Nitrogen-vacancy Magnetometer Research Articles

Imaging the Magnetic Anisotropy in Ultrathin Fe4GeTe2 with a Nitrogen-Vacancy Magnetometer.

Two-dimensional (2D) FenGeTe2, with n = 3, 4, and 5, has been realized in experiments, showing strong magnetic anisotropy with enhanced critical temperature (Tc). The understanding of its magnetic anisotropy is crucial for the exploration of more stable 2D magnets and its spintronic applications. Here, we report a quantitative reconstruction of the magnetization magnitude and its direction in ultrathin Fe4GeTe2 using nitrogen vacancy centers. Through imaging stray magnetic fields, we identified the spin-flop transition at approximately 80 K, resulting in a change of the easy axis from the out-of-plane direction to the in-plane direction. Moreover, by analyzing the thermally activated escape behavior of the magnetization near Tc in terms of the Ginzburg-Landau model, we observed the in-plane magnetic anisotropy effect and the formation capability of magnetic domains at ∼0.4 μm2 μT-1. Our findings contribute to the quantitative understanding of the magnetic anisotropy effect in a vast range of 2D van der Waals magnets.

High-frequency resolution diamond nitrogen-vacancy center wide-spectrum imaging technology

High-resolution wide-spectrum measurement techniques have important applications in fields such as astronomy, wireless communication, and medical imaging. Nitrogen-vacancy (NV) center in diamond is well known for its high stability, high sensitivity, real-time monitoring, single-point detection, and suitability for long-term measurement, and has an outstanding choice for spectrum analyzers. Currently, spectrum analyzers based on NV centers as detectors can perform real-time spectrum analysis in the range of several tens of gigahertz, but their frequency resolution is limited to a MHz level. In this study, we construct a quantum diamond microwave spectrum imaging system by combining continuous wave-mixing techniques. According to the spin-related properties of the NV center in diamond, we implement optical pumping by 532 nm green laser light illuminating the diamond NV center. A spherical magnet is used to produce a magnetic field gradient along the direction of the diamond crystal. By adjusting the size and direction of the magnetic field gradient, spatial encoding of the resonance frequency of the NV center is achieved. The magnetic field gradient induces the Zeeman effect on the diamond surface at different positions, generating corresponding ODMR signals. Through accurate programming, we coordinate the frequency scanning step size of the microwave source with the camera exposure and image storage time, and synchronize them circularly according to the order of image acquisition. Ultimately, after algorithmic processing, we successfully obtain comprehensive spectrum data in a range from 900 MHz to 6.0 GHz. Within the measurable spectrum range, the system employs continuous wave-mixing, simultaneously applying resonant microwaves and slightly detuning auxiliary microwaves to effectively excite the NV center. This method triggers off microwave interference effects, disrupting the balance between laser-induced polarization and microwave-induced spontaneous relaxation. Specifically, microwave interference causes the phase and amplitude of the fluorescence signal to change, leading to the generation of alternating current fluorescence signals. This further enhances the response of the NV magnetometer to weak microwave signals. The method enables the system to achieve a frequency resolution of 1 Hz in the measurable spectrum range, and it can separately measure the frequency resolution of multiple frequency points with a frequency step size of 1 MHz. The research results indicate that the wide-spectrum measurement based on NV centers can achieve sub-hertz frequency resolution, providing robust technical support for future spectrum analysis and applications.

Noise Suppression of Nitrogen-Vacancy Magnetometer in Lock-In Detection Method by Using Common Mode Rejection.

Nitrogen-vacancy (NV) centers in diamonds are promising solid-state magnetic sensors with potential applications in power systems, geomagnetic navigation, and diamond NV color center current transformers, in which both high bandwidth and high magnetic field resolution are required. The wide bandwidth requirement often necessitates high laser power, but this induces significant laser fluctuation noise that affects the detection magnetic field resolution severely. Therefore, enhancement of the magnetic field resolution of wide-bandwidth NV center magnetic sensors is highly important because of the reciprocal effects of the bandwidth and magnetic field resolution. In this article, we develop a common mode rejection (CMR) model to eliminate the laser noise effectively. The simulation results show that the noise level of the light-detected magnetic resonance signal is significantly reduced by a factor of 6.2 after applying the CMR technique. After optimization of the laser power and modulation frequency parameters, the optimal system bandwidth was found to be 75 Hz. Simultaneously, the system's detection magnetic field resolution was enhanced significantly, increasing from 4.49 nT/Hz1/2 to 790.8 pT/Hz1/2, which represents an improvement of nearly 5.7 times. This wide-bandwidth, high-magnetic field resolution NV color center magnetic sensor will have applications including power systems, geomagnetic navigation, and diamond NV color center current transformers.

Compact multipass-laser-beam antenna for NV sensor sensitivity enhancement.

Large-area, highly uniform microwave field radiation and efficient excitation of fluorescence are the key to achieving high sensitivity sensing of the NV (nitrogen-vacancy) magnetometer. In this paper, we report a compact multipass-laser-beam antenna for NV ensemble color centers sensing. The antenna not only provides a tridimensional uniform magnetic field, but also can be used for efficient excitation of the NV fluorescence. The optimal size of the antenna and the angle of laser incidence are determined by the multi-physics field simulation software COMSOL. For an equivalent excitation power, the designed structure increases the path length of the excitation beam by up to three orders of magnitude, up to the level of m, compared to the conventional direct beam mode. Finally, this method increased the sensitivity by a factor of 60 realized a magnetic field sensitivity of 2.8 nT/√Hz in the range of 10-100 Hz. This work provides an experimental method for the design of integrated NV magnetometers.

High sensitivity of diamond nitrogen-vacancy magnetometer with magnetic flux concentrators via enhanced fluorescence collection

Diamond nitrogen-vacancy (NV) centers magnetometer is attracting more and more attention due to their room-temperature solid-state property. High sensitivity is crucial for practical applications of diamond NV magnetometers. However, the sensitivity of the diamond magnetometer needs to be improved for weak magnetic measurement. Recent studies have shown that diamond with magnetic flux concentrators (MFC) makes it feasible to achieve low-frequency sensitivity at fT/Hz level. Here, we demonstrate an enhanced fluorescence collection method to improve the sensitivity of diamond NV magnetometer with MFC. A high magnetic field sensitivity of 0.47 pT/Hz (>100Hz) is achieved under resonance conditions, which is about two times higher than that without improved fluorescence collection. We then integrate the diamond magnetometer (probe size ∼8×9×13 cm3) with a shielded sensitivity of 1 pT/Hz (>30Hz). Our method offers a solution for the high-sensitivity diamond NV magnetometer. Thus, this work enables an ultra-compact and scalable platform for quantum sensing based on the diamond NV center technique. Future, diamond NV magnetometers could be proposed used in shipwreck detection, cardiac magnetic field measurement, explosives detection, etc.

Hybrid NV Center Magnetometer With Directional Vector Resolution Based on Magnetic Flux Concentrators

Since the introduction of Hall magnetometers, different types of magnetometers have been widely used. Among them, magnetometers based on nitrogen-vacancy (NV) centers in diamonds are promising room-temperature solid-state sensors. Their sensitivity to low-frequency (< 10 Hz) magnetic fields has been reported to be currently at the nT level. Still, limitations in the detectable spatial range hinder their potential applications in medical imaging, earth sciences, and navigation. The most recent sensitivity enhancement technique involves using high-permeability flux concentrators with a unique magnetic field amplification structure that can cause the NV magnetometer to lose its vector detection capabilities. To address this limitation, we proposed a hybrid magnetometer based on a diamond-based hetero-NV center, which collects magnetic flux over a larger area and concentrates it in the diamond magnetometer, from which the magnetic field vector can be calculated by measuring the specific magnetic field direction vector. The low-frequency sensitivity of 90 pT∙Hz-1/2 under conventional ambient conditions has been solved by inserting NV-doped diamond blocks between four permalloy concentrators in a triangular structure, which solves the vector deficiency problem of traditional magnetic concentrators. High-sensitivity real-time variation acquisition of the magnetic field signal has been achieved by applying an AC magnetic field in combination with modulation and demodulation methods.

Robustness improvement of a nitrogen-vacancy magnetometer by a double driving method.

The nitrogen vacancy (NV) color center in diamonds is an electron spin that can measure magnetic fields with high sensitivity and resolution. Furthermore, the robustness of an NV-based quantum system should be improved for further application in other sensing methods and in the exploration of basic physics. In this work, the robustness of an NV magnetometer is improved by the double driving method. The sensitivity of the NV magnetometer was improved 2.1 times by strengthening the pumping power from 100 to 600 mW. In this process, thermal drift was introduced, which affects the measurement accuracy. The temperature drift of a diamond matrix was measured using an infrared camera, and the temperature change of a diamond host drifted to ∼80K under high laser and microwave power. To address the drift of temperature owing to sensitivity improvement by pumping enhancement, the double driving method was introduced, to suppress the drift of the resonance frequency, to improve the robustness of a continuous-wave NV magnetometer. The magnetic noise density was improved from 10 to 1.2 nT/Hz1/2. This study checked the source of temperature noise in the process of measuring with the NV color centers and proposes a double driving measurement method to track the resonant frequency change due to environmental temperature drift and improve sensitivity. The findings of this study are useful in applying complex pulse protocols in high-level sensing applications based on solid-state spin.

Solid-State Microwave Magnetometer with Picotesla-Level Sensitivity

Magnetometry with diamond nitrogen-vacancy (NV) ensembles has enabled devices with picotesla sensitivity at static and low-frequency fields, but their performance at higher frequencies lags far behind. The authors solve the technical challenges and demonstrate a microwave-frequency NV magnetometer with picotesla sensitivity, by implementing a pulse scheme for noise cancellation and employing a custom-grown diamond. This sensitivity enhancement could be extended into a far broader range of frequencies using spin-locking and quantum frequency mixing. These results could lead to applications such as near-field antenna characterization and microwave circuitry imaging.

Pico-tesla magnetic field detection with integrated flux concentrators using a multi-frequency modulation technique on the solid nuclear spin in diamonds.

In this paper, we implement integrated magnetic flux concentrators (MFCs) combined with a multi-frequency modulation method to achieve high-magnetic-detection sensitivity using a nuclear spin on the solid nuclear spin in diamonds. First, we excited the nuclear spin in diamonds using a continuous-wave technique, and a linewidth of 1.37MHz and frequency resolution of 79Hz were successfully obtained, which is reduced by one order of the linewidth, and increased by 56 times in frequency resolution compared to that excited by an electron spin. The integrated high-permeability MFC was designed to magnify the magnetic field near the diamond, with a magnification of 9.63 times. Then, the multi-frequency modulation technique was used to fully excite the hyperfine energy level of Nitrogen Vacancy (NV) centers along the four axes on the diamond with MFC, and magnetic detection sensitivity of 250p T/H z 1/2 was realized. These techniques should allow designing an integrated NV magnetometer with high sensitivity in a small volume.

Design of NV Centers Integrated Magnetometer and High-Resolution Output Module Based on the ODMR System

In recent years, magnetic sensing based on the nitrogen-vacancy (NV) centers in diamonds has been considered to be a promising candidate for highly sensitive, spatial resolution solid-state quantum sensors. With the advancement of technology, scholars are committed to the introduction of miniaturized, highly integrated NV magnetometers. In this article, we integrate a focus system in the traditional laboratory into the sensor, propose a dual-focus magnetometer module (DFM), and design a signal-processing module (SPM) to perform current/voltage conversion–subtraction–amplification and filtering on the output signal of the DFM. Comparing the signal directly output by the DFM with the signal output after passing through the SPM the magnetic sensitivity and resolution are improved by 419 times and 256 times, respectively, and the magnetic resolution of the entire sensor reaches 24.6 nT in an area of 10.6 cm2. It has achieved a breakthrough in the resolution limit of integrated magnetometers in smaller volumes.

Design of an ultra-sensitive and miniaturized diamond NV magnetometer based on a nanocavity structure

The ensemble of nitrogen-vacancy (NV) centers in diamond allows for the potential realization of the sensitive magnetometers by leveraging their excellent spin properties. However, the NV-based magnetometers are limited by their experimental magnetic field sensitivity owing to its inefficient photon collection. Moreover, they are a disadvantage to the reduced spatial resolution and excessive excitation power. To overcome these issues, we propose a ultra-sensitive diamond magnetometer based on nanocavities. The device structure can attain a high collective efficiency and enhance the photon emission intensity of the NV ensemble. This device can allow the efficient photon collection even when considering the positional distribution of the NV centers. Our theoretical analysis indicates that the minimum expected sensitivity is 60 fT/ The proposed design can achieve a volume-normalized sensitivity of 0.92 aT/ along with the required power of 7 μW, both of which are superior to those of bulk diamond. The proposed approach offers a promising route towards highly sensitive and energy-efficient magnetometers.

Temperature Fluctuations Compensation with Multi-Frequency Synchronous Manipulation for a NV Magnetometer in Fiber-Optic Scheme.

Nitrogen-vacancy (NV) centers in diamonds play a large role in advanced quantum sensing with solid-state spins for potential miniaturized and portable application scenarios. With the temperature sensitivity of NV centers, the temperature fluctuations caused by the unknown environment and the system itself will mix with the magnetic field measurement. In this research, the temperature-sensitive characteristics of different diamonds, alongside the temperature noise generated by a measurement system, were tested and analyzed with a homemade NV magnetometer in a fiber-optic scheme. In this work, a multi-frequency synchronous manipulation method for resonating with the NV centers in all axial directions was proposed to compensate for the temperature fluctuations in a fibered NV magnetic field sensing scheme. The symmetrical features of the resonance lines of the NV centers, the common-mode fluctuations including temperature fluctuations, underwent effective compensation and elimination. The fluorescence change was reduced to 1.0% by multi-frequency synchronous manipulation from 5.5% of the single-frequency manipulation within a ±2 °C temperature range. Additionally, the multi-frequency synchronous manipulation improved the fluorescence contrast and the magnetic field measurement SNR through an omnidirectional manipulation scheme. It was very important to compensate for the temperature fluctuations, caused by both internal and external factors, to make use of the NV magnetometer in fiber-optic schemes’ practicality. This work will promote the rapid development and widespread applications of quantum sensing based on various systems and principles.

Towards the design and operation of a uniformly illuminated NV detector for magnetic field mapping applications

Detectors based on an ensemble of nitrogen-vacancy (NV) centres in diamond are the latest addition to the list of magnetometers capable of detecting weak magnetic fields. Here, we report the design, construction, and testing of an NV magnetometer with a wide spatial area excitation and collection capability. The system was developed for the purpose of magnetic field measurement in an Halbach magnet configuration. The designed system performance was comparable to a conventional NV confocal system in the low power excitation region. From the dip profiles observed in the optically detected magnetic resonance (ODMR) spectrum, each NV centre experienced a static magnetic field in the 2.14–6.07 mT range of the Halbach magnet. The sensitivity of the field mapping system using the continuous wave (CW)-ODMR approach was estimated and found to be approximately 0.2 μT/Hz at 170 MHz. Extensive details of the setup and the experimental procedures are presented, and the system can be easily reproduced by those who are not familiar with NV centre instrumentation. The results obtained pave way for the development of scalable NV sensors using wide area illumination and collection methods, especially for the mapping B0 and B1 field in magnetic resonance imaging (MRI) to facilitate non-FFT image reconstruction.

Sub-second temporal magnetic field microscopy using quantum defects in diamond

Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it’s current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50–200 frames per s (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy.

Portable Diamond NV Magnetometer Head Integrated With 520 nm Diode Laser

The integrated diamond nitrogen vacancy (NV) magnetometer has attracted considerable attention in recent years and presents high potential for its use in magnetic field detection in fields such as aerospace, medical, and agriculture. A laser with a large volume seriously hampers the integration and performance of magnetometers. To solve this problem, we proposed a 520-nm laser diode with an integrated NV magnetometer. The 520-nm laser diode has a high excitation efficiency and good performance with a small size. We introduced the key steps of designing an integrated magnetometer head and tested the performance, a magnetic field sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20.77\,\,{n}T\,\,\cdot \,\,Hz^{-1/2}$ </tex-math></inline-formula> , and a shot noise sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.43\,\,{n}T\,\,\cdot \,\,Hz^{-1/2}$ </tex-math></inline-formula> . The measurable magnetic field range is from −107 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$107 ~\mu \text{T}$ </tex-math></inline-formula> , the system bandwidth is 15 Hz at the laser power of 10 mW, and the magnetic field conversion coefficient is 16.88 V/T. Thus, this paper provides a new method to minimise the light source volume and integrate it with a NV magnetometer.

Burst eddy current testing with diamond magnetometry

In this work, a burst eddy current testing technique based on the employment of a diamond nitrogen vacancy (NV) center magnetometer with the Hahn echo (HE) sequence is demonstrated. With the confocal experiment apparatus, the HE-based NV magnetometer attains a magnetic sensitivity of 4.3 nT/Hz and a volume-normalized sensitivity of 3.6 pT/Hz mm−3, which are ∼five times better than the already existing method under the same conditions. Based on the proposed magnetometer configuration, a burst eddy current testing prototype achieves a minimum detectable sample smaller than 300 μm and a spatial resolution of 470 μm, which is employed to image different metallic specimens and detect layered internal structures. Since this prototype comprises remarkable high sensitivity, it exhibits various potential applications in the fields of security screening and quality control. Moreover, its biocompatibility and promising nanoscale resolution pave the way for electromagnetic testing in the fields of biomaterials.

Scanning diamond NV center magnetometer probe fabricated by laser cutting and focused ion beam milling

The nitrogen-vacancy (NV) centers in diamond have been applied to scanning magnetometer probes combined with atomic force microscopy (AFM) to demonstrate nanometer-scale magnetic sensing and imaging. However, the scanning diamond NV center probe fabrication requires complicated processes including electron-beam lithography and photolithography. In this study, we introduce an alternative method to fabricate a scanning NV probe using laser cutting and focused ion beam (FIB) milling from a bulk diamond hosting an ensemble of NV centers. A few tens of micrometer-sized diamond pieces, cut by laser processing, were attached to the probe end of a quartz tuning-fork-based AFM. Then, it was fabricated into a few-micrometer-sized diamond NV center probe by using a donut-shaped milling pattern in the FIB processing to avoid damage to the diamond probe surface to degrade the NV− charged state at the tip apex. By using a home-built scanning NV magnetometer probe microscopy setup, an optically detected magnetic resonance was measured to detect stray magnetic fields demonstrating the imaging of a magnetic structure of approximately 5-μm periodicity from a magnetic tape. This study offers a method with a higher degree of probe-shape control for scanning NV probe that will broaden its application capabilities.

The Possibility of Integrating NV Magnetometer Array by Using Wireless Microwave Excitation and Its Application in Remote Heart Sound Records

Nitrogen-vacancy (NV) centers have potential applications in medical imaging and distributed sensing, for which the integrated NV magnetometer array is needed. However, the microwave (MW) drive module limits the development of the compact NV magnetometer system and array. In this paper, wireless MW is applied to two NV magnetometer systems (Sensor I and Sensor II) innovatively. We verified that the magnetic-field sensitivity was approximately equal to 77.1 nT•Hz−1/2 and 44.6 nT•Hz−1/2 for Sensor I and Sensor II, respectively, when the distance between the transmitting and receiving antennas was 1 m. This method can be extended to the NV magnetometer array. The receiving antenna and power amplifier are integrated into the NV magnetometer array system for miniaturization. This expands the utility of NV magnetometers in sensing and medical imaging. Furthermore, we achieved remote heart sound records with a signal-to-noise ratio (SNR) of 5.93 dB and 4.68 dB for Sensor I and Sensor II, respectively, by applying the principle of NV-based demodulator to the system. This would further extend the advantages of the wireless MW in the NV magnetometer. The NV magnetometer could receive the frequency point carrying heart sound signal to reach up to 16, which can avoid the interference of the 2.4 GHz frequency band of Bluetooth and Wi-Fi in complex environments.

Research on the common mode suppression method of temperature noise in an NV color center spin magnetometer

In this paper, two symmetrical magnetic formants in a nitrogen-vacancy (NV) ensemble are used to detect the two symmetrical spin states independently by means of microwave modulation and demodulation and frequency locking. Through the method of differential common mode cancellation, the spin information noise caused by temperature can be suppressed, and the measurement sensitivity of the NV magnetometer can be improved. In the experiment, a double formant magnetic detection system is built. By giving the same feedback to two symmetrical magnetic formants, we achieve differential common mode cancellation. From the experimental results, the temperature noise is reduced by 10 times, and the magnetic field sensitivity is increased from 126.8 pT Hz−1/2 to 13 pT Hz−1/2. The research realizes the suppression of temperature noise for the NV color center spin magnetometer, which provides technical support for improving the sensitivity and bandwidth of magnetometers.

Preferential coupling of diamond NV centres in step-index fibres.

Diamonds containing the negatively charged nitrogen-vacancy centre are a promising system for room-temperature magnetometry. The combination of nano- and micro-diamond particles with optical fibres provides an option for deploying nitrogen-vacancy magnetometers in harsh and challenging environments. Here we numerically explore the coupling efficiency from nitrogen-vacancy centres within a diamond doped at the core/clad interface across a range of commercially available fibre types so as to inform the design process for a diamond in fibre magnetometers. We determine coupling efficiencies from nitrogen-vacancy centres to the guided modes of a step-index fibre and predict the optically detected magnetic resonance (ODMR) generated by a ensemble of four nitrogen-vacancy centres in this hybrid fibre system. Our results show that the coupling efficiency is enhanced with a high refractive index difference between the fibre core and cladding and depends on the radial position of the nitrogen-vacancy centres in the fibre core. Our ODMR simulations show that due to the preferential coupling of the nitrogen-vacancy emission to the fibre guided modes, certain magnetometry features such as ODMR contrast can be enhanced and lead to improved sensitivity in such diamond-fibre systems, relative to conventional diamond only ensemble geometries.