Micrometer‐scale magnetic imaging of geological samples using a quantum diamond microscope

At a Glance

Metadata	Details
Publication Date	2017-07-26
Journal	Geochemistry Geophysics Geosystems
Authors	D. R. Glenn, Roger R. Fu, P. Kehayias, D. Le Sage, E.A. Lima
Institutions	Center for Astrophysics Harvard & Smithsonian, Massachusetts Institute of Technology
Citations	184
Analysis	Full AI Review Included

Technical Documentation and Analysis: Quantum Diamond Microscopy (QDM) for Paleomagnetism

Based on: Micrometer-scale Magnetic Imaging of Geological Samples Using a Quantum Diamond Microscope (Glenn et al.)

Executive Summary

This research validates the Quantum Diamond Microscope (QDM), leveraging Nitrogen-Vacancy (NV) centers in diamond, as a high-resolution tool for paleomagnetic analysis of complex geological samples. This application is highly relevant to 6CCVD’s core capability in producing specialized single-crystal (SCD) and polycrystalline (PCD) diamond materials.

Core Achievement: Development of a QDM achieving 5 µm spatial resolution and superior image-area-normalized magnetic sensitivity (< 20 µT·µm/Hz^1/2) for room-temperature rock magnetometry.
Critical Material Requirement: The device relies on specialized diamond chips featuring NV layers ranging from ultra-thin (10 nm, ion-implanted) for maximizing spatial resolution to thick (up to 13 µm, CVD-grown) for maximizing magnetic signal/sensitivity.
Future Sensitivity Goal: Researchers project a ~30x sensitivity improvement by transitioning to thicker CVD-grown NV layers (~10 µm), directly aligning with 6CCVD’s expertise in customized MPCVD doping recipes.
Engineering Challenges: High-resolution imaging requires exceptionally low sensor-sample standoff distances (1-3 µm), mandating ultra-smooth polishing (Ra < 1 nm) of the diamond sensing surface.
Thermal Management: Significant laser-induced heating (up to 30 °C) observed, highlighting the need for diamond’s inherent superior thermal conductivity and robust heatsinking solutions.
6CCVD Solution: 6CCVD specializes in providing the necessary custom CVD diamond materials (SCD and PCD) in the required thicknesses (nm to µm) and achieving the nanoscale surface quality necessary to replicate and enhance QDM performance.

Technical Specifications

Key performance metrics and material properties extracted from the QDM research.

Parameter	Value	Unit	Context
Spatial Resolution (Optimum)	5	µm	Magnetic field and reflected-light imaging limit
Magnetic Sensitivity (Image-Area-Normalized)	20	µT·µm/Hz^1/2	Typical VMM performance (1 mm² FOV, long averaging)
Best Noise Floor (RMS)	20	nT	Demonstrated on ALHA81001 eucrite sample
Field of View (FOV) Max	4	mm	Wide-field surveying capability
Diamond Chip Thickness (Used)	500	µm	Standard thickness, noted as limiting optical resolution
NV Layer Thickness (Implanted, D1/D2)	10	nm	Used for maximum spatial resolution (τ << 1)
NV Layer Thickness (CVD Doped, D3/D4)	4 - 13	µm	Used for increased magnetic sensitivity
Future NV Thickness Target	10	µm	Projected to deliver ~30x SNR improvement
Excitation Wavelength	532	nm	Continuous Green Laser (NV pumping)
Operating Temperature Increase (Max)	~30	°C	Observed heating above room temperature
VMM/PMM Bias Field (B₀)	> 1	mT	Required for resolving ODMR lines
CPMM Bias Field (B₀) Low Limit	< 10	µT	Operation at very small ambient fields
NV Density (Implanted)	2 x 10¹¹	cm^-2	Surface density (Diamonds D1)
NV Density (CVD Doped)	2 x 10¹⁷	cm^-3	Volume density (Diamonds D3, D4)

Key Methodologies

The core QDM methodology centers on preparing specialized diamond sensors and coupling them with microwave and optical excitation fields.

Diamond Sensor Fabrication:
- Ion implantation (14 keV ¹⁴N or ¹⁵N) used to create extremely thin NV layers (10 nm) near the surface, optimizing for resolution (D1, D2).
- MPCVD growth with nitrogen doping used to create thicker, deeper NV layers (4-13 µm thickness, 2-6.5 µm depth) for enhanced signal (D3, D4).
Sample and Sensor Configuration:
- Geological samples polished to minimize surface roughness (Roughness ~1-3 µm, setting the standoff distance d_s-s).
- Diamond chips are placed NV-layer-face-down directly onto the sample surface to maintain minimal standoff distance.
Microwave Excitation:
- A printed circuit board patterned with crossed stripline resonators provides the GHz-frequency magnetic drive field (B₁) for continuous spin manipulation.
- Microwaves are configured for linear or circular polarization depending on the desired measurement mode.
Magnetic Field Control (B₀):
- Three orthogonal pairs of Helmholtz coils apply a homogeneous static bias field (B₀) to control the Zeeman splitting and cancel ambient fields (Earth’s field).
Optically Detected Magnetic Resonance (ODMR):
- Green laser (532 nm) provides continuous optical excitation. NV center fluorescence (637-800 nm) is monitored by an sCMOS camera while the microwave frequency is swept near the 2.87 GHz Zero-Field Splitting (ZFS).
- Fluorescence decrease at resonance is used to map the local magnetic field (B_||) projected onto the NV axis.
QDM Operating Modes:
- Vector Magnetic Microscopy (VMM): Three-axis vector field measurement, requires B₀ > 1 mT, utilizes field reversal to distinguish remanent vs. induced magnetization.
- Projective Magnetic Microscopy (PMM): Single-axis measurement optimized for faster data acquisition and sensitivity enhancement (factor of ~2-3) when dynamic range is small.
- Circularly-Polarized Magnetic Microscopy (CPMM): Single-axis measurement operating at very low bias field (B₀ < 10 µT), avoiding the need for precise bias reversal.
Calibration and Optimization:
- Absolute calibration performed using a characterized solenoid coil (0.6% accuracy).
- Post-processing includes spatial filtering (Gaussian/Butterworth filters) to maximize Signal-to-Noise Ratio (SNR).

6CCVD Solutions & Capabilities

This research confirms the essential role of highly specified MPCVD diamond for developing state-of-the-art quantum sensors like the QDM. 6CCVD is uniquely positioned to supply the required specialized NV diamond materials and integrated engineering services needed to optimize next-generation quantum magnetometry systems.

Applicable Materials for QDM Research

Research Requirement	6CCVD Material Recommendation	Value Proposition for QDM Optimization
High Sensitivity/SNR (Future 30x improvement goal)	High N-Doped SCD or PCD	Provides the high-concentration, thick NV layers (~10 µm) needed for maximized signal integration, crucial for paleomagnetic surveys.
High Resolution/Surface NV (10 nm implanted layers)	Optical Grade SCD Precursors	Ultra-high purity single-crystal substrates (up to 500 µm thick) optimized for consistent, low-defect ion implantation and subsequent annealing.
Reduced Aberration (Need thinner chips)	Custom Thin SCD/PCD Wafers	SCD or PCD wafers available with custom thickness control (down to 0.1 µm), allowing researchers to overcome the 500 µm chip thickness limitation noted in the study (which causes spherical aberration).
Thermal Dissipation (Required Heatsinking)	Thick Thermal Grade Substrates	SCD or PCD substrates available up to 10 mm thickness, providing superior thermal anchoring (2-3 x 10³ W/m·K) to manage the laser-induced heating (< 30 °C noted) and enable higher laser power operation for better sensitivity.

Customization Potential and Engineering Services

6CCVD offers the critical physical processing capabilities required to fabricate research-grade QDM sensors that push the limits of resolution and sensitivity:

Custom Wafer Dimensions: We provide diamond plates and wafers up to 125mm (PCD) and custom SCD sizes, ensuring optimal geometries (e.g., 2x2 mm² or 4x4 mm² chips, as used in this study) can be laser-cut to tight tolerances.
Nanoscale Polishing: Achieving a minimal sensor-sample standoff distance (d_s-s) of < 5 µm is crucial for resolution. 6CCVD provides ultra-low roughness polishing (Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD) necessary to minimize this gap and maximize spatial fidelity.
Integrated Metalization: QDM setups require integrated microwave structures (striplines) and potentially calibration coils. 6CCVD offers in-house precision metalization capabilities, including Ti, Pt, Au, Pd, W, and Cu, allowing for direct integration of microwave delivery systems onto the diamond sensor surface or substrate.

Engineering Support

6CCVD’s in-house PhD team can assist with material selection and optimization for similar Quantum Diamond Sensing and Paleomagnetism projects. We offer consultation on:

Optimizing nitrogen doping concentrations and post-processing protocols (irradiation and annealing) for maximizing NV density and coherence time (T₂).
Selecting the ideal diamond type (SCD vs. PCD) based on the required NV layer thickness and target magnetic sensitivity profile.
Designing sensor geometries and thermal interfaces to maintain stable, low-temperature operation essential for accurate magnetic field measurements (mitigating the observed 30 °C heating effect).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract Remanent magnetization in geological samples may record the past intensity and direction of planetary magnetic fields. Traditionally, this magnetization is analyzed through measurements of the net magnetic moment of bulk millimeter to centimeter sized samples. However, geological samples are often mineralogically and texturally heterogeneous at submillimeter scales, with only a fraction of the ferromagnetic grains carrying the remanent magnetization of interest. Therefore, characterizing this magnetization in such cases requires a technique capable of imaging magnetic fields at fine spatial scales and with high sensitivity. To address this challenge, we developed a new instrument, based on nitrogen‐vacancy centers in diamond, which enables direct imaging of magnetic fields due to both remanent and induced magnetization, as well as optical imaging, of room‐temperature geological samples with spatial resolution approaching the optical diffraction limit. We describe the operating principles of this device, which we call the quantum diamond microscope (QDM), and report its optimized image‐area‐normalized magnetic field sensitivity (20 µT⋅µm/Hz 1/2 ), spatial resolution (5 µm), and field of view (4 mm), as well as trade‐offs between these parameters. We also perform an absolute magnetic field calibration for the device in different modes of operation, including three‐axis (vector) and single‐axis (projective) magnetic field imaging. Finally, we use the QDM to obtain magnetic images of several terrestrial and meteoritic rock samples, demonstrating its ability to resolve spatially distinct populations of ferromagnetic carriers.

Tech Support

Original Source

DOI: https://doi.org/10.1002/2017gc006946

References

1989 - Corrosion of Fe‐Ni alloys by Cl‐containing akaganeite (beta‐FeOOH): The Antarctic meteorite case