles, e.g., resulting from SARS-CoV-2 antibodies binding to the S antigen on the surface of functionalized MNPs. With a sensitivity of 0.33 fmole/50 µl (≙7 pM) for SARS-CoV-2-S1 antibodies, measured with a low-cost portable COMPASS device, the proposed technique is competitive with respect to sensitivity while providing flexibility, robustness, and a measurement time of seconds per sample. In addition, initial results with blood serum demonstrate high specificity.

Frequency Mixing Magnetic Detection Setup Employing Permanent Ring Magnets as a Static Offset Field Source.

Frequency mixing magnetic detection (FMMD) has been explored for its applications in fields of magnetic biosensing, multiplex detection of magnetic nanoparticles (MNP) and the determination of core size distribution of MNP samples. Such applications rely on the application of a static offset magnetic field, which is generated traditionally with an electromagnet. Such a setup requires a current source, as well as passive or active cooling strategies, which directly sets a limitation based on the portability aspect that is desired for point of care (POC) monitoring applications. In this work, a measurement head is introduced that involves the utilization of two ring-shaped permanent magnets to generate a static offset magnetic field. A steel cylinder in the ring bores homogenizes the field. By variation of the distance between the ring magnets and of the thickness of the steel cylinder, the magnitude of the magnetic field at the sample position can be adjusted. Furthermore, the measurement setup is compared to the electromagnet offset module based on measured signals and temperature behavior.

A New Design of a CMOS Vertical Hall Sensor with a Low Offset.

Vertical Hall sensors (VHSs), compatible with complementary metal oxide semiconductor (CMOS) technology, are used to detect magnetic fields in the plane of the sensor. In previous studies, their performance was limited by a large offset. This paper reports on a novel CMOS seven-contact VHS (7CVHS), which is formed by adding two additional contacts to a traditional five-contact VHS (5CVHS) to alleviate the offset. The offset voltage and offset magnetic field of the 7CVHS are reduced by 90.20% and 88.31% of those of the 5CVHS, respectively, with a 16.16% current-related sensitivity loss. Moreover, the size and positions of the contacts are optimized in standard GLOBALFOUNDRIES 0.18 μm BCDliteTM technology by scanning parameters using FEM simulations. The simulation data are analyzed in groups to study the influence of the size and contact positions on the current-related sensitivity, offset voltage, and offset magnetic field.

Strontium Ferromolybdate-Based Magnetic Tunnel Junctions

Thin-film strontium ferromolybdate is a promising material for applications in room-temperature magnetic tunnel junction devices. These are spin-based, low-power-consuming alternatives to CMOS in non-volatile memories, comparators, analog-to-digital converters, and magnetic sensors. In this work, we consider the main tasks to be solved when creating such devices based on strontium ferromolybdate: (i) selecting an appropriate tunnel barrier material, (ii) determining the role of the interface roughness and its quantification, (iii) determining the influence of the interface dead layer, (iv) establishing appropriate models of the tunnel magnetoresistance, and (v) promoting the low-field magnetoresistance in (111)-oriented thin films. We demonstrate that (i) barrier materials with a lower effective electronegativity than strontium ferromolybdate are beneficial, (ii) diminution of the magnetic offset field (the latter caused by magnetic coupling) requires a wavy surface rather than solely a surface with small roughness, (iii) the interface dead-layer thickness is of the order of 10 nm, (iv) the tunnel magnetoresistance deteriorates due to spin-independent tunneling and magnetically disordered interface layers, and (v) antiphase boundaries along the growth direction promote the negative low-field magnetoresistance by reducing charge carrier scattering in the absence of the field.

Comparative Modeling of Frequency Mixing Measurements of Magnetic Nanoparticles Using Micromagnetic Simulations and Langevin Theory.

Dual frequency magnetic excitation of magnetic nanoparticles (MNP) enables enhanced biosensing applications. This was studied from an experimental and theoretical perspective: nonlinear sum-frequency components of MNP exposed to dual-frequency magnetic excitation were measured as a function of static magnetic offset field. The Langevin model in thermodynamic equilibrium was fitted to the experimental data to derive parameters of the lognormal core size distribution. These parameters were subsequently used as inputs for micromagnetic Monte-Carlo (MC)-simulations. From the hysteresis loops obtained from MC-simulations, sum-frequency components were numerically demodulated and compared with both experiment and Langevin model predictions. From the latter, we derived that approximately 90% of the frequency mixing magnetic response signal is generated by the largest 10% of MNP. We therefore suggest that small particles do not contribute to the frequency mixing signal, which is supported by MC-simulation results. Both theoretical approaches describe the experimental signal shapes well, but with notable differences between experiment and micromagnetic simulations. These deviations could result from Brownian relaxations which are, albeit experimentally inhibited, included in MC-simulation, or (yet unconsidered) cluster-effects of MNP, or inaccurately derived input for MC-simulations, because the largest particles dominate the experimental signal but concurrently do not fulfill the precondition of thermodynamic equilibrium required by Langevin theory.

A Non-Contact Micro-Ampere DC Current Digital Sensor Based on the Open-Loop Structure

The non-contact accurate measurement of weak direct current (dc) is very important for various fields, such as the online insulation monitoring of power cables. Compared with other dc current sensors, the magnetic modulator is more suitable for dc measurement with its high sensitivity, low noise, and stable working. Most magnetic modulators adopt the closed-loop structure to achieve accurate measurement of large current. And the magnetic modulator only realized the accurate measurement of mA current in the filed of weak dc current, which is difficult to meet the $\mu \text{A}$ current measurement requirements of the online insulation monitoring. Therefore, this article proposes a new structure of open-loop digital magnetic modulator for precision measurement of $\mu \text{A}$ dc current. It adopts an annular shielding layer to weaken the zero offset and stray magnetic field. And a low-frequency excitation source, which can be adjusted with the ambient temperature, is used to improve the temperature stability. Instead of the traditional analog demodulation method, a digital demodulation algorithm is designed to reduce the measurement error caused by the zero offset and excitation source fluctuation. Measurement results show that the new sensor with the absolute error of 0.5 $\mu \text{A}$ can measure 5-1000 $\mu \text{A}$ dc current with a relative error of 0.05% in the full scale. And the sensor can effectively suppress the measurement error caused by the fluctuation of the excitation source and the ambient temperature to less than 0.5%.

The upper atmospheres of Uranus and Neptune.

We review the current understanding of the upper atmospheres of Uranus and Neptune, and explore the upcoming opportunities available to study these exciting planets. The ice giants are the least understood planets in the solar system, having been only visited by a single spacecraft, in 1986 and 1989, respectively. The upper atmosphere plays a critical role in connecting the atmosphere to the forces and processes contained within the magnetic field. For example, auroral current systems can drive charged particles into the atmosphere, heating it by way of Joule heating. Ground-based observations of H3+ provides a powerful remote diagnostic of the physical properties and processes that occur within the upper atmosphere, and a rich dataset exists for Uranus. These observations span almost three decades and have revealed that the upper atmosphere has continuously cooled between 1992 and 2018 at about 8 K/year, from approximately 750 K to approximately 500 K. The reason for this trend remain unclear, but could be related to seasonally driven changes in the Joule heating rates due to the tilted and offset magnetic field, or could be related to changing vertical distributions of hydrocarbons. H3+ has not yet been detected at Neptune, but this discovery provides low-hanging fruit for upcoming facilities such as the James Webb Space Telescope and the next generation of 30 m telescopes. Detecting H3+ at Neptune would enable the characterization of its upper atmosphere for the first time since 1989. To fully understand the ice giants, we need dedicated orbital missions, in the same way the Cassini spacecraft explored Saturn. Only by combining in situ observations of the magnetic field with in-orbit remote sensing can we get the complete picture of how energy moves between the atmosphere and the magnetic field. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

Investigation of Electromagnetic Characteristic in Series-Connected Augmented Quadrupole Rail Launcher

The weight of projectiles and magnetic shield are two important factors influencing an electromagnetic launch (EML). The quadrupole rail launcher forms a cylindrical area in the middle of a launcher with very small magnetic field intensity, which can shield electromagnetic radiation and protect precision electronic components inside a projectile. However, the offset magnetic field weakens the electromagnetic thrust and decreases the launch efficiency. To solve the mentioned problem, the structure of a series-connected augmented quadrupole rail launcher is presented, the electromagnetic model based on the numerical analysis and the finite element method for nonaugmented and augmented quadrupole rail launcher is built. In addition, the distribution characteristic of current density, magnetic field intensity, and electromagnetic force of the two launchers is analyzed, respectively. The simulation results show that the current density distribution of rail in the nonaugmented launcher is O-like shape and the augmented launcher is C-like shape. Under given conditions, the electromagnetic thrust of the augmented launcher is larger about 3.7 times than that of the nonaugmented launcher. Meanwhile, a large magnetic shielding area can be kept in the augmented launcher.

High-fidelity Imaging of the Inner AU Mic Debris Disk: Evidence of Differential Wind Sculpting?

Abstract We present new high-fidelity optical coronagraphic imagery of the inner ∼50 au of AU Mic’s edge-on debris disk using the BAR5 occulter of the Hubble Space Telescope Imaging Spectrograph (HST/STIS) obtained on 2018 July 26–27. This new imagery reveals that “feature A,” residing at a projected stellocentric separation of 14.2 au on the southeast side of the disk, exhibits an apparent “loop-like” morphology at the time of our observations. The loop has a projected width of 1.5 au and rises 2.3 au above the disk midplane. We also explored Transiting Exoplanet Survey Satellite photometric observations of AU Mic that are consistent with evidence of two starspot complexes in the system. The likely co-alignment of the stellar and disk rotational axes breaks degeneracies in detailed spot modeling, indicating that AU Mic’s projected magnetic field axis is offset from its rotational axis. We speculate that small grains in AU Mic’s disk could be sculpted by a time-dependent wind that is influenced by this offset magnetic field axis, analogous to co-rotating solar interaction regions that sculpt and influence the inner and outer regions of our own Heliosphere. Alternatively, if the observed spot modulation is indicative of a significant misalignment of the stellar and disk rotational axes, we suggest that the disk could still be sculpted by the differential equatorial versus polar wind that it sees with every stellar rotation.

Optical Sensing of Broadband RF Magnetic Field Using a Micrometer-Sized Diamond

High-resolution field imaging with minimized invasiveness is of growing interest with the development of radio frequency (RF) integrated circuits and planar antenna characterization. In this paper, we propose an optical broadband sensor with static offset magnetic field using a diamond crystal that contains an ensemble of nitrogen-vacancy (NV) centers. The spatial resolution of micrometer scale is demonstrated by using a diamond crystal of this size. This technique is based on the modulation of fluorescence emitting by NV center under green laser excitation, by an off-axis magnetic field at radio frequencies. We demonstrate experimentally the detection of an RF magnetic field, with minimum measurable amplitude on the order of 0.1 Gauss, which is limited by the noise floor of the avalanche photodetector. The sensitivity can be further improved by noise suppression in optics. The bandwidth of the sensor is measured to be 15 MHz.

Mercury\u2019s anomalous magnetic field caused by a symmetry-breaking self-regulating dynamo

The discovery of Mercury’s unusually axisymmetric, anomalously axially offset dipolar magnetic field reveals a new regime of planetary magnetic fields. The cause of the offset dipole remains to be resolved, although some exotic models have been proposed. Deciphering why Mercury has such an anomalous field is crucial not only for understanding the internal dynamics, evolutionary history and origin of the planet, but also for establishing the general dynamo theory. Here we present numerical dynamo models, where core convection is driven as thermo-compositional, double-diffusive convection surrounded by a thermally stably stratified layer. We show that the present models produce magnetic fields similar in morphology and strength to that of Mercury. The dynamo-generated fields act on the flow to force interaction between equatorially symmetric and antisymmetric components that results in north-south asymmetric helicity. This symmetry-breaking magnetic feedback causes the flow to generate and maintain Mercury’s axially offset dipolar field.

The Construction of a Graphene Hall Effect Magnetometer

This paper deals with design and fabrication of graphene micro-Hall devices along with driving and processing electronics to construct a highly sensitive graphene magnetometer system. Graphene micro-Hall elements were fabricated by implementing microfabrication techniques. Custom driving and processing circuitry was designed and built on a printed circuit board and integrated with fabricated graphene Hall devices; reducing the offset equivalent magnetic field and greatly improving the sensitivity. The performance parameters were quantitatively analyzed for characterizing the sensitivity in terms of linearity, current-related sensitivity, and magnetic field resolution. The potential of graphene magnetometers for detection capability of low fields has been demonstrated with a current related sensitivity of 2540 V/AT and magnetic field resolution of 162 nT/ $\surd $ Hz.

Estimation of gradients in quantum metrology

We develop a general theory to estimate magnetic field gradients in quantum metrology. We consider a system of $N$ particles distributed on a line whose internal degrees of freedom interact with a magnetic field. Usually gradient estimation is based on precise measurements of the magnetic field at two different locations, performed with two independent groups of particles. This approach, however, is sensitive to fluctuations of the off-set field determining the level-splitting of the particles and results in collective dephasing. In this work we use the framework of quantum metrology to assess the maximal accuracy for gradient estimation. For arbitrary positioning of particles, we identify optimal entangled and separable states allowing the estimation of gradients with the maximal accuracy, quantified by the quantum Fisher information. We also analyze the performance of states from the decoherence-free subspace (DFS), which are insensitive to the fluctuations of the magnetic offset field. We find that these states allow to measure a gradient directly, without the necessity of estimating the magnetic offset field. Moreover, we show that DFS states attain a precision for gradient estimation comparable to the optimal entangled states. Finally, for the above classes of states we find simple and feasible measurements saturating the quantum Cram\'er-Rao bound.

From Three-Contact Vertical Hall Elements to Symmetrized Vertical Hall Sensors with Low Offset

This paper gives a detailed account of vertical Hall sensors (VHS) with an electrical symmetry equivalent to that of planar Hall plates (PHP). Conventional five-contact VHS (5CVHS) have shown strongly nonlinear offset characteristics due to the junction field effect combined with an inherent electrical asymmetry which have prevented efficient offset compensation by current spinning. We show that this deficit can be cured by appropriate device design. The novel sensor achieves a higher symmetry by four identical three-contact vertical Hall elements (3CVHE) interconnected into what we call the fully symmetric VHS (FSVHS). In this device offset compensation by current spinning is found to be at least ten times more efficient than in comparable 5CVHS. Quantitatively, at a drive current of 0.5mA the residual magnetic offset field of the FSVHS extracted from 50 samples is 0.14±0.41mT. In 5CVHS devices the corresponding value was found to be 2.04±1.32mT. Moreover, by orthogonal coupling of four FSVHS the device is symmetrized even further, which results in an equivalent magnetic offset field of 0.04±0.05mT. This represents the best results to date achieved by a stand-alone VHS in standard CMOS technology. For comparison the magnetic offset of conventional PHP fabricated in CMOS technology lies in the range of 0.05 to 0.1 mT, between the results of the two novel concepts.

Easily magnetic anomalies earthquake prediction

Low power consumption long time offset magnetic field detector (earthquake prediction) .The design of the hardware circuit of the magnetic field detector seismic geomagnetic acquisition and pre processing module mainly includes. Electronic compass, compass. monitoring device while the magnetic azimuth for monitoring and analyzing the object, GSM, but it can also be applied to other seismic precursor information analysis, such as earthquake precursory infrasound abnormality, only need infrasound abnormality intelligent sensor replace geomagnetic anomaly intelligent sensor, and modify the relevant parameters can be.

Experimental evidence of electrification processes during the 2009 L'Aquila earthquake main shock

AbstractTwo types of coseismic magnetic field events are simultaneously observed: transient offset events and magnetic field signal that occurred at the destructive, Mw6.1 L'Aquila earthquake main shock. The offset event, conventionally interpreted as a signature of piezomagnetic effects, however, could not be explained as such. In the second type of coseismic event, the transient magnetic signal starts simultaneously with the offset event and reaches amplitude of 0.8 nT in the total magnetic field. The signal is a local one; its amplitude shape resembles diffusion‐like form with time scale characteristics that are indicative for a source deep in the crust. The polarity of the transient signal is in the horizontal plane and nearly parallel to the L'Aquila fault strike.

Stability of a trapped-atom clock on a chip

We present a compact atomic clock interrogating ultracold 87Rb magnetically trapped on an atom chip. Very long coherence times sustained by spin self-rephasing allow us to interrogate the atomic transition with 85% contrast at 5 s Ramsey time. The clock exhibits a fractional frequency stability of $5.8\times 10^{-13}$ at 1 s and is likely to integrate into the $1\times10^{-15}$ range in less than a day. A detailed analysis of 7 noise sources explains the measured frequency stability. Fluctuations in the atom temperature (0.4 nK shot-to-shot) and in the offset magnetic field ($5\times10^{-6}$ relative fluctuations shot-to-shot) are the main noise sources together with the local oscillator, which is degraded by the 30% duty cycle. The analysis suggests technical improvements to be implemented in a future second generation set-up. The results demonstrate the remarkable degree of technical control that can be reached in an atom chip experiment.

Controlling the magnetic-field sensitivity of atomic-clock states by microwave dressing

We demonstrate control of the differential Zeeman shift between clock states of ultracold rubidium atoms by means of non-resonant microwave dressing. Using the dc-field dependence of the microwave detuning, we suppress the first and second order differential Zeeman shift in magnetically trapped $^{87}$Rb atoms. By dressing the state pair 5S$_{1/2} F= 1, m_F = -1$ and $F= 2, m_F = 1$, a residual frequency spread of <0.1 Hz in a range of 100 mG around a chosen magnetic offset field can be achieved. This is one order of magnitude smaller than the shift of the bare states at the magic field of the Breit-Rabi parabola. We further identify double magic points, around which the clock frequency is insensitive to fluctuations both in the magnetic field and the dressing Rabi frequency. The technique is compatible with chip-based cold atom systems and allows the creation of clock and qubit states with reduced sensitivity to magnetic field noise.