Conventional Magnetometer Research Articles

Obtaining High‐Resolution Magnetic Records From Speleothems Using Magnetic Microscopy

AbstractSpeleothems are mineral deposits capable of recording detrital and/or chemical remanent magnetization at annual timescales. They can offer high‐resolution paleomagnetic records of short‐term variations in Earth's magnetic field, crucial for understanding the evolution of the dynamo. Owing to limitations on the magnetic moment sensitivity of commercial cryogenic rock magnetometers (∼10−11 Am2), paleomagnetic studies of speleothems have been limited to samples with volumes of several hundreds of mm3, averaging tens to hundreds of years of magnetic variation. Nonetheless, smaller samples (∼1–10 mm3) can be measured using superconducting quantum interference device (SQUID) microscopy, with a sensitivity better than ∼10−15 Am2. To determine the application of SQUID microscopy for obtaining robust high‐resolution records from small‐volume speleothem samples, we analyzed three different stalagmites collected from Lapa dos Morcegos Cave (Portugal), Pau d'Alho Cave (Brazil), and Crevice Cave (United States). These stalagmites are representative of a range of magnetic properties and have been previously studied with conventional rock magnetometers. We show that by using SQUID microscopy we can achieve a five‐fold improvement in temporal resolution for samples with higher abundances of magnetic carriers (e.g., Pau d'Alho Cave and Lapa dos Morcegos Cave). In contrast, speleothems with low abundances of magnetic carriers (e.g., Crevice Cave) do not benefit from higher resolution analysis and are best analyzed using conventional rock magnetometers. Overall, by targeting speleothem samples with high concentrations of magnetic carriers we can increase the temporal resolution of magnetic records, setting the stage for resolving geomagnetic variations at short time scales.

Optimizing magnetometers arrays and analysis pipelines for multivariate pattern analysis

BackgroundMultivariate pattern analysis (MVPA) has proven an excellent tool in cognitive neuroscience. It also holds a strong promise when applied to optically-pumped magnetometer-based magnetoencephalography. New methodTo optimize OPM-MEG systems for MVPA experiments this study examines data from a conventional MEG magnetometer array, focusing on appropriate noise reduction techniques for magnetometers. We determined the least required number of sensors needed for robust MVPA for image categorization experiments. ResultsWe found that the use of signal space separation (SSS) without a proper regularization significantly lowered the classification accuracy considering a sub-array of 102 magnetometers or a sub-array of 204 gradiometers. We also found that classification accuracy did not improve when going beyond 30 sensors irrespective of whether SSS has been applied. Comparison with existing methodsThe power spectra of data filtered with SSS has a substantially higher noise floor that data cleaned with SSP or HFC. Consequently, MVPA decoding results obtained from the SSS-filtered data are significantly lower compared to all other methods employed. ConclusionsWhen designing MEG system based on SQUID magnetometers optimized for multivariate analysis for image categorization experiments, about 30 magnetometers are sufficient. We advise against applying SSS filters without a proper regularization to data from MEG and OPM systems prior to performing MVPA as this method, albeit reducing low-frequency external noise contributions, also introduces an increase in broadband noise. We recommend employing noise reduction techniques that either decrease or maintain the noise floor of the data like signal-space projection, homogeneous field correction and gradient noise reduction.

Suppression of spin-exchange relaxation in Bell-Bloom magnetometer

The atomic spin-exchange relaxation is the most important limitation of the sensitivity for the optically pump magnetometers operated on non-zero magnetic field environment. In this paper, a method with 2π magnetic field pulse modulation is proposed to suppress the spin-exchange relaxation by counteracting the Larmor precession effect caused by the external magnetic field, which allows being operated under non-zero magnetic field. This method applies an additional periodic 2π pulse magnetic field repeated with the Larmor precession frequency on the conventional Bell-Bloom magnetometer. The theoretical model of the Bell-Bloom magnetometer responding with 2π pulse magnetic field is established, and the suppression of the spin-exchange relaxation is demonstrated experimentally. This method reduces the spin-exchange relaxation by 91.4% and improves the magnetometer response by 74.0%, which is directly beneficial to the sensitivity of the atomic magnetometer applied in μT-scale magnetic field.

Measurement of Kerr rotation and ellipticity in magnetic thin films by MOKE magnetometry

When polarized light is incident on a magnetic material, the magneto-optical Kerr effect (MOKE) rotates the polarization and induces ellipticity in the reflected light, which allows the magnetization direction to be probed optically. The Kerr rotation and ellipticity determine the magnitude of the effect and are usually measured using dedicated ellipsometers. Here, we demonstrate a simple method for extracting Kerr rotation and ellipticity in magnetic thin films using a conventional MOKE magnetometer consisting of two polarizers and a quarter waveplate. Using this technique, we report the longitudinal Kerr angle of BiYIG, GdCo, and TbCo. We additionally observe a linear decrease in polar complex Kerr angle magnitude in 3 nm GdCo films as the atomic fraction of Gd is increased.

Fourier-space generalized magneto-optical ellipsometry

The magneto-optical Kerr effect (MOKE) is widely exploited in laboratory-based setups for the study of thin films and nanostructures, providing magnetic characterization with good spatial and temporal resolutions. Due to the complex coupling of light with a magnetic sample, conventional MOKE magnetometers normally work by selecting a small range of incident wave-vector values, focusing the incident light beam to a small spot, and recording the reflected intensity at that angular range by means of photodetectors. Using this approach, additional methodologies and measurements are required for full vectorial magnetic characterization. Here, we computationally investigate a Fourier-space MOKE setup, where a focused beam ellipsometer using high numerical aperture optics and a camera detector is employed to simultaneously map the intensity distribution for a wide range of incident and reflected wave vectors. We employ circularly incident polarized light and no analyzing optics, in combination with a fitting procedure of the light intensity maps to the analytical expression of the Kerr effect under linear approximation. In this way, we are able to retrieve the three unknown components of the magnetization vector as well as the material' s optical and magneto-optical constants with high accuracy and short acquisition times, with the possibility of single-shot measurements. Fourier MOKE is thus proposed as a powerful method to perform generalized magneto-optical ellipsometry for a wide range of magnetic materials and devices.

Sandwich structure magnetometer with a high sensitivity and signal-to-noise ratio based on an ultrahigh-Q CaF2 resonator.

Whispering gallery mode (WGM) resonators with an ultrahigh quality (Q) factor provide an extremely high resolution for high-precision sensing. However, it is difficult to use them directly in magnetic sensors because of the transparency to the magnetic field. In this paper, a sandwich structure consisting of a neodymium iron boron magnet and calcium fluoride resonator with a Q factor of 109 is proposed. The experimental results show that, compared with the conventional magnetometer, the signal-to-noise ratio of the optical WGM magnetometer reaches 62dB, with the direct current sensitivity of 42.59MHz/mT and the AC sensitivity of 794p T H z -1/2 at 42kHz.

Direct Magnetic Evidence, Functionalization, and Low-Temperature Magneto-Electron Transport in Liquid-Phase Exfoliated FePS3.

Magnetism and the existence of magnetic order in a material is determined by its dimensionality. In this regard, the recent emergence of magnetic layered van der Waals (vdW) materials provides a wide playground to explore the exotic magnetism arising in the two-dimensional (2D) limit. The magnetism of 2D flakes, especially antiferromagnetic ones, however, cannot be easily probed by conventional magnetometry techniques, being often replaced by indirect methods like Raman spectroscopy. Here, we make use of an alternative approach to provide direct magnetic evidence of few-layer vdW materials, including antiferromagnets. We take advantage of a surfactant-free, liquid-phase exfoliation (LPE) method to obtain thousands of few-layer FePS3 flakes that can be quenched in a solvent and measured in a conventional SQUID magnetometer. We show a direct magnetic evidence of the antiferromagnetic transition in FePS3 few-layer flakes, concomitant with a clear reduction of the Néel temperature with the flake thickness, in contrast with previous Raman reports. The quality of the LPE FePS3 flakes allows the study of electron transport down to cryogenic temperatures. The significant through-flake conductance is sensitive to the antiferromagnetic order transition. Besides, an additional rich spectra of electron transport excitations, including secondary magnetic transitions and potentially magnon-phonon hybrid states, appear at low temperatures. Finally, we show that the LPE is additionally a good starting point for the mass covalent functionalization of 2D magnetic materials with functional molecules. This technique is extensible to any vdW magnetic family.

3-D Visualization of Magnetic Field Using In-Line Holographic Microscopy for Micro-Magnetofluidic Applications

The increasing attentions paid to magnetofluidic devices with their applications in cell manipulation, drug delivery, immunoassay, etc., call for higher requirements for precise detection of the magnetic field parameters at micro-scale for more flexible manipulations in precision. Nevertheless, conventional magnetometers can only measure magnetic field strength or flux off the device, while it still remains a challenge to determine the real field distribution within the microchips with high precision. Herein, a digital holography-based technique capable of on- chip measurement and visualization of magnetic field distribution is presented to solve this problem. The interference pattern of the ferromagnetic chain is firstly recorded by an in-line holographic microscopy system with the assistance of the magnetic particles which can be aligned and form a chain along the field direction. Each part digitally reconstructed in different depths is then auto-focalized in an extended focus image with corresponding colors. The detection angle of the ferromagnetic chain is presented in three dimensions, covering a range from -40 to 40 degrees and allowing for an automatic recording and visualization of the magnetic field direction. Our on- chip measuring approach can pronouncedly enhance the magnetic control for magnetofluidic devices with complex 3-D architectures, facilitating their effective applications in the near future.

Enhancement of bandwidth in spin-exchange relaxation-free (SERF) magnetometers with amplitude-modulated light

We demonstrate the bandwidth enhancement of an all-optical spin-exchange relaxation-free (SERF) magnetometer based on amplitude-modulated (AM) light. Alkali metal atoms are modulated directly by the pump beam instead of the modulation field or radio frequency field. The first harmonic demodulation of an AM SERF magnetometer with a modulation intensity of 15 kHz results in a high bandwidth of over 11 kHz with a sensitivity of 20 fT/Hz1/2 at 30 Hz and 60 fT/Hz1/2 at 10 kHz. Meanwhile, the AM SERF magnetometer with DC demodulation presents the same sensitivity as a traditional DC SERF magnetometer (6 fT/Hz1/2 at 30 Hz). The presented technique for modulating the amplitude of the pump beam allows AM SERF magnetometers to enter the domain of high-bandwidth magnetometers and opens the door to many areas that are inaccessible to conventional magnetometers.

Surpassing the Energy Resolution Limit with Ferromagnetic Torque Sensors.

We discuss the fundamental noise limitations of a ferromagnetic torque sensor based on a levitated magnet in the tipping regime. We evaluate the optimal magnetic field resolution taking into account the thermomechanical noise and the mechanical detection noise at the standard quantum limit. We find that the energy resolution limit, pointed out in recent literature as a relevant benchmark for most classes of magnetometers, can be surpassed by many orders of magnitude. Moreover, similarly to the case of a ferromagnetic gyroscope, it is also possible to surpass the standard quantum limit for magnetometry with independent spins, arising from spin-projection noise. Our finding indicates that magnetomechanical systems optimized for magnetometry can achieve a magnetic field resolution per unit volume several orders of magnitude better than any conventional magnetometer. We discuss possible implications, focusing on fundamental physics problems such as the search for exotic interactions beyond the standard model.

Roadmap on Magnetoelectric Materials and Devices

The possibility of tuning the magnetic properties of materials with voltage (converse magnetoelectricity) or generating electric voltage with magnetic fields (direct magnetoelectricity) has opened new avenues in a large variety of technological fields, ranging from information technologies to healthcare devices and including a great number of multifunctional integrated systems, such as mechanical antennas, magnetometers, and radio frequency (RF) tunable inductors, which have been realized due to the strong strain-mediated magnetoelectric (ME) coupling found in ME composites. The development of single-phase multiferroic materials (which exhibit simultaneous ferroelectric and ferromagnetic or antiferromagnetic orders), multiferroic heterostructures, as well as progress in other ME mechanisms, such as electrostatic surface charging or magneto-ionics (voltage-driven ion migration), have a large potential to boost energy efficiency in spintronics and magnetic actuators. This article focuses on existing ME materials and devices and reviews the state of the art in their performance. The most recent progress on different ME devices based on ME heterostructures is presented but with a larger emphasis on ME antennas and sensors due to the significant advances achieved in these applications. The rapid development of mechanically actuated ME antennas has been observed over the past several years, producing ME antennas that are miniaturized by 1-2 orders compared to conventional antenna size. Magnetic sensors based on simple ME composites are potentially promising alternatives to conventional magnetometers due to their very good detectivity (<; pT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> ) at low frequencies. Other ME devices reviewed in this article include RF tunable inductors with high inductance tunability and quality (Q) factor; non-reciprocal microelectromechanical system (MEMS) bandpass filters with dual H- and E-field tunability; passive isolators and gyrators in the low-frequency (LF) range; and ME random access memories for low-power data storage. All these compact and lightweight ME devices are also promising for future biomedical and wireless applications. Finally, some open questions and future directions where the community might be headed are provided.

A nanosecond-resolved atomic hydrogen magnetometer.

We introduce a novel and sensitive ns-resolved atomic magnetometer, which is at least three orders of magnitude faster than conventional magnetometers. We use the magnetic field dependence of the hyperfine beating of high-density spin-polarized H atoms, produced from the rapid photodissociation of HCl gas with sub-ns laser pulses and measured with a pick-up coil, to demonstrate ns-resolved magnetometry, and project sensitivity of a few nT for a spin-projection-limited sensor with 10 nl measurement volume after 1 ns measurement time. The magnetometer will allow ultrafast continuous B-field measurements in many fields, including spin chemistry, spin physics, and plasma physics.

Wide-bandwidth atomic magnetometry via instantaneous-phase retrieval

We develop and demonstrate a new protocol that allows sensing of magnetic fields in an extra-ordinary regime for atomic magnetometry. Until now, the demonstrated bandwidth for atomic magnetometry has been constrained to be slower than the natural precession of atomic spins in a magnetic field---the Larmor frequency. We demonstrate a new approach that tracks the instantaneous phase of atomic spins to measure arbitrarily modulated magnetic fields with frequencies up to fifty times higher than the Larmor frequency. By accessing this regime, we demonstrate magnetic-field measurements across four decades in frequency up to 400 kHz, over three orders of magnitude wider than conventional atomic magnetometers. Furthermore, we demonstrate that our protocol can linearly detect transient fields 100--fold higher in amplitude than conventional methods. We highlight the bandwidth and dynamic range of the technique by measuring a magnetic field with a broad and dynamical spectrum.

Measuring the Magnetization from the Image of the Stripe Magnetic Domain

Magnetization, the magnetic moment per unit volume, is the basic quantity for describing a magnetic system. Generally, magnetic imaging techniques show the relative magnitude of magnetization, and its direction. Determining the $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e$ value of magnetization requires separate measurement with a reference sample, though. This study describes the stripe state of magnetization, in which the absolute value can be measured simply by magneto-optical imaging. In particular, this approach is applicable to samples so small that they cannot be measured by conventional magnetometers.

Determination of the Dzyaloshinskii-Moriya interaction in exchange biased Au/Co/NiO systems

The determination of the interfacial Dzyaloshinskii-Moriya interaction for perpendicularly magnetized thin layered films is not trivial and therefore needs some experimental efforts to determine its sign and magnitude, especially in the exchange biased system. Here, we developed a method proposed by D.-S. Han et al. [Nano Lett. 16, (2016) 4438], which opens a way to investigate this interaction using a conventional PMOKE magnetometer, also in exchange biased systems. Using our approach we demonstrated that the Au/Co/NiO layered system has a strong negative Dzyaloshinskii-Moriya interaction, which is independent of the direction of perpendicular interlayer exchange bias coupling.

Application of SQUIDs to low temperature and high magnetic field measurements-Ultra low noise torque magnetometry.

Torque magnetometry is a key method to measure the magnetic anisotropy and quantum oscillations in metals. In order to resolve quantum oscillations in sub-millimeter sized samples, piezo-electric micro-cantilevers were introduced. In the case of strongly correlated metals with large Fermi surfaces and high cyclotron masses, magnetic torque resolving powers in excess of 104 are required at temperatures well below 1 K and magnetic fields beyond 10 T. Here, we present a new broadband read-out scheme for piezo-electric micro-cantilevers via Wheatstone-type resistance measurements in magnetic fields up to 15 T and temperatures down to 200 mK. By using a two-stage superconducting-quantum interference device as a null detector of a cold Wheatstone bridge, we were able to achieve a magnetic moment resolution of Δm = 4 × 10-15 J/T at maximal field and 700 mK, outperforming conventional magnetometers by at least one order of magnitude in this temperature and magnetic field range. Exemplary de Haas-van Alphen measurement of a newly grown delafossite, PdRhO2, was used to show the superior performance of our setup.

Scanning SQUID microscope with an in-situ magnetization/demagnetization field for geological samples

Magnetic properties of rocks are crucial for paleo-, rock-, environmental-magnetism, and magnetic material sciences. Conventional rock magnetometers deal with bulk properties of samples, whereas scanning microscope can map the distribution of remanent magnetization. In this study, a new scanning microscope based on a low-temperature DC superconducting quantum interference device (SQUID) equipped with an in-situ magnetization/demagnetization device was developed. To realize the combination of sensitive instrument as SQUID with high magnetizing/demagnetizing fields, the pick-up coil, the magnetization/demagnetization coils and the measurement mode of the system were optimized. The new microscope has a field sensitivity of 250 pT/√Hz at a coil-to-sample spacing of ∼350 µm, and high magnetization (0–1 T)/ demagnetization (0–300 mT, 400 Hz) functions. With this microscope, isothermal remanent magnetization (IRM) acquisition and the according alternating field (AF) demagnetization curves can be obtained for each point without transferring samples between different procedures, which could result in position deviation, waste of time, and other interferences. The newly-designed SQUID microscope, thus, can be used to investigate the rock magnetic properties of samples at a micro-area scale, and has a great potential to be an efficient tool in paleomagnetism, rock magnetism, and magnetic material studies.

基于激光偏振调制的全光Cs原子磁力仪研究

Modulation of laser's left-handed and right-handed polarization was used in the paper. This method has realized all-optical detection of magnetic field and avoided cross talk existing in conventional atomic magnetometer. The detection principle of precession frequency of atomic magnetic moment in magnetic field was demonstrated based on the method, and the signal was analyzed, then the detection and processing system was designed based on the analysis. The in-phase signal was displayed as Lorenz lineshape. The quadrature signal was displayed as dispersion lineshape, and the magnitude was 0 at resonance frequency. The phase signal was displayed as monotonically decreasing function with frequency, and the magnitude was 0 at resonance frequency. Both the quadrature and phase signals can be used to lock resonance frequency and realize measurement of magnetic field.

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.

Flexible Microwire Residence Times Difference Fluxgate Magnetometer

We present a novel realization of the microwire fluxgate magnetometer based on a flexible CoFeSiB core. The readout is underpinned by the residence times difference, wherein the target dc magnetic field information is derived from the temporal positions of the output waveform “spikes” generated by the crossing of two thresholds. This readout is known to be simple, and less noisy, as well as less computationally intensive compared to traditional frequency domain readouts. The design of the magnetometer is discussed with emphasis on the sensor configuration, sensing capabilities, resolution, sensitivity, and hysteresis. We describe a specific device that has been realized using the flexible core technology; in particular, we compare it to a “classical” microwire fluxgate (having a rigid core), previously developed by us. As a part of the evaluation of the sensor performance, its ability to locate the position of a dc quasi-static magnetic source is characterized and related to the radius of curvature of the core. Our results demonstrate the possibility of using the flexible architecture as the sensing element for the detection of weak (dc) magnetic targets with a very good performance in terms of resolution, sensitivity, and magnetic coupling; the performance is far superior to conventional fluxgate magnetometers constructed with nonflexible sensing elements.