Magnetic Noise Sources Research Articles

Simulation of pulse width modulation strategies: validation based on sound quality evaluations

Though the noise emitted by an electrical machine is less loud than the one from a combustion engine, the existing literature shows it is not always more pleasant. This is because of its strong tonal content, due to deterministic excitations (e.g. electromagnetic slotting and switching effects, mechanical gear mesh effects). The main source of magnetic noise is the radiation of the stator submitted to electromagnetic forces, which are harmonic by nature. Their highest frequency components are generated by the Pulse Width Modulation (PWM) technique designed to vary the supply frequency and operating speed of the electric machine. Several PWM strategies exist, leading to specific harmonic spectra. A temporal simulation model has been developed to auralize the corresponding electromagnetic noise emitted by the machine, considering its mechanical and electrical parameters. A testbench has been adapted to test the relevance of the model, both from an electrical and acoustic point of view. The simulation model and the testbench can be used to generate virtual and real sound stimuli to be assessed in a jury testing. The subjective evaluations allow to assess the relevance of the auralization model, but also to identify the least annoying strategies.

P1 Center Electron Spin Clusters Are Prevalent in Type Ib Diamonds.

Understanding the spatial distribution of the P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as polarization sources for dynamic nuclear polarization (DNP) quantum sensing and play a significant role in the relaxation of nitrogen vacancy (NV) centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized DNP and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric line shapes. The 13C DNP frequency profile at a high magnetic field revealed a pattern that requires an asymmetric EPR line shape of the P1 clusters with electron-electron (e-e) coupling strengths exceeding the 13C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e-e couplings. We employed a two-frequency pump-probe pulsed electron double resonance technique to show cross-talk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all of the P1 populations. Direct observation of clustered P1 centers and their asymmetric line shape offers a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for 13C and other nuclear spins of analytes. We propose that room temperature 13C DNP at a high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects.

A Magnetoelastic Twist on Magnetic Noise: The Connection with Intrinsic Nonlinearities

AbstractIntrinsic magnetic noise limits the functionality of all magnetic field sensors and related devices using magnetic films as sensing elements. A novel origin of magnetic noise due to ferromagnetic material's magnetostriction is revealed by implementing a comprehensive multi‐level signal‐noise model and thoroughly validating it experimentally on magnetoelectric composite ΔE‐effect sensors. From electrical measurements and operando magnetic domain visualization, magnetic contributions are shown to dominate the noise floor and limit the overall performance of multi‐domain magnetoelastic sensors. The newly introduced noise contribution is correlated with the nonlinearity of the magnetostrictive properties. The effect on sensor output is particularly pronounced in magnetoelastic and magnetoelectric composite devices, but the results generally apply to all sensor devices incorporating magnetic materials. Therefore, the identified magnetic noise source makes a significant contribution to understanding the limitations of magnetic sensors and provides important guidelines for optimizing future magnetic sensors for low detectivity.

Observation of Magnetic Domains in Amorphous Magnetic Wires with a Diameter of 10 μm Used in GSR Sensors.

The core of a Gigahertz Spin Rotation (GSR) sensor, a compact and highly sensitive magnetic sensor, is composed of Co-Fe-based amorphous magnetic wire with a diameter of 10 μm. Observations of the magnetic domain structure showed that this magnetic wire has unusual magnetic noise characteristics. Bamboo-shaped magnetic domains a few hundred micrometers in width were observed to form inside the wire, and smaller domains a few micrometers across were observed to form inside these larger domains. The magnetic domain pattern changed abruptly when an external magnetic field was applied to the wire. Herein is shown how these changes may be a source of magnetic noise in the wire.

Possible Origin of Low-Frequency Magnetic Flux Noise in Superconducting Devices

The magnetic flux noise caused by surface spin fluctuations in superconducting quantum interference devices (SQUIDs) limits their development. In this work, we report that different adsorbents such as H, O2, NO, and NO2 that adsorb on the surfaces of Mg-based and Pb-based SQUIDs, respectively, producing large local magnetic moments ranging from 0.7-1.6 μB, with energy barriers for thermal spin fluctuation as low as 10-30 mK. Moreover, we observe that the presence of H atoms on the surface of MgO can cause the coadsorption of other molecules, which generates additional spin sources. Monte Carlo simulations of the weakly coupled spin on a two-dimensional square lattice produce a low-frequency flux noise spectrum. We suggest eliminating the surface magnetism by coating the surface with monolayer indium phosphide or protecting the surface from other molecules by nonmagnetic preoccupants with a larger adsorption energy. The work provides important physical insights and feasible strategies for reducing magnetic noise sources in superconducting circuits.

Decoherence of nitrogen-vacancy spin ensembles in a nitrogen electron-nuclear spin bath in diamond

Nitrogen-vacancy (NV) centers in diamond have been developed into essential hardware units for a wide range of solid-state-based quantum technology applications. While such applications require the long spin coherence times of the NV centers, they are often limited due to decoherence. In this study, we theoretically investigate the decoherence of NV-spin ensembles induced by nitrogen impurities (P1 centers), which are one of the most dominant and inevitable magnetic field noise sources in diamond. We combined cluster correlation expansion and density functional theory to compute the Hahn-echo spin-coherence time of the NV centers for a broad range of P1 concentrations. Results indicate a clear linear dependence of T2 on P1 concentrations on a log scale with a slope of −1.06, which is in excellent agreement with previous experimental results. The interplay between the Jahn–Teller effect and the hyperfine interaction in the P1 center plays a critical role in determining the bath dynamics and the resulting NV decoherence. Our results provide a theoretical upper bound for the NV-spin T2 over a wide range of P1 densities, serving as a key reference for materials optimization and spin bath characterization to develop highly coherent NV-based devices for quantum information technology.

Quantitative study of the response of a single NV defect in diamond to magnetic noise

The nitrogen-vacancy (NV) defect in diamond is an efficient quantum sensor of randomly fluctuating signals via relaxometry measurements. In particular, the longitudinal spin relaxation of the NV defect accelerates in the presence of magnetic noise with a spectral component at its electron spin resonance frequency. We look into this effect quantitatively by applying a calibrated and tunable magnetic noise on a single NV defect. We show that an increase of the longitudinal spin relaxation rate translates into a reduction of the photoluminescence (PL) signal emitted under continuous optical illumination, which can be explained using a simplified three-level model of the NV defect. This PL quenching mechanism offers a simple, all-optical method to detect magnetic noise sources at the nanoscale.

Magnetic field noise analyses generated by the interactions between a nitrogen vacancy center diamond and surface and bulk impurities

We investigated the mechanism of magnetic noise due to both surface and bulk impurities. For surface noise, we apply the Langevin method to spin fluctuation theory to calculate the noise for paramagnetic surface impurities absorbed in a thin layer of water. We find that the mechanisms generating noise are spin flip and spin precession which depend on impurity spin relaxation and spin precession time. For the bulk noise, we consider carbon-13 and nitrogen as impurities and employ the correlated-cluster expansion to calculate noise. Carbon-13 noise is a few orders of magnitude larger than nitrogen due to the higher impurity density in the typical NV center diamond system. We also find that the noise in the secular approximation underestimates noise under low applied magnetic field. Overall, the major source of magnetic field noise is spin precession noise, which is more than five orders of magnitude larger than the spin flip noise.

Development of noise correction method of environmental magnetic field with reference magnetic sensor for magnetocardiography

AbstractIn recent years, a magnetocardiography (MCG) with superconducting quantum interference device with noise level of 50 fT/√ Hz is used for arrhythmia diagnosis. The MCG is usually installed in a multilayered magnetically shielded room (MSR). In the MCG close to some magnetic noise sources like direct current (dc)‐electrified railway, the MCG can stably measure the magnetic field by increasing the number of layers of MSR. However, increasing the number of layers in the MSR increases the cost and weight. We developed the method to correct the environmental magnetic field (EMF) in the MCG that considers the reduction response of each EMF reduction technology of the MCG including the MSR by referring to the signal measured by the reference magnetic sensor outside the MSR. The effect of the correction was evaluated for each channel of the MCG by using the EMF signal from the dc‐electrified railway as the magnetic noise source. The average value of the correction effect was 18.1 dB, the standard deviation (SD) was 4.1 dB, and the 2SD range was from 9.8 to 26.4 dB. From these results, it is suggested that the developed method has the possibility to improve the EMF in the MCG by more than 9 dB.

Magnetic shielding of long paraboloid structures in the inhomogeneous magnetic field

Shielding efficacy of the high-temperature superconducting (HTS) magnetic shields depends on the superconductor properties and on the orientation of the external magnetic field. For precise magnetic field measurements in areas with changing direction of magnetic noise it is important to reduce both the parallel and perpendicular components of the magnetic field. We have designed and fabricated magnetic shields of 25 cm long paraboloid shape with closed sides from second-generation HTS tapes. We have characterized HTS shields in DC and variable frequency AC magnetic fields at 77 K above a copper electromagnet acting as the source of inhomogeneous magnetic noise. The HTS magnetic shields reduce the magnetic field noise penetration and enhance the sensitivity of magnetic field sensors. The measurements were performed with the magnetic shield placed between the noise source and the sensor. 2D finite element analysis using Comsol model was generated and the results were compared with the experimental data of magnetic field dependences of the shielding factor (SF).

A Novel Method for Wide Range Electric Current Measurement in Gas-Insulated Switchgears With Shielded Magnetic Measurements

Magnetic field (MF) sensing and interpretation remains an active area of research for source current reconstruction and measurement applications. One such area is large current measurement for power system components with contact less MF sensing. The output of recently made available magnetic sensors is favorable for large current measurement applications. This stimulates development of a noncontact magnetic sensor-based large electric current measurement system for fixed conductors to be employed in power systems. However, the presence of strong magnetic noise from near equipment imposes restriction on the use of sensitive magnetic sensor in open air where magnetic noise sources operating at the same frequency may distort the sensor output. The novelty of this paper is the application of MF sensor inside magnetic shielding layers for large current measurement. High-permeability Mu-metal is employed and arranged in three arc-shaped layers for designing a semicircular special shielding arrangement. The application of the method and design is demonstrated for large current carrying conductor arranged inside a gas-insulated switchgear. The measurement system is first validated by the finite-element analysis method and then tested with a comprehensive experimental setup in the presence of strong magnetic noise stimulus of 0.28 T generated by a strong neodymium magnet. The resultant apparatus is tested for current of up to 1000 A with a resolution of 0.01% and error less than 3% in the worst case scenario. The prototype demonstrated excellent linearity during experimental validation of current measurement of 100–1000 A in the presence of magnetic noise stimuli at different positions where the standard deviation between slopes value for sensor response to input current (100–1000 A) curve for six different test cases remains only 0.83%.

Magnetic noise from ultrathin abrasively deposited materials on diamond

Sensing techniques based on the negatively charged nitrogen-vacancy (NV) center in diamond have emerged as promising candidates to characterize ultrathin and $2\mathrm{D}$ materials. An outstanding challenge to this goal is isolating the contribution of $2\mathrm{D}$ materials from undesired contributions arising from surface contamination and changes to the diamond surface induced by the sample or transfer process. Here we report on such a scenario, in which the abrasive deposition of trace amounts of materials onto a diamond gives rise to a previously unreported source of magnetic noise. By deliberately scratching the diamond surface with macroscopic blocks of various metals (Fe, Cu, Cr, Au), we are able to form ultrathin structures (i.e., with thicknesses down to $<1\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$, and find that these structures give rise to a broadband source of noise. Explanation for these effects are discussed, including spin and charge noise native to the sample and/or induced by sample-surface interactions, and indirect effects, where the deposited material affects the charge stability and magnetic environment of the sensing layer. This work illustrates the high sensitivity of NV noise spectroscopy to ultrathin materials down to subnanometer regimes---a key step toward the study of $2\mathrm{D}$ electronic systems---and highlights the need to passivate the diamond surface for future sensing applications in ultrathin and 2D materials.

Impact of Surface Functionalization on the Quantum Coherence of Nitrogen-Vacancy Centers in Nanodiamonds.

Nanoscale quantum probes such as the nitrogen-vacancy (NV) center in diamonds have demonstrated remarkable sensing capabilities over the past decade as control over fabrication and manipulation of these systems has evolved. The biocompatibility and rich surface chemistry of diamonds has added to the utility of these probes but, as the size of these nanoscale systems is reduced, the surface chemistry of diamond begins to impact the quantum properties of the NV center. In this work, we systematically study the effect of the diamond surface chemistry on the quantum coherence of the NV center in nanodiamonds (NDs) 50 nm in size. Our results show that a borane-reduced diamond surface can on average double the spin relaxation time of individual NV centers in nanodiamonds when compared to thermally oxidized surfaces. Using a combination of infrared and X-ray absorption spectroscopy techniques, we correlate the changes in quantum relaxation rates with the conversion of sp2 carbon to C-O and C-H bonds on the diamond surface. These findings implicate double-bonded carbon species as a dominant source of spin noise for near surface NV centers. The link between the surface chemistry and quantum coherence indicates that through tailored engineering of the surface, the quantum properties and magnetic sensitivity of these nanoscale systems may approach that observed in bulk diamond.

Localization of a magnetic moment using a two-qubit probe

A nanomagnet precessing in an external magnetic field can be treated as a source of narrow-bandwidth magnetic noise, that leaves characteristic fingerprints in decoherence of a nearby spin qubit undergoing dynamical decoupling. We show how, by measurements of two-qubit coherence, a noise sensor composed of qubit pair can be used to reconstruct the position of the nanomagnet. Such localization of noise source is possible with only two qubit probes, because the course of coherence decay under appropriately designed dynamical decoupling sequences contain information not only about noises experienced by each qubit, but also about their cross-correlations. We test the applicability of the proposed protocol on an example of two qubits coupled to the nanomagnet via dipolar interaction. We also show how, using a two-qubit sensor possessing a particular symmetry, one can localize the nanomagnet even when the sensor-magnet coupling law is unknown.

BabyMEG: A whole-head pediatric magnetoencephalography system for human brain development research

We developed a 375-channel, whole-head magnetoencephalography (MEG) system ("BabyMEG") for studying the electrophysiological development of human brain during the first years of life. The helmet accommodates heads up to 95% of 36-month old boys in the USA. The unique two-layer sensor array consists of: (1) 270 magnetometers (10 mm diameter, ∼15 mm coil-to-coil spacing) in the inner layer, (2) thirty-five three-axis magnetometers (20 mm × 20 mm) in the outer layer 4 cm away from the inner layer. Additionally, there are three three-axis reference magnetometers. With the help of a remotely operated position adjustment mechanism, the sensor array can be positioned to provide a uniform short spacing (mean 8.5 mm) between the sensor array and room temperature surface of the dewar. The sensors are connected to superconducting quantum interference devices (SQUIDs) operating at 4.2 K with median sensitivity levels of 7.5 fT/√Hz for the inner and 4 fT/√Hz for the outer layer sensors. SQUID outputs are digitized by a 24-bit acquisition system. A closed-cycle helium recycler provides maintenance-free continuous operation, eliminating the need for helium, with no interruption needed during MEG measurements. BabyMEG with the recycler has been fully operational from March, 2015. Ongoing spontaneous brain activity can be monitored in real time without interference from external magnetic noise sources including the recycler, using a combination of a lightly shielded two-layer magnetically shielded room, an external active shielding, a signal-space projection method, and a synthetic gradiometer approach. Evoked responses in the cortex can be clearly detected without averaging. These new design features and capabilities represent several advances in MEG, increasing the utility of this technique in basic neuroscience as well as in clinical research and patient studies.

Identification of the Local Sources of Paramagnetic Noise in Superconducting Qubit Devices Fabricated onα−Al2O3Substrates Using Density-Functional Calculations

Effective methods for decoupling superconducting qubits (SQs) from parasitic environmental noise sources are critical for increasing their lifetime and phase fidelity. While considerable progress has been made in this area, the microscopic origin of noise remains largely unknown. In this work, first principles density functional theory calculations are employed to identify the microscopic origins of magnetic noise sources in SQs on an α-Al2O3 substrate. The results indicate that it is unlikely that the existence of intrinsic point defects and defect complexes in the substrate are responsible for low frequency noise in these systems. Rather, a comprehensive analysis of extrinsic defects shows that surface aluminum ions interacting with ambient molecules will form a bath of magnetic moments that can couple to the SQ paramagnetically. The microscopic origin of this magnetic noise source is discussed and strategies for ameliorating the effects of these magnetic defects are proposed.

Tuning a Spin Bath through the Quantum-Classical Transition

We study decoherence of a single nitrogen-vacancy (NV) center induced by the 13C nuclear spin bath of diamond. By comparing Hahn-Echo experiments on single and double-quantum transitions of the NV triplet ground state we demonstrate that this bath can be tuned into two different regimes. At low magnetic fields, the nuclei behave as a quantum bath which causes decoherence by entangling with the NV central spin. At high magnetic fields, the bath behaves as a source of classical magnetic field noise, which creates decoherence by imprinting a random phase on the NV central spin.

Measurements, characteristics, and origin of new electromagnetic interference on magnetocardiographic measurements

In order to eliminate the influence of the large-amplitude magnetic field noise that has complicated magnetocardiographic studies since October 2009, we have performed high-accuracy measurement of the environmental magnetic field noise in the vicinity of Beijing Subway Line 4 using a three-component height Tc radio frequency (rf) superconducting quantum interference device (SQUID). By analysing the spatial form and other characteristics of the time and the frequency domains and by calculating the circumferential magnetic field distribution based on a duel-end feeding system model, we reach the following conclusions: (i) the main source of magnetic field noise is the magnetic field generated by the subway trains, and (ii) the magnetic field interference results mainly from the imbalance between traction current and return rail current that is caused by the leakage current.

2D Hybrid Yttrium Iron Garnet Magnetic Sensor Noise Characterization

This paper deals with the noise characterization of a magnetic field hybrid sensor based on flux-gate-like magnetometer. In the used layout, a magnetic core, like an Yttrium-Iron-Garnet (YIG) thin film, is driven to saturation by a rotating magnetization field, which induces a modulated magnetic field. The latter is sensed, by means of one or more punctual sensors, as an image of the applied magnetic field vector components. Both theoretical principles and main equivalent magnetic noise sources are explained. Sensor performance is given and compared with measurements. The experimental setup and limiting factors, such as the coupling factor between the magnetization and the sensed signal, are also evaluated and analyzed. That leads to the illustration of the system performance in term of noise and sensitivity.

Lagrangian and energy forms for retrieving the impulse response of the Earth due to random electromagnetic forcing

We distinguish between trivial and nontrivial differences in retrieving the real or imaginary parts of the Green's function. Trivial differences come from different Green's function definitions. The energy and lagrangian forms constitute nontrivial differences. Magnetic noise sources suffice to extract the quasistatic electromagnetic-field Earth impulse response in the lagrangian form. This is of interest for Earth subsurface imaging. A numerical example demonstrates that all source vector components are necessary to extract a single-field vector component.