Diamond magnetometer enhanced by ferrite flux concentrators
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2020-06-24 |
| Journal | Physical Review Research |
| Authors | Ilja Fescenko, Andrey Jarmola, Igor Savukov, Pauli Kehayias, JÄnis Å mits |
| Institutions | Los Alamos National Laboratory, University of California, Berkeley |
| Citations | 145 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive Summaryā- Core Achievement: Demonstrated a Nitrogen-Vacancy (NV) diamond magnetometer achieving a sensitivity of ~0.9 pT s-1/2 across a broad frequency range (10-1000 Hz).
- Enhancement Mechanism: Utilized microstructured MN60 ferrite cones (relative permeability Āµr ā 6500) in a bowtie configuration, providing a ~250-fold amplification of the external magnetic field within the diamond sensor.
- Efficiency: The device operates at ambient temperature and requires low power: 200 mW of 532 nm laser light and ā¤20 mW of microwave power.
- Noise Suppression: A dual-resonance modulation technique was implemented to extract the magnetic field signal (f+ - f-), effectively suppressing noise caused by thermal shifts of the NV zero-field splitting parameter D(ĪT).
- Limiting Factor: Current sensitivity is primarily limited by photoelectron shot noise (projected limit: 0.72 pT s-1/2).
- Future Potential: With optimization, the noise floor could reach ~0.02 pT s-1/2 at 1 kHz, limited by the thermal magnetization noise of the ferrite concentrators.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Demonstrated Sensitivity | 0.9 | pT s-1/2 | 10-1000 Hz frequency range |
| Photoelectron Shot-Noise Limit (Projected) | 0.72 | pT s-1/2 | Calculated minimum noise floor |
| Ferrite Thermal Magnetization Noise Limit | 0.02 | pT s-1/2 | At 1 kHz, scales as f-1/2 |
| Magnetic Field Enhancement (Īµ) | 254 Ā± 19 | - | Experimental factor |
| Ferrite Material | MN60 | - | MnZn Ferrite (Āµr ā 6500) |
| Diamond Type | Type Ib HPHT | - | NV-doped membrane |
| Diamond Membrane Dimensions | ~300 x 300 x 43 | Āµm3 | Active sensing volume |
| Laser Wavelength | 532 | nm | Optical excitation |
| Laser Power (Popt) | 200 | mW | Continuous-wave operation |
| Microwave Power | ā¤20 | mW | Delivered via two-turn copper loop |
| Local Diamond Temperature (Inferred) | ~385 | K | Based on D(ĪT) ā 2862 MHz |
| Excitation-to-Photoelectron Efficiency (Ī¾) | ā 0.01 | - | Conversion efficiency |
| FDMR Linewidth (Ī) | ā 9 | MHz | Measured resonance width |
| Spatial Resolution (FWHM) | ~11 | mm | Estimated point spread function |
| Lock-in Modulation Frequency (fmod) | 15 | kHz | Used for dual-resonance technique |
Key Methodologies
Section titled āKey MethodologiesāThe experimental design combines advanced diamond processing, microstructured magnetic materials, and specialized microwave modulation techniques:
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Diamond Preparation:
- A commercially-available Type Ib HPHT diamond was used.
- It was irradiated with 2-MeV electrons at a dose of ~1019 cm-2.
- Subsequent annealing was performed in a vacuum furnace between 800-1100 Ā°C to convert nitrogen impurities into NV centers.
- The resulting membrane was mechanically polished and cut to ~300 x 300 x 43 Āµm3 dimensions.
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Flux Concentrator Assembly:
- Two MN60 ferrite cones (10 mm height, 10 mm base diameter) were micro-machined.
- The cones were arranged in a bowtie configuration with the diamond membrane positioned in the ~43 Āµm gap (Ī“).
- A two-turn copper loop was wound around one cone to deliver the microwave magnetic field, which was also enhanced by the ferrite structure (ā„2-fold enhancement).
-
Optical and Magnetic Excitation:
- A 532 nm laser (200 mW) was focused to a ~40 Āµm beam traversing the diamond edge.
- NV fluorescence (650-800 nm) was collected and measured using a balanced photodetector to minimize laser intensity noise.
- External bias and test fields (Bext) were generated by Helmholtz coils (radius: 38 mm) and surrounded by a mu-metal shield (shielding factor ~100).
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Dual-Resonance Magnetometry:
- The technique uses continuous-wave Fluorescence-Detected Magnetic Resonance (FDMR).
- Two microwave frequencies (F+ and F-), centered about the NV ms=0 to ms=Ā±1 transitions, were modulated at the same frequency (fmod = 15 kHz) and depth (fa = 3.3 MHz).
- A relative Ļ phase shift was applied between the F+ and F- modulation functions.
- The lock-in amplifier demodulates the signal, making the output proportional to (f+ - f-), thus isolating magnetic field changes from temperature-induced shifts in the zero-field splitting D(ĪT).
Commercial Applications
Section titled āCommercial ApplicationsāThe demonstrated sub-picotesla sensitivity and low-power operation make this technology highly relevant for several high-value engineering and scientific fields:
- Biomagnetic Sensing (MEG/MCG): The sensitivity achieved (0.9 pT s-1/2) is approaching the requirements for Magnetoencephalography (MEG) and Magnetocardiography (MCG), offering a potential room-temperature, solid-state alternative to superconducting quantum interference devices (SQUIDs).
- Geoscience and Navigation: The broadband, high-sensitivity detection capability is ideal for long-range Magnetic Anomaly Detection (MAD) and precision navigation systems where high spatial resolution is not the primary constraint (device size ~10 mm).
- Integrated Quantum Sensors: The low optical and microwave power requirements facilitate the miniaturization and parallelization of NV sensor arrays, crucial for developing compact, field-deployable quantum sensors.
- Materials Characterization: The use of micro-structured magnetic materials (ferrites) to manipulate and concentrate magnetic fields opens new avenues for magnetic microscopy and characterization of magnetic phenomena in condensed matter systems.
- Fluid NMR/MRI: High-sensitivity diamond magnetometers are already used for Nuclear Magnetic Resonance (NMR) spectroscopy of fluids; this enhancement technique could improve the signal-to-noise ratio for microfluidic NMR applications.
View Original Abstract
Magnetometers based on nitrogen-vacancy (NV) centers in diamond are promising room-temperature, solid-state sensors. However, their reported sensitivity to magnetic fields at low frequencies (ā¾1 kHz) is presently āæ10 pT s<sup>1/2</sup>, precluding potential applications in medical imaging, geoscience, and navigation. Here we show that high-permeability magnetic flux concentrators, which collect magnetic flux from a larger area and concentrate it into the diamond sensor, can be used to improve the sensitivity of diamond magnetometers. By inserting an NV-doped diamond membrane between two ferrite cones in a bowtie configuration, we realize a ~250-fold increase of the magnetic field amplitude within the diamond. We demonstrate a sensitivity of ~0.9 pT s<sup>1/2</sup> to magnetic fields in the frequency range between 10 and 1000 Hz. This is accomplished using a dual-resonance modulation technique to suppress the effect of thermal shifts of the NV spin levels. The magnetometer uses 200 mW of laser power and 20 mW of microwave power. This work introduces a new degree of freedom for the design of diamond sensors by using structured magnetic materials to manipulate magnetic fields.