Vector Magnetometer Research Articles

Ground-based Observations of the Polar Region Space Environment at the Jang Bogo Station, Antarctica

Jang Bogo Station (JBS), the second Korean Antarctic research station, was established in Terra Nova Bay, Antarctica (74.62°S 164.22°E) in February 2014 in order to expand the Korea Polar Research Institute (KOPRI) research capabilities. One of the main research areas at JBS is space environmental research. The goal of the research is to better understand the general characteristics of the polar region ionosphere and thermosphere and their responses to solar wind and the magnetosphere. Ground-based observations at JBS for upper atmospheric wind and temperature measurements using the Fabry-Perot Interferometer (FPI) began in March 2014. Ionospheric radar (VIPIR) measurements have been collected since 2015 to monitor the state of the polar ionosphere for electron density height profiles, horizontal density gradients, and ion drifts. To investigate the magnetosphere and geomagnetic field variations, a search-coil magnetometer and vector magnetometer were installed in 2017 and 2018, respectively. Since JBS is positioned in an ideal location for auroral observations, we installed an auroral all-sky imager with a color sensor in January 2018 to study substorms as well as auroras. In addition to these observations, we are also operating a proton auroral imager, airglow imager, global positioning system total electron content (GPS TEC)/scintillation monitor, and neutron monitor in collaboration with other institutes. In this article, we briefly introduce the observational activities performed at JBS and the preliminary results of these observations.

A hexagonal-framed magnetoelectric composite for magnetic vector measurement

In this work, a multi-terminal hexagonal-framed magnetoelectric composite (HFMEC) made of amorphous FeBSi alloy (Metglas) ribbons and three sandwiched [011]-oriented PMN-PT single-crystal fibers (Pb(Mg1/3Nb2/3)O3-PbTiO3) arranged with a 120° interval is presented, and the directional magnetoelectric (ME) effect of the HFMEC operating in the L-T mode under a constant-amplitude AC magnetic field is investigated. It is found that the HFMEC exhibits threefold symmetric directional ME coupling in response to an applied in-plane DC magnetic field HDC, while each ME terminal shows a twofold symmetry due to the magnetization and demagnetization effect of HDC. Moreover, a valid formula for calculating the angular direction of the magnetic field is also presented. It is further revealed that the HFMEC exhibits a “V” shaped ME voltage output as a response to HDC due to the geomagnetic field (HGeo) effect, and the knee point of the curve literally reveals the magnitude of local HGeo, while the direction of HDC indicates a reverse direction of HGeo. The proposed HFMEC provides a great potential for a vector magnetometer as well as geomagnetic sensor application.

Resolution limit of anisotropic magnetoresistance(AMR) based vector magnetometer

An AMR based vector magnetometer was developed for the Separated Payload program to explore the field-aligned currents in Earth polar region. It has the advantages of low volume and CubeSat platform compatible mass while still maintains adequate resolution. In order to determine the resolution limit of AMR based magnetometer, we measured the sensor’s intrinsic noise. This characteristic was in conflict with our prototype magnetoresistive magnetometer since phase sensitive detection(PSD) could only reject noise from interfacing circuit without affecting the signal to be measured. We believe that besides flipping the magnetization direction periodically, the set/reset technique also functions as a modulation to the magnetic field. The sensor operates at the modulation frequency and its noise level is much lower than that at low frequency. This technique is analyzed theoretically and a description from frequency perspective is given. By modulating the signal beyond corner frequency of the sensor and amplifier, both the sensor and interfacing circuit work in white noise region. Then we calculate the theoretical resolution limit of HMC1001 based magnetometer and the result shows that the noise floor is 24.9 pT/Hz0.5@1 Hz under 5 V bias with proper-matched amplifier.

Sensitive and stable vector magnetometer for operation in zero and finite fields.

We report on an optically pumped magnetometer that uses multiple laser beams to pump and probe spin-polarized Cs atoms. The selected sensor geometry allows for operation in finite magnetic fields as well as close to zero field. In finite fields the magnetometer employs free spin precession signals to determine the field modulus and direction as described in a separate publication. This publication focuses on the magnetometer operation close to zero field, which is based on a ground state Hanle resonance. The four laser beams permit the simultaneous measurement of two orthogonal magnetic field components in a differential detection scheme that greatly suppresses technical laser power noise. Sensitivities better than 54 fT/Hz1/2 could be demonstrated simultaneously for both measurement channels in a well shielded environment. A minimum Allan deviation, limited by residual field fluctuations, of better than 40 fT was observed for integration times of 2s. The magnetometer achieves high sensitivity and stability in offset fields as well as close to zero field and is, thus, a universal tool for low frequency magnetic field measurements.

Robust high-dynamic-range vector magnetometry with nitrogen-vacancy centers in diamond

We demonstrate a robust, scale-factor-free vector magnetometer, which uses a closed-loop frequency-locking scheme to simultaneously track Zeeman-split resonance pairs of nitrogen-vacancy (NV) centers in diamond. This technique offers a three-orders-of-magnitude increase in dynamic range compared to open-loop methodologies; is robust against fluctuations in temperature, resonance linewidth, and contrast; and allows for simultaneous interrogation of multiple transition frequencies. By directly detecting the resonance frequencies of NV centers oriented along each of the diamond's four tetrahedral crystallographic axes, we perform full vector reconstruction of an applied magnetic field.

Vector magnetometer based on synchronous manipulation of nitrogen-vacancy centers in all crystal directions

Negatively charged nitrogen vacancy (NV−) centers in diamond have been extensively studied as high-sensitivity magnetometers, showcasing a wide range of applications. This study experimentally demonstrates a vector magnetometry scheme based on synchronous manipulation of NV− center ensembles in all crystal directions using double frequency microwaves (MWs) and multi-coupled-strip-lines (mCSL) waveguide. The application of the mCSL waveguide ensures a high degree of synchrony (99%) for manipulating NV− centers in multiple orientations in a large volume. Manipulation with double frequency MWs makes NV− centers of all four crystal directions involved, and additionally leads to an enhancement of the manipulation field. In this work, by monitoring the changes in the slope of the resonance line consisting of multi-axes NV− centers, measurement of the direction of the external field vector was demonstrated with a sensitivity of . Based on the scheme, the fluorescence signal contrast was improved by four times higher and the sensitivity to the magnetic field strength was improved by two times. The method provides a more practical way of achieving vector sensors based on NV− center ensembles in diamond.

Flux trapping in multi-loop SQUIDs and its impact on SQUID-based absolute magnetometry

The effect of flux trapping on the flux-voltage characteristics of multi-loop SQUID magnetometers was investigated by means of repeated cool-down cycles in a stepwise increased magnetic background field. For a SQUID with N parallel loops, N different flux offsets, each separated by were observed even in zero magnetic field. These flux offsets further split into a so called fine structure, which can be explained by minor asymmetries in the SQUID design. The observed results are discussed with particular regard to their impact on the previously presented absolute SQUID cascade vector magnetometer.

Planarization, Fabrication, and Characterization of Three-Dimensional Magnetic Field Sensors

Nanomagnetism deals with magnetic phenomena in nanoscale structures, involving processes at the atomic level. Magnetic sensors, which exhibit the surprising giant magnetoresistance (GMR) effect, are some of the first real applications of nanotechnology, and have become very important in the last two decades. In addition, high-performance magnetoresistance (MR) measurement is a critical technique in modern electrical applications, including electronic compasses, aviation navigation, motion tracking, noncontact current sensing, rotation sensing, and vehicle detection. Both GMR and tunneling magnetoresistance (TMR) sensors have been used in the state-of-art electronic compasses. A new planar design layout of a vector magnetometer is proposed in this report. It can sense variations in three-dimensional (3-D) magnetic fields. The planarization of a vector magnetometer is carried out with consideration of materials, magnetic schematics, as well as transducer circuit designs. The optimization of an advanced magnetic material for use in GMR and TMR sensors and its planarization in a 3-D design are crucial practical issues. This paper presents an overview of the planarization of vector magnetometers and the development of its applications. It focuses on recent works, covers an analytic model of magnetoresistive sensors, and methods of thin film fabrication. It also addresses the planar vector magnetometer with a flux-guide, the chopping technique, and techniques for microfabrication of substrates. Planarization in magnetic sensors will become increasingly exploited as nanomagnetism grows in importance.

Theory of atomic vector magnetometer using linearly polarized resonant light

As is well known a linearly polarized resonant laser will cause atoms to generate a magnetic tensor moment (MTM) by polarizing them. When there exists an external magnetic field, it is possible that the moment will precess around the field. In the presence of a radio frequency (RF) exciting source, we investigate theoretically the dependence of time-independent (direct current, DC), the first and second harmonic signal of the MTM precession on magnetic vector field, and obtain its analytical solution by solving the Liouville equation. The results show that the interference of both harmonic components will result in the precession spectrum evidently varying. A detailed explanation is described in the following. For the DC signal, Rabi frequency Ω of 1/(2√2) is a spectral splitting threshold. When it is greater than the threshold, the interference will cause single resonant absorption dip characterized usually to split into two dips, which has not been reported before to the best of our knowledge, and the separation between both the dips may be expressed as √3√Ω2+Ω4 -Ω2-1. For the first harmonic signal including symmetric and antisymmetric component, an interference fringe will appear near the center of antisymmetric part when Ω >1/(2√2), simultaneously its symmetric part behaves like the above dc component, such as splitting threshold and separation between both dips. With regard to the second harmonic signal, it is found that the interference can also lead to the width of the second harmonic decreasing to 38% compared with the case of the first harmonic signal. At the optimum RF Rabi frequency, on the assumption that noise spectral density is constant, it is theoretically shown that the most sensitive magnetometer, realized by the DC component or the first or second harmonic signal of the precession, depends only on the angle between the light polarization and the measured magnetic field.In fact, we are able to obtain the modules of the measured magnetic vector by RF resonant frequency. The angle between the magnetic field and the laser polarization is determined just by the ratio of the intensity of the DC component to the intensity of the second harmonic signal and the ratio between the intensities of the symmetric parts of two harmonic signals in resonance, and another orientation angle between the measured field projection at the plane perpendicular to the light polarization and the direction of RF source depends on the phase difference between the antisymmetric components of both harmonic signals. Consequently, we demonstrate a vectorial atomic magnetometer that is realized by using the RF source and the linearly polarized resonant laser without rotating laser polarization. This kind of atomic magnetometer with simple sensor structure is easy to integrate vector magnetometer array which will be suitable for solving the inverse problem and geomagnetic navigation.

Integrated Calibration for the Cross Magnetic Gradiometer

The magnetic gradiometer onboard a maneuverable vehicle is subjected to not only magnetometer errors and misalignment errors but also magnetic interference of vehicle. Measurement precision of the magnetic gradiometer is greatly affected by those errors, so an integrated error calibration method is proposed in this paper. Firstly, considering vector magnetometer errors and magnetic interference, an integrated error calibration model for vector magnetometer is established, and ellipsoid fitting parameters are calculated by the least square algorithm under ellipsoid restriction, then the error calibration matrices are solved by the Cholesky factorization. Secondly, the misalignment error calibration matrices are obtained by solving the Orthogonal Procrustes problem. Finally, simulations and experiments with a cross magnetic gradiometer are performed to verify effectiveness and robustness of the proposed method. Results show that the proposed method can effectively calibrate the cross magnetic gradiometer, and measurement accuracy of the cross magnetic gradiometer is increased greatly.

In situ vector calibration of magnetic observatories

Abstract. The goal of magnetic observatories is to measure and provide a vector magnetic field in a geodetic coordinate system. For that purpose, instrument set-up and calibration are crucial. In particular, the scale factor and orientation of a vector magnetometer may affect the magnetic field measurement. Here, we highlight the baseline concept and demonstrate that it is essential for data quality control. We show how the baselines can highlight a possible calibration error. We also provide a calibration method based on high-frequency absolute measurements. This method determines a transformation matrix for correcting variometer data suffering from scale factor and orientation errors. We finally present a practical case where recovered data have been successfully compared to those coming from a reference magnetometer.

Automated observatory in Antarctica: real-time data transfer on constrained networks in practice

Abstract. In 2013 a project was started by the geophysical centre in Dourbes to install a fully automated magnetic observatory in Antarctica. This isolated place comes with specific requirements: unmanned station during 6 months, low temperatures with extreme values down to −50 °C, minimum power consumption and satellite bandwidth limited to 56 Kbit s−1. The ultimate aim is to transfer real-time magnetic data every second: vector data from a LEMI-25 vector magnetometer, absolute F measurements from a GEM Systems scalar proton magnetometer and absolute magnetic inclination–declination (DI) measurements (five times a day) with an automated DI-fluxgate magnetometer. Traditional file transfer protocols (for instance File Transfer Protocol (FTP), email, rsync) show severe limitations when it comes to real-time capability. After evaluation of pro and cons of the available real-time Internet of things (IoT) protocols and seismic software solutions, we chose to use Message Queuing Telemetry Transport (MQTT) and receive the 1 s data with a negligible latency cost and no loss of data. Each individual instrument sends the magnetic data immediately after capturing, and the data arrive approximately 300 ms after being sent, which corresponds with the normal satellite latency.

The equatorial electrojet (EEJ) current deduced from CHAMP satellite and ground magnetic measurements in West Africa

The equatorial electrojet (EEJ) is an intense daytime ionospheric current that circulates eastward along the geomagnetic dip equator. In this work, the EEJ current intensity was studied from ground-based magnetic data and CHAllenging Minisatellite Playoad (CHAMP) satellite observations. The ground-based magnetic data were recorded along a meridian chain of three stations across the dip equator in West Africa. These stations were operated by the Institut de Physique du Globe de Paris (IPGP). For the CHAMP satellite data, vector magnetometer records during geomagnetically quiet periods with Kp ≤ 2 were used. To estimate the EEJ current intensity, we have first isolated its magnetic effect from CHAMP satellite observations for 10 years (from July 2000 to September 2010). The results were used to analyze the EEJ seasonal variability and the local time and longitude dependence. In addition, a comparison between ground-based and satellite observations of the EEJ current intensity was made. A good correlation was found between satellite and ground-based EEJ current intensity, with an average correlation coefficient of r 2 ~0.93. This performed correlation show that the technique used in this study can be extended to all ground magnetic stations for modeling the EEJ phenomena.

High-speed optical three-axis vector magnetometry based on nonlinear Hanle effect in rubidium vapor

The magnetic-field-compensation optical vector magnetometer based on the nonlinear Hanle effect in alkali metal vapor allowing two-axis measurement operation has been further elaborated for three-axis performance, along with significant reduction of measurement time. The upgrade was achieved by implementing a two-beam resonant excitation configuration and a fast maximum searching algorithm. Results of the proof-of-concept experiments, demonstrating 1 μTB-field resolution, are presented. The applied interest and capability of the proposed technique is analyzed.

A Fast Calibration Method for Magnetometer Array and the Application of Ferromagnetic Target Localization

The vector magnetometer array is commonly used in ferromagnetic target detection and localization. Due to technological limitations, the accuracy of measurements is restricted by magnetometer errors, misalignment errors between magnetometers and the interference of soft iron and hard iron. In order to correct the measurement errors, a calibration method with less computation time is proposed. A magnetometer array is designed for ferromagnetic target detection and localization, which is calibrated by the proposed method. The results of our empirical evaluations confirm that the proposed method outperforms conventional calibration method both in terms of computation cost and accuracy. Different test specimens are used to test the detection range and the localization accuracy of the system. The array system compensated by the proposed method has a larger detection range.

All-fiber-optic vector magnetometer based on anisotropic magnetism-manipulation of ferromagnetism nanoparticles

The development of simple and sensitive sensors with the capability of simultaneously detecting magnetic field intensity and its direction in three-dimensional (3D) space is a technical imperative for magnetic field detection. In this paper, we first demonstrated the mechanism of anisotropic manipulation of ferromagnetism nanoparticles (FMNPs) in a non-continuous magnetic fluid film with the aid of varying magnetic fields. Second, based on the anisotropic distribution of the FMNPs around an optical fiber, we fabricated a vector magnetometer with a direction error of ±1.9° and an intensity sensitivity of 222.0 pm/mT, respectively. The sensing mechanism relies on the magnetism-controllable effective refractive index modification of asymmetric cladding modes in an in-line fiber-optic Mach-Zehnder interferometer. Compared to the previously reported intensity-based magnetometers, the as-fabricated magnetometer also provides an avenue to monitor the vector direction of the magnetic field in 3D space.

Low-noise tunneling-magnetoresistance vector magnetometers with flux chopping technique

A concept for a low-noise three-axis magnetometer consisting of tunneling magnetoresistance (TMR) sensors and a flux chopper was designed, implemented, and characterized in this work. The TMR sensors used in this study were the model of TMR2102D from Multidimension Technology Inc. Three TMR sensors were aligned orthogonally on a printed circuit board (PCB) and mounted inside a cylindrical flux chopper. The cylindrical flux chopper including a soft magnetic shielding tube and enameled copper wires was 16mm in length and 8mm in diameter. The shielding tube was made of a cobalt-based soft magnetic ribbon, Metglas-2714A, from Metglas Inc. The flux chopper modulated the external magnetic flux density using the fluxgate effect, which made the sensors respond to the quasi-static field at the chopping frequency. The demodulated output showed a reduction in low-frequency noise to the level of 0.17nT/√Hz@1Hz. To demonstrate the technical feasibility for electronic compass application, the demodulated output of the vector magnetometer prototype was recorded by rotating the device in Earth’s magnetic fields about a fixed axis. The Cartesian components Bx, By, and Bz of the Earth’s fields at various azimuth angles were retrieved by performing a calibration algorithm to correct the non-orthogonality caused by the misalignment of TMR sensors and the chopper. The calibrated outputs were linear and orthogonal to each other with an angle error less than 1° and nonlinearity of 0.7%, indicating that the chopping technique is useful to realize an extremely low-noise three-dimensional magnetometer for the geomagnetic application.

Alignment of a vector magnetometer to an optical prism.

A method for alignment of a vector magnetometer to a rigidly attached prism is presented. This enables optical comparison of the magnetometer axes to physical surfaces in the apparatus, and thus an absolute determination of the magnetic field direction in space. This is in contrast with more common techniques, which focus on precise determination of the relative angles between magnetometer axes, and so are more suited to measuring differences in the direction of magnetic fields. Here we demonstrate precision better than 500 μrad on a fluxgate magnetometer, which also gives the coil orthogonality errors to a similar precision. The relative sensitivity of the three axes is also determined, with a precision of about 5 × 10-4.

All-Optical Vector Cesium Magnetometer**Supported by the National Natural Science Foundation of China under Grant Nos U1631239 and U1331114, and the 111 Project to Harbin Engineering University under Grant No B13015.

Based on power modulation of a pump laser and precessional projection detection, we present an all-optical vector magnetometer of cesium, which has a demonstrated magnitude sensitivity of 80 fT/Hz1/2 and an orientation sensitivity of 0.1°/Hz1/2. In the device, four main factors are measured experimentally, which are the Larmor precession frequency of a polarized magnetic moment that depends on the modulus of the measured magnetic field only, two phase shifts and amplitude ratio of the precession projection in the two probe directions relative to the magnetic field orientation. This kind of magnetometer with high sensitivity in the range of the spatial angle is suitable for solving the inverse problem and geomagnetic navigation.

Characterization and compensation of thermo-elastic instability of SWARM optical bench on micro Advanced Stellar Compass attitude observations

Launched into orbit on November 22, 2013, the Swarm constellation of three satellites precisely measures magnetic signal of the Earth. To ensure the high accuracy of magnetic observation by vector magnetometer (VFM), its inertial attitude is precisely determined by µASC (micro Advanced Stellar Compass). Each of the three Swarm satellites is equipped with three µASC Camera Head Units (CHU) mounted on a common optical bench (OB), which has a purpose of transference of the attitude from the star trackers to the magnetometer measurements. Although substantial pre-launch analyses were made to maximize thermal and mechanical stability of the OB, significant signal with thermal signature is discovered when comparing relative attitude between the three CHU's (Inter Boresight Angle, IBA). These misalignments between CHU's, and consequently geomagnetic reference frame, are found to be correlated with the period of angle between Swarm orbital plane and the Sun (ca. 267 days), which suggests sensitivity of optical bench system on temperature variation.In this paper, we investigate the propagation of thermal effects into the µASC attitude observations and demonstrate how thermally induced attitude variation can be predicted and corrected in the Swarm data processing. The results after applying thermal corrections show decrease in IBA RMS from 6.41 to 2.58″. The model significantly improves attitude determination which, after correction, meets the requirements of Swarm satellite mission. This study demonstrates the importance of the OB pre-launch analysis to ensure minimum thermal gradient on satellite optical system and therefore maximum attitude accuracy.