Imaging AC magnetization response of soft magnetic thin films using diamond quantum sensors
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-05-23 |
| Journal | Communications Materials |
| Authors | Ryota Kitagawa, Aoi Nakatsuka, Teruo KĂŽhashi, Takeyuki Tsuji, H. Nitta |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research introduces a novel diamond quantum imaging system utilizing Nitrogen-Vacancy (NV) centers to analyze the AC magnetization response of soft magnetic thin films, addressing a critical bottleneck in high-frequency power electronics: energy loss in inductor cores.
- Wideband Imaging Capability: The system achieves simultaneous imaging of both the amplitude and phase of AC stray magnetic fields across an unprecedented range, from 100 Hz up to 2.3 MHz.
- Dual Measurement Protocols: Two distinct protocols were developed: Qubit Frequency Track (Qurack) for the kilohertz (kHz) range (up to 200 kHz), and Quantum Heterodyne (Qdyne) for the megahertz (MHz) range.
- Negligible Hard Axis Loss: Soft magnetic CoFeB-SiO2 thin films, when driven along the hard axis, exhibited near-zero phase delay (0°) up to 2.3 MHz, confirming negligible energy loss and suitability for high-frequency inductors.
- Anisotropy-Dependent Loss: When driven along the easy axis, the phase delay increased significantly with frequency (up to -60°), directly indicating increased hysteresis loss and energy dissipation, likely due to domain-wall movement suppression.
- Direct Energy Loss Metric: By imaging the phase delay between the magnetic field (H) and magnetization (M), the system provides a direct, localized metric for hysteresis loss, which correlates to the enclosed area of the M-H loop.
- High Resolution: The system demonstrated spatial resolution between 2 ”m and 5 ”m, with potential for reduction down to the optical diffraction limit (~400 nm).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Frequency Range | 100 Hz to 2.3 | MHz | Simultaneous amplitude and phase imaging demonstrated. |
| Qurack Frequency Limit | Up to 200 | kHz | Limited by MW frequency modulation deviation (±40 MHz). |
| Qdyne Frequency Limit | Up to 2.3 | MHz | Limited by Rabi frequency (spin manipulation speed). |
| Qdyne Lower Frequency Limit | ~300 | kHz | Limited by spin coherence time (T2). |
| Spatial Resolution (Demonstrated) | 2 to 5 | ”m | Dependent on NV layer thickness and measurement type. |
| Spatial Resolution (Potential) | ~400 | nm | Restricted by optical diffraction limit. |
| Soft Magnet Material | CoFeB-SiO2 | Thin Film | Deposited via facing-target sputtering. |
| Film Thickness | 150 | nm | Thickness of the CoFeB-SiO2 layer. |
| NV Center Alignment | [111] | Direction | Perfectly aligned NV centers in CVD-grown diamond. |
| NV Layer Thickness | 5 (or 2) | ”m | Thickness used for easy (or hard) axis measurements. |
| NV Density | 4 x 1017 | cm-3 | Density of NV centers in the diamond substrate. |
| Spin Coherence Time (T2) | 3.6 (5.8) | ”s | Measured under XY8 (N=1) for hard (easy) axis. |
| Max Measurable Field (Qurack) | ~1429 | ”T | Limited by signal generator deviation (±40 MHz). |
| Max Measurable Field (Qdyne, 1 MHz) | 2.2 | ”T | Limited by the Ï/2 fold-back amplitude. |
| Phase Delay (Hard Axis) | Near 0 | ° | Observed up to 2.3 MHz (negligible energy loss). |
| Phase Delay (Easy Axis) | -30 to -60 | ° | Observed from 100 Hz to 50 kHz (increasing energy loss). |
Key Methodologies
Section titled âKey MethodologiesâThe imaging system combines specialized diamond material engineering with two distinct quantum measurement protocols to cover a wide frequency spectrum.
-
NV Center Fabrication:
- NV centers were formed in Ib (111) diamond single crystals using Microwave Plasma Chemical Vapor Deposition (MPCVD).
- N2 gas was introduced during MPCVD to serve as the nitrogen source, ensuring NV centers were perfectly aligned along the [111] direction, simplifying signal analysis.
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Soft Magnetic Film Preparation:
- CoFeB-SiO2 thin films (150 nm thick) were deposited onto thermally oxidized silicon substrates using facing-target sputtering.
- A DC bias field (4 mT) was applied along the z-direction during measurement to resolve the degeneracy of the NV centers.
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Qubit Frequency Track (Qurack) Protocol (100 Hz to 200 kHz):
- This protocol uses continuous green laser and microwave (MW) irradiation.
- The MW frequency is modulated to track the oscillation of the NV qubit frequency caused by the AC magnetic field.
- The amplitude and phase of the AC field are derived by sweeping the modulation parameters to find the âmatching condition,â which results in a minimum in the photoluminescence (PL) intensity.
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Quantum Heterodyne (Qdyne) Protocol (MHz Range):
- This method employs heterodyne undersampling of the AC field.
- A sequence train of MW pulses (specifically, the XY8 dynamic decoupling sequence) and laser readout pulses are used.
- The MW pulses are synchronized with the period of the target AC magnetic field, allowing the camera frame rate to capture the down-converted, undersampled waveform.
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Data Processing and Stray Field Isolation:
- The total magnetic field measured by the NV center includes the stray field from the magnet (M) and the linear offset field from the drive coil (H).
- The offset field is subtracted (assuming uniformity within the FOV) to isolate the stray magnetic field generated by the magnetization (M) of the soft magnetic thin film.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to locally map energy loss mechanisms in magnetic materials across a wide frequency range is crucial for advancing several high-technology sectors.
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High-Frequency Power Electronics:
- Evaluation and optimization of soft magnetic materials (e.g., CoFeB-SiO2) for use in miniaturized, high-efficiency inductors and transformers operating above 100 kHz.
- Feedback loop for material development to minimize frequency-dependent energy dissipation (hysteresis and eddy current losses).
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Magnetic Material Research and Development (R&D):
- Direct imaging of local non-uniformities, magnetic domains, and domain-wall motion under high-frequency AC excitation.
- Quantification of M-H loop characteristics and hysteresis loss at operating frequencies, replacing traditional B-H loop measurements.
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Quantum Sensing and Metrology:
- Advancement of wide-field diamond quantum imagers for industrial metrology, leveraging the high sensitivity and spatial resolution of NV centers.
- Development of nanoscale imaging techniques (e.g., NV scanning probes) for detailed analysis of domain-wall dynamics.
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Magnetic Recording Technology:
- Analysis of high-frequency magnetic responses relevant to read/write heads and media, where magnetic field characteristics are often more critical than magnetization itself.
View Original Abstract
Abstract The energy loss in inductor core is a significant limitation in high-frequency power electronics. For evaluating and optimizing soft magnets, simultaneous imaging of both amplitude and phase of AC stray fields beyond 10 kHz is crucial. Here, we develop an imaging technique for analyzing AC magnetization response using diamond quantum sensors. For frequencies up to 200 kHz, we propose a measurement protocol, Qubit Frequency Track (Qurack), where microwave frequency modulation tracks qubit frequency oscillations. For higher frequencies above MHz, quantum heterodyne (Qdyne) imaging is employed. The soft magnetic CoFeB-SiO2 thin films, developed for high-frequency inductors, exhibit near-zero phase delay up to 2.3 MHz, indicating negligible energy loss. Moreover, the energy loss depends on the anisotropy: when the magnetization is driven along the easy axis, phase delay increases with frequency, signifying higher energy dissipation. These results suggest potential applications in analyzing soft magnets and improving the performance of power electronics.