🔬 applied physics

Wide-field NV magnetometry under simultaneous high-pressure and high-temperature conditions

This paper demonstrates the feasibility of wide-field optically detected magnetic resonance (ODMR) using nitrogen-vacancy (NV) centers to spatially visualize magnetic fields under simultaneous extreme conditions of high pressure (up to 7 GPa) and high temperature (500 K), establishing a new platform for imaging magnetic phenomena in such environments.

Original authors: Masahiro Ohkuma, Eikichi Kimura, Shumpei Ohyama, Miu Tezuka, Ryo Matsumoto, Shinobu Onoda, Yoshihiko Takano, Shintaro Azuma, Kenji Ohta, Keigo Arai

Published 2026-06-25✓ Author reviewed ⓘ

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CC BY 4.0

Original authors: Masahiro Ohkuma, Eikichi Kimura, Shumpei Ohyama, Miu Tezuka, Ryo Matsumoto, Shinobu Onoda, Yoshihiko Takano, Shintaro Azuma, Kenji Ohta, Keigo Arai

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a tiny, magical compass inside a diamond that can tell you not just where North is, but also how strong the magnetic field is right next to it. Scientists call this a "Nitrogen-Vacancy (NV) center." Usually, these diamond compasses are great at room temperature and normal pressure, but they struggle when things get really hot or when you squeeze them with immense force.

This paper is about teaching these diamond compasses to survive and work in an environment that is both scorching hot and crushingly heavy at the same time—conditions similar to what you might find deep inside the Earth or in the core of a star.

Here is how they did it and what they found, explained simply:

1. The Setup: A Tiny Pressure Cooker

To create these extreme conditions, the researchers used a Diamond Anvil Cell (DAC). Think of this as a microscopic pressure cooker made of two tiny diamonds. They put a sample (in this case, a tiny piece of iron) between the diamonds and squeezed them together.

The Squeeze: They applied pressure equivalent to about 5 to 7 times the weight of a car pressing down on a postage stamp (5 to 7 Gigapascals).
The Heat: They heated the sample up to 500 K (about 227°C / 440°F), which is hotter than a pizza oven.
The Problem: Usually, when you squeeze and heat something this much, the delicate "compass" inside the diamond stops working or gets confused.

2. The Solution: A Diamond with a Built-in Heater

To keep the diamond compass working, the team didn't just use an external heater. They built a micro-heater directly onto the diamond itself using a special type of conductive diamond (boron-doped).

The Analogy: Imagine trying to listen to a whisper (the magnetic signal) while standing next to a roaring fire. Usually, the fire drowns out the whisper. Here, they built the fire inside the room where the whisper is happening, but they engineered it so the whisper could still be heard clearly.

3. The Magic Trick: "Reading" the Diamond

The diamond compass works by glowing with light when you shine a green laser on it. The brightness of this glow changes slightly depending on the magnetic field and the temperature. This is called ODMR (Optically Detected Magnetic Resonance).

The Test: They shone a laser and sent microwave signals (like radio waves) into the diamond. Even at 500 K and 5 GPa of pressure, the diamond still "sang" its magnetic song. The researchers could clearly hear the signal, proving the compass still worked.

4. The Big Picture: Seeing the Invisible

Once they proved the compass worked in these extreme conditions, they did something even cooler: Magnetic Imaging.

The Experiment: They put a tiny piece of iron inside the pressure cooker. Iron is magnetic, so it creates a "stray field" (an invisible magnetic aura) around it.
The Result: Using the diamond compass, they took a "picture" of this invisible aura. They could see exactly where the magnetic field was strong and where it was weak, even though the iron was trapped under huge pressure and heat.
The Metaphor: It's like being able to see the wind blowing around a tree during a hurricane, even though you are inside a sealed, heated box looking through a tiny window.

5. What They Learned

It Works: They confirmed that you can read the magnetic state of materials at 500 K and 5-7 GPa of pressure.
The Signal Changes: As they got hotter, the "song" of the diamond changed pitch (a natural effect of heat), but it didn't stop singing.
Pressure Matters: They noticed that the pressure inside the tiny chamber wasn't perfectly stable; it shifted slightly as they heated it up. This taught them that to get perfect measurements in the future, they need to be very careful about tracking how pressure changes while heating.

The Bottom Line

This paper doesn't claim to cure diseases or build new engines yet. Instead, it simply proves that a specific tool (the NV center in a diamond) is tough enough to survive the "extreme sports" of high pressure and high temperature. This opens the door for scientists to take "magnetic photos" of materials in conditions that were previously too harsh to study, helping us understand how materials behave deep underground or in extreme industrial processes.

Technical Summary: Wide-field NV Magnetometry under Simultaneous High-Pressure and High-Temperature Conditions

Problem Statement
While nitrogen-vacancy (NV) center magnetometry is a well-established tool for spatially resolved magnetic imaging, its application to environments combining simultaneous high pressure and high temperature has remained largely unexplored. Existing magnetic characterization techniques face significant limitations in these extreme conditions: Mössbauer spectroscopy is restricted to specific nuclei, and X-ray magnetic circular dichroism (XMCD) is constrained by X-ray transmission through diamond anvils and accessible absorption edges. Furthermore, standard superconducting quantum interference device (SQUID) magnetometers typically operate only up to approximately 400 K and struggle with the small sample volumes and background signals inherent to diamond anvil cell (DAC) experiments. There is a need for a broadly applicable technique capable of probing local magnetic fields under concurrent high-pressure and high-temperature conditions.

Methodology
The authors utilized a diamond anvil cell (DAC) setup equipped with nitrogen-vacancy (NV) centers in the diamond anvils to perform optically detected magnetic resonance (ODMR).

Sample Preparation: NV centers were created via $^{12}\text{C}^+$ ion implantation (30 keV, $5 \times 10^{12} \text{ cm}^{-2}$ ) followed by annealing at 1000°C. The vacancies were distributed within approximately 50 nm of the diamond surface.
Heating and Control: To achieve simultaneous high pressure and temperature, the authors employed a boron-doped diamond (BDD) heater. For point-detection experiments, the BDD heater was fabricated on the NV-containing (100)-oriented diamond anvil. For wide-field imaging, the heater was placed on the opposing (111)-oriented anvil. Temperature was monitored using a K-type thermocouple attached to the diamond anvil slope.
Pressure Measurement: Pressure was estimated using ruby fluorescence for the (100)-oriented setup and the temperature-corrected shift of the NV zero-field splitting ( $D$ ) for the (111)-oriented setup.
Measurement Setup: A custom-built microscope with a 532-nm laser was used for excitation. Continuous-wave (CW) ODMR spectra were acquired using either an avalanche photodiode (point detection) or an EM-CCD camera (wide-field imaging). Microwave excitation was delivered via a patterned BDD antenna or a platinum foil.
Samples: Experiments were conducted on the diamond anvils themselves to verify signal stability, and subsequently on an iron foil loaded into the sample chamber to demonstrate magnetic imaging.

Key Contributions

Demonstration of Feasibility: The study establishes that NV spin readout via ODMR is feasible under simultaneous high-pressure (up to 7 GPa) and high-temperature (up to 500 K) conditions.
Wide-Field Imaging in Extreme Environments: The authors successfully performed wide-field ODMR imaging of an iron sample at 7 GPa and 500 K, visualizing the stray magnetic field through the pressure cell.
Characterization of Signal Stability: The work provides empirical data on the behavior of ODMR parameters (zero-field splitting $D$ , linewidth, and contrast) under combined stress and thermal loads, confirming that magnetic resonance signals persist even above 500 K.

Results

Signal Persistence: Clear ODMR signals were observed at 5 GPa and 500 K, and at 12 GPa up to 426 K. This confirms that NV spin coherence and optical readout are maintained in these extreme environments.
Temperature and Pressure Dependence: The zero-field splitting parameter $D$ showed a temperature dependence consistent with previous reports at ambient pressure, though deviations were observed under pressure, suggesting a contribution from pressure-induced shifts during heating cycles.
Magnetic Imaging of Iron: At 7 GPa and 500 K, the authors mapped the ODMR splitting across the sample chamber. The resulting spatial map revealed regions of enhanced and reduced splitting relative to the background, corresponding to the stray magnetic field of the iron foil projected along the NV axis. This demonstrated the ability to image magnetic phenomena in materials under combined extreme conditions.
Signal Quality: While the ODMR contrast decreased with increasing temperature (consistent with prior literature), the linewidth showed only weak temperature dependence. The authors noted that pressure variations during heating cycles (due to sample chamber deformation) affected the absolute pressure calibration but did not preclude the observation of magnetic features.

Significance
The paper concludes that NV-center magnetometry serves as a promising platform for imaging magnetic phenomena in materials under high-pressure and high-temperature environments. By providing spatially resolved magnetic information from within a DAC, this approach offers a complementary capability to element-specific probes like XMCD and Mössbauer spectroscopy. The results extend the application of NV-based magnetometry across the pressure–temperature phase space, enabling the investigation of magnetic ordering and phase transitions in condensed matter physics, materials science, and geoscience under conditions previously difficult to access with local magnetic probes. The authors note that further quantitative work on the in situ calibration of the zero-field splitting under these conditions is essential for precise magnetometry.