Detection Of Weak Magnetic Fields Research Articles

Calibration of Room Temperature Magnetic Sensor Array for Biomagnetic Measurement

Biomagnetic field measurement is a promising tool for the investigation of electrical activities in a living body. Room temperature (RT) magnetic sensors with an improved resolution such as magnetoresistance (MR) devices have been recently employed for the detection of weak magnetic fields, which were earlier detected solely using superconducting interference device magnetic sensors. The position, orientation, and sensitivity of each magnetic sensor in a sensor array must be precisely determined for accurate magnetic source analysis. We proposed a calibration method using an array of multiple coils, which is applicable to an RT magnetic sensor array. To demonstrate the validity of the proposed calibration method, we applied it to an MR device-based magnetocardiography (MCG) system equipped with an L-shaped planar sensor array, which was newly developed for the simultaneous observation of both anterior and lateral sides of the body. The deviation of the sensor parameters from the designed values was estimated via calibration. The result of the marker coil localization test indicated that the calibration considerably improved the accuracy of the magnetic source analysis. Finally, we demonstrated a preliminary MCG measurement using the calibrated magnetic sensor array.

Fast and Sensitive Detection of Paramagnetic Species Using Coupled Charge and Spin Dynamics in Strongly Fluorescent Nanodiamonds.

Sensing of a few unpaired electron spins, such as in metal ions and radicals, is a useful but difficult task in nanoscale physics, biology, and chemistry. Single negatively charged nitrogen-vacancy (NV-) centers in diamond offer high sensitivity and spatial resolution in the optical detection of weak magnetic fields produced by a spin bath but often require long acquisition times on the order of seconds. Here, we present an approach based on coupled spin and charge dynamics in dense NV ensembles in strongly fluorescent nanodiamonds (NDs) to sense external magnetic dipoles. We apply this approach to various paramagnetic species, including gadolinium complexes, magnetite nanoparticles, and hemoglobin in whole blood. Taking advantage of the high NV density, we demonstrate a dramatic reduction in acquisition time (down to tens of milliseconds) while maintaining high sensitivity to paramagnetic centers. Strong luminescence, high sensitivity, and short acquisition time make dense NV- ensembles in NDs a potentially promising tool for biosensing and bioimaging applications.

The ultrasensitive anomalous Hall effect induced by interfacial oxygen atoms redistribution

Improved weak magnetic field detection has been achieved by the ultrasensitive linear Anomalous Hall effect (AHE) in MgO/CoFeB/X/Ta/MgO (X: Hf or Gd) multilayers. The Anomalous Hall sensitivity (Sv) has been tuned by effective oxygen migration assisted interfacial oxygen atoms redistribution. For the sample with 0.2 nm Hf insertion, the Anomalous Hall sensitivity (Sv) is 16 182 Ω/T at the as-deposited state, 560% larger than that of the sample with no insertion (2470 Ω/T). However, the value of Sv for the sample with 0.2 nm Gd insertion is only up to 6837 Ω/T. An ultrahigh value of Sv (28 282 Ω/T) is obtained for the sample with 0.2 nm Hf insertion by a further annealing process. Interfacial structural results indicate that different linear AHE behavior originate from various interfacial oxygen atoms distribution states, which can be ascribed to different oxygen migration processes. The strong oxygen affinity capability of metallic Hf leads to the bidirectional interfacial oxygen migration. However, only unidirectional interfacial oxygen migration can be observed in the sample with 0.2 nm Gd insertion. This study will provide an effective way to improve the AHE-based sensors for weak magnetic field detection.

Effects of remanent magnetization on dynamic magnetomechanical and magnetic-sensing characteristics in bi-layer multiferroics

Influences of remanent magnetization on dynamic magnetomechanical mechanisms in a bi-layer asymmetric magnetoelectric (ME) laminate consisting of lead zirconate titanate and samarium iron alloy has been studied systematically, and the underlying hysteresis physics involved in dynamic magnetomechanical process as well as its magnetic-sensing characteristics was intensively characterized. To appreciate the distinct magnetostriction and ferromagnetism simultaneously in samarium iron alloy, key magnetomechanical parameters of dynamic piezomagnetic coefficient, Young's modulus and mechanical quality factors exhibit hysteresis behaviors under magnetic field application. Consequently, high sensitivity in proposed bi-payer laminate for field detection can be reached without the facilitation of additional bias field. Experimental results show that the ME output has an approximately linear correlation with the applied AC magnetic field, and the low-frequency and the detection limits at 1 kHz and 120 kHz can reach 2.3 × 10−6 T and 2.2 × 10−8 T, respectively. These unique features provide such an asymmetric ME composite great potentials for weak magnetic field detection without DC bias field.

Rotating Magnetoelectric Sensor for DC Magnetic Field Measurement

Magnetoelectric (ME) sensors have a significant potential for detection of weak magnetic fields if the mechanical resonance is utilized. Generally, magnetic and electric modulations can be employed for frequency conversion during dc or small ac signal measurements. In this paper, we perform frequency conversion using a rotating ME sensor, which enables us to detect a dc magnetic field without any additional excitation. The sensor, consisted of L–T mode ME laminates whose magnetostrictive layers are magnetized along the length directions but piezoelectric layers are poled along the thickness directions, is more sensitive to a magnetic field along its longitudinal direction. By rotation, the dc magnetic field to be measured generates a sinusoid component in the direction of the sensor’s longitudinal axis, which is equivalent to applying an excitation. The experimental results are consistent with the theoretical calculations and demonstrate the feasibility of this proposed modulation method.

Self-biased vector magnetic sensor based on a Love-type surface acoustic wave resonator

Magnetic sensors based on the interaction between the delta-E effect and surface acoustic waves (SAWs) are promising devices due to their simple driving circuits. In this study, a magnetic sensor composed of a (Fe90Co10)78Si12B10 magnetic layer and Love-type SAW resonator was investigated. The resonance frequency sensitivities (RFS) of the sensor resulting from the delta-E effect are 76.06 Hz/μT and 364.28 Hz/μT in the L-direction (along the easy axis) and W-direction (along the hard axis), respectively. The maximum RFS achieved by this magnetic sensor is 663.98 Hz/μT along the hard axis, with a resonance frequency of 221.76 MHz. The shift of resonance frequency shows a good linearity along with the external magnetic field in the L-direction. The curves of frequency response demonstrate point symmetry rather than the expected axial symmetry in both directions, due to the existence of residual magnetization. Non-zero RFS at zero magnetic field also appears in this device, because of the non-zero change of the Young's modulus at zero magnetic field. The good linearity and high sensitivity mean that the investigated magnetic sensor can be applied for weak DC magnetic field detection.

Magnetic field detections in Herbig Ae SB2 systems

AbstractStudies of the presence of magnetic fields in Herbig Ae/Be stars are extremely important because they enable us to improve our insight into how the magnetic fields of these stars are generated and how they interact with their environment, including their impact on the planet formation process and the planet-disk interaction. We report new detections of weak mean longitudinal magnetic fields in the close Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD 95881 (Järvinen et al. 2018) based on observations obtained with HARPSpol attached to ESO’s 3.6 m telescope. Such studies are important because only very few close spectroscopic binaries with orbital periods below 20 d are known among Herbig Ae stars. Our detections favour the conclusion that the previously suggested low incidence (5-10%) of magnetic Herbig Ae stars can be explained by the weakness of these fields and the limited accuracy of the published measurements. The search for magnetic fields and the determination of their geometries in close binary systems will play an important role for understanding the mechanisms that are responsible for the magnetic field generation.

DC Magnetometer Based on Ensembles of Nitrogen Vacancy Centers in Diamond

The solid spin of ensembles of the nitrogen vacancy (NV) centers in diamond offers unique features to be employed as atomic-scale sensors for detection of weak magnetic field. Here, we mainly demonstrate two types of DC magnetometer based on ensembles of the nitrogen vacancy (NV) centers in diamond with the method of analyzing optically detected magnetic resonance (ODMR) spectra and microwave frequency modulation technology, respectively and experimentally. Firstly, we demonstrate ODMR spectra and resonance frequency of ODMR with the various external magnetic field magnitudes applied to the NV centers in diamond. The minimum detectable magnetic field of the magnetometer is 0.0498 Gauss, and a magnetic sensitivity of 5.22 μT/Hz obtained with the analysis of ODMR spectra. Further, we utilize microwave frequency modulation technology and analyze the power spectra of noise of demodulated signal of the Lock in Amplifier. The sensitivity of the magnetometer can reach as low as 14.3 nT/Hz with microwave frequency modulation. This study lays a foundation for the advancement of magnetic detection research with higher sensitivity based on ensembles of NV centers.

Multi−interface spin exchange regulated biased magnetoelectric coupling in Cluster−assembled multiferroic heterostructural films

Cluster−assembled Ni−NiO core−shell structures were periodically immersed into multiferroic Bi5Ti3FeO15 (BTFO) film matrices to form pseudo 1–3 BTFO/Ni−NiO/BTFO heterostructural films with multiferroic/antiferromagnetic/ferromagnetic (MF/AFM/FM) multi−interfaces. A biased magnetoelectric coupling effect was obtained at room temperature by a magnetic annealing process. The biased direction depends on the orientation of the annealing magnetic field, with magnetoelectric coefficient curves moving to left after positive annealing magnetic field, while moving to right with negative annealing magnetic field. This biased magnetoelectric effect originates from the multi−interface spin exchange coupling. The immersion of cluster−assembled Ni−NiO core−shells in the multiferroic matrices enhance the interfacial coupling effect due to the large surface-to-volume ratio. Thus, the interfacial spin exchange induced biased magnetoelectric effect is enhanced and reversible. The work provides an avenue on the weak magnetic field detection, spintronic devices and multistate switching memories.

Weak Magnetic Fields in Two Herbig Ae Systems: The SB2 AK Sco and the Presumed Binary HD 95881

Abstract We report the detection of weak mean longitudinal magnetic fields in the Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD 95881 using observations with the High Accuracy Radial velocity Planet Searcher polarimeter (HARPSpol) attached to the European Southern Observatory’s (ESO’s) 3.6 m telescope. Employing a multi-line singular value decomposition method, we detect a mean longitudinal magnetic field G in the secondary component of AK Sco on one occasion. For HD 95881, we measure G and G at two different observing epochs. For all the detections the false alarm probability is smaller than 10−5. For AK Sco system, we discover that accretion diagnostic Na i doublet lines and photospheric lines show intensity variations over the observing nights. The double-lined spectral appearance of HD 95881 is presented here for the first time.

High Performance Single Element MI Magnetometer With Peak-to-Peak Voltage Detector by Synchronized Switching

We propose a new high performance magnetometer using a single magneto-impedance (MI) sensor with a peak-to-peak voltage detector by synchronized switching (shortened: Pk-pk VD-type MI magnetometer), which is aimed to measure an extremely weak magnetic field ranging from nT ( $10^{-9}$ T) to pT ( $10^{-12}$ T), based upon the conventional MI magnetometer, which is used in the electronic compass. In order to enhance sensitivity and reduce noise, the Pk-pk VD-type MI magnetometer detects both the positive peak and the negative peak in induced voltage waveform of pickup coil. In this paper, for demonstrating the performances of the Pk-pk VD-type MI magnetometer, we investigate the magnetic field sensitivity and the magnetic noise spectral density. We demonstrated that the noise level of the Pk-pk VD-type MI magnetometer is lower than 2 pT/Hz $^{1/2}$ for the frequency range from 1 to 100 Hz, which is much better than the conventional MI magnetometer. We discuss the noise performance of the Pk-pk VD-type MI magnetometer. According to the results obtained in this paper, we expect that this proposed MI magnetometer can be widely used for extremely weak magnetic field detection.

A magnetoelectric flux gate: new approach for weak DC magnetic field detection

The magnetic flux gate sensors based on Faraday’s Law of Induction are widely used for DC or extremely low frequency magnetic field detection. Recently, as the fast development of multiferroics and magnetoelectric (ME) composite materials, a new technology based on ME coupling effect is emerging for potential devices application. Here, we report a magnetoelectric flux gate sensor (MEFGS) for weak DC magnetic field detection for the first time, which works on a similar magnetic flux gate principle, but based on ME coupling effect. The proposed MEFGS has a shuttle-shaped configuration made of amorphous FeBSi alloy (Metglas) serving as both magnetic and magnetostrictive cores for producing a closed-loop high-frequency magnetic flux and also a longitudinal vibration, and one pair of embedded piezoelectric PMN-PT fibers ([011]-oriented Pb(Mg,Nb)O3-PbTiO3 single crystal) serving as ME flux gate in a differential mode for detecting magnetic anomaly. In this way, the relative change in output signal of the MEFGS under an applied DC magnetic anomaly of 1 nT was greatly enhanced by a factor of 4 to 5 in comparison with the previous reports. The proposed ME flux gate shows a great potential for magnetic anomaly detections, such as magnetic navigation, magnetic based medical diagnosis, etc.

Giant magnetoelectric coupling effect in lead-free perovskite BiFeO3/Na0.5Bi4.5Ti4O15 composite films

Lead-free BiFeO3/Na0.5Bi4.5Ti4O15 composite films were deposited on Pt(100)/Ti/SiO2/Si substrates using chemical solution deposition. The surface, phase and electric domain structure, as well as ferroelectric, leakage, piezoelectric, magnetic and magnetoelectric properties were investigated systematically. The good ferroelectric, leakage properties, piezoelectric and magnetic properties were obtained in composite films. More importantly, a giant magnetoelectric voltage coefficient is observed in BiFeO3/Na0.5Bi4.5Ti4O15 composite films with the maximum αE = 136 mV/cm·Oe at Hbias = 8.0 kOe, which is comparable with lead-based composites films. Such giant magnetoelectric effect is derived from good single-phase characteristics and similar crystal structure between BiFeO3 and Na0.5Bi4.5Ti4O15 film layers. The present work provides a feasible way of preparing magnetoelectric composite films and facilitating their applications in micro-electro-mechanical system and information storage devices and weak magnetic field detection.

Cavity-enhanced room-temperature high sensitivity optical Faraday magnetometry

We propose a cavity QED system with two-photon Doppler-free configuration for weak magnetic field detection with high sensitivity at room temperature based on cavity electromagnetically induced transparency. Owing to the destructive interference induced by the control and driving fields, two transparency channels are opened. The Faraday rotation within two transparency channels can be used to detect weak magnetic field with high sensitivity at room temperature. The sensitivity with single photon and multiphoton probe inputs is analyzed. With single photon measurement, our numerical calculations demonstrate that the sensitivity with 3.8nT/Hz and 6.4nT/Hz could be achieved. When we measure the magnetic field with multiphoton input, the sensitivity can be improved to 7.7fT/Hz and 25.6fT/Hz under the realistic experimental conditions.

Ultra-sensitive magnetic field sensor with resolved temperature cross-sensitivity employing microfiber-assisted modal interferometer integrated with magnetic fluids

A compact and ultra-sensitive magnetic field sensor has been proposed by exploiting a microfiber-assisted Mach-Zehnder interferometer functionalized by magnetic fluids. We have experimentally investigated the transmission spectral responses of the proposed sensor to the variation of applied magnetic field intensity and environmental temperature. The interference dips exhibit a magnetic field sensitivity as large as −1.193 nm/Oe for a low magnetic field intensity range of 3 Oe to 21 Oe. By using the sensing matrix containing the magnetic field as well as temperature sensitivities for different interference dips, the temperature cross-sensitivity issue could be effectively resolved. Our proposed sensor is anticipated to find potential applications in weak magnetic field detection, and moreover, the immunity to temperature cross-sensitivity effect ensures its applicability in temperature-fluctuated environments.

Theoretical investigation of magnetoelectric surface acoustic wave characteristics of ZnO/Metglas layered composite

The surface acoustic wave properties of piezoelectric/magnetostrictive layered structures consisting of insulating ZnO and metallic Metglas with giant Δ E effect were studied based on a stable scattering matrix method. Only the first Rayleigh mode was found with phase velocity between 2200 m/s and 2650 m/s, and the maximum electro-mechanical coupling coefficient about 1%. It was found that the center frequency of ZnO/Metglas is highly sensitive on the change of magnetic field, up to 440 MHz/Oe. However, there is a cutoff Young’s modulus of Metglas for different designs of SAW, below which the Rayleigh mode will disappear. For a magnetoelectric SAW design with the center frequency of 335 MHz and covering a full magnetic field range from -1.4 to +1.4 Oe, the frequency sensitivity is 212 MHz/Oe, equivalent to a magnetic field sensitivity of 5 × 10−12 Tesla. Unlike conventional magnetoelectric bulk laminates or film stacks, the detection of frequency shift instead of electrical charge allows not only shrinkage of device volume but also a broad frequency band detection of weak magnetic field.

Design of Tunnel Magnetoresistive-Based Circular MFL Sensor Array for the Detection of Flaws in Steel Wire Rope

Tunnel magnetoresistive (TMR) devices have superior performances in weak magnetic field detection. In this study, TMR devices were first employed to form a circular magnetic flux leakage (MFL) sensor for slight wire rope flaw detection. Two versions of this tailor-made circular TMR-based sensor array were presented for the inspection of wire ropes with the diameters of 14 mm and 40 mm, respectively. Helmholtz-like coils or a ferrite magnet-based magnetizer was selected to provide the proper magnetic field, in order to meet the technical requirements of the TMR devices. The coefficient of variance in the flaw detection performance of the sensor array elements was experimentally estimated at 4.05%. Both versions of the MFL sensor array were able to detect multiple single-broken wire flaws in the wire ropes. The accurate axial and circumferential positions of these broken wire flaws were estimated from the MFL scanning image results. In addition, the proposed TMR-based sensor array was applied to detect the MFL signal induced by slight surface wear defects. A mutual correlation analysis method was used to distinguish the signals caused by the lift-off fluctuation from the MFL scanning image results. The MFL sensor arrays presented in this study provide inspiration for the designing of tailor-made TMR-based circular sensor arrays for cylindrical ferromagnetic structural inspections.

HARPS spectropolarimetry of three sharp-lined Herbig Ae stars: New insights

Several arguments have been presented that favour a scenario in which the low detection rate of magnetic fields in Herbig Ae stars can be explained by the weakness of these fields and rather large measurement uncertainties. Spectropolarimetric studies involving sharp-lined Herbig Ae stars appear to be promising for the detection of such weak magnetic fields. These studies offer a clear spectrum interpretation with respect to the effects of blending, local velocity fields, and chemical abundances, and allow us to identify a proper sample of spectral lines appropriate for magnetic field determination. High-resolution spectropolarimetric observations of the three sharp-lined Herbig Ae stars HD101412, HD104237, and HD190073 have been obtained in recent years with the HARPS. We used these archival observations to investigate the behaviour of their longitudinal magnetic fields. To carry out the magnetic field measurements, we used the multi-line singular value decomposition method. We discovered that different line lists for HD101412 yield differences in both the shape of the Stokes V signatures and their field strengths. They could be interpreted in the context of the impact of the circumstellar matter and elemental abundance inhomogeneities on the measurements of the magnetic field. Due to the small size of the Zeeman features on the first three epochs and the lack of near-IR observations, circumstellar and photospheric contributions cannot be estimated unambiguously. In the Stokes V spectrum of HD104237, we detect that the secondary component, a T Tauri star, possesses a rather strong magnetic field <B_z>=129G, while no significant field is present in the primary component. Our measurements of HD190073 confirm the presence of a variable magnetic field and indicate that the circumstellar environment may have a significant impact on the observed polarization features.

Experimental demonstration of all-optical weak magnetic field detection using beam-deflection of single-mode fiber coated with cobalt-doped nickel ferrite nanoparticles.

We experimentally demonstrate single-mode optical-fiber-beam-deflection configuration for weak magnetic-field-detection using an optimized (low coercive-field) composition of cobalt-doped nickel ferrite nanoparticles. Devising a fiber-double-slit type experiment, we measure the surrounding magnetic field through precisely measuring interference-fringe yielding a minimum detectable field ∼100 mT and we procure magnetization data of the sample that fairly predicts SQUID measurement. To improve sensitivity, we incorporate etched single-mode fiber in double-slit arrangement and recorded a minimum detectable field, ∼30 mT. To further improve, we redefine the experiment as modulating fiber-to-fiber light-transmission and demonstrate the minimum field as 2.0mT. The device will be uniquely suited for electrical or otherwise hazardous environments.

B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type starsβ CMa andϵ CMa

Within the context of the "B fields in OB stars (BOB)" collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars beta CMa (HD44743; B1 II/III) and epsilon CMa (HD52089; B1.5 II). For both stars, we consistently detected the signature of a weak (<30 G in absolute value) longitudinal magnetic field. We determined the physical parameters of both stars and characterise their X-ray spectrum. For beta CMa, our mode identification analysis led to determining a rotation period of 13.6+/-1.2 days and of an inclination angle of the rotation axis of 57.6+/-1.7 degrees, with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of ~22 degrees and a dipolar magnetic field strength (Bd) of ~100 G (60<Bd<230 G within 1 sigma), below what is typically found for other magnetic massive stars. For epsilon CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Both stars are expected to have a dynamical magnetosphere. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2/3 of their main sequence lifetime. A histogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field "desert" observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic fields in massive stars with the currently available instrumentation and techniques imply that weak fields might be more common than currently observed. Our results show that, if present, even relatively weak magnetic fields are detectable in massive stars and that more observational effort is probably still needed to properly access the magnetic field incidence.