| Metadata | Details |
|---|
| Publication Date | 2024-12-22 |
| Journal | Earth Planets and Space |
| Authors | Hirokuni Oda, Seiji Kumagai, Kosuke Fujiwara, Hitoshi Matsuzaki, Hiroshi Wagatsuma |
| Institutions | Tohoku University, Geological Survey of Japan |
| Citations | 2 |
| Analysis | Full AI Review Included |
- Core Value Proposition: This research successfully demonstrates a high-sensitivity, room-temperature Scanning Magnetic Microscope (SMM) utilizing Tunnel Magneto-Resistance (TMR) sensors, offering a non-cryogenic alternative to Superconducting Quantum Interference Devices (SQUIDs) for geological applications.
- Sensor Performance: TMR sensors (CoFeB/MgO/CoFeB-MTJs) achieved magnetic field sensitivities of approximately 30 nT/âHz (1073 ”m sensor) and 90 nT/âHz (357 ”m sensor) at 1 Hz after 10-point averaging.
- Noise Reduction: The system demonstrated RMS noise levels as low as 5.87 nT in the 0.1-2.5 Hz band (10-point average), representing a factor of seven reduction compared to raw data and an order of magnitude improvement over comparable previous TMR systems.
- Geological Validation: Magnetic maps of a vertically magnetized Hawaiian basalt thin section were consistent with reference SQUID images after computational adjustments (upward continuation and convolution) to account for TMR sensor geometry and lift-off differences (0.25 mm to 0.4 mm total distance).
- Spatial Resolution: The TMR sensors, despite their elongated geometry (up to 1073 ”m), provided magnetic images comparable to SQUID data, validating the calibration and post-processing methods.
- Future Goal: The practical target is achieving magnetic field sensitivity less than 5 nT with a spatial resolution of 50-200 ”m, which is necessary for ultra-fine scale magnetostratigraphic dating.
| Parameter | Value | Unit | Context |
|---|
| Sensor Type | TMR (MTJ) | N/A | CoFeB/MgO/CoFeB structure |
| Operating Condition | Room Temperature | N/A | Eliminates cryogenic requirements |
| Sensor #1 Length | 1073 | ”m | Long sensor variant |
| Sensor #2 Length | 357 | ”m | Short sensor variant |
| TMR Element Width | 0.1 | ”m | Sensing region width |
| TMR Element Vertical Extent | 100 | ”m | Sensing region height |
| Raw Sampling Frequency | 50 | Hz | Data acquisition rate |
| Sensitivity (S#1, Raw) | ~200 | nT/âHz @ 1 Hz | Background noise level |
| Sensitivity (S#1, 10-pt Avg) | ~30 | nT/âHz @ 1 Hz | Achieved sensitivity |
| RMS Noise (S#1, 0.1-2.5 Hz, 10-pt Avg) | 5.87 | nT | Low-frequency noise performance |
| Maximum Measurable Field | 3 | ”T | At 100 dB amplification range |
| Scanning Grid Resolution | 0.1 | mm | Step size for magnetic mapping |
| Estimated Lift-off (S#1) | 0.265 | mm | Distance from sample surface |
| SQUID Reference Lift-off | 0.216 | mm | Used for upward continuation comparison |
| Target Sensitivity | < 5 | nT | Desirable for magnetostratigraphy |
| Target Spatial Resolution | 50-200 | ”m | Desirable for magnetostratigraphy |
- System Adaptation: An existing XYZ stage and controller designed for a Scanning SQUID Microscope (SSM) were repurposed. TMR sensors were mounted in a plastic holder and placed above the sample holder within a two-layered magnetic shield.
- Electronics and Signal Chain: TMR sensors were connected to the analog voltage input via a DC-preamplifier (Wheatstone bridge circuit) and a DC-amplifier, utilizing a precision DC power supply (Agilent E3620A) for bias voltage.
- Calibration Procedure: Calibration factors (290 nT/V for S#1; 315 nT/V for S#2) were determined by measuring the magnetic field generated by a 3 mA line current and fitting the data to a theoretical infinite line current curve.
- Noise Characterization: Background measurements were conducted continuously at 50 Hz. Power Spectral Density (PSD) was calculated using periodograms after dividing the dataset into segments and applying a Hanning window, comparing raw data versus 10-point averaged data.
- Sample Measurement: A Hawaiian basalt thin section, prepared with Anhysteretic Remanent Magnetization (ARM), was scanned on a 0.1 mm grid. The stage moved at 50 mm/min, and data acquisition (0.3 s) occurred during stop-and-go steps.
- Drift Correction: Linear drift correction was applied by registering and averaging measurements taken at the starting (lower) and ending (upper) margins of each line scan, assuming these areas represented zero magnetic field.
- SSM Data Processing (Comparison): Reference SQUID magnetic images (216 ”m lift-off) were processed using Upward Continuation (using GMT grafft command) to simulate the higher lift-offs of the TMR measurements (up to 0.4 mm).
- Integration Simulation (Convolution): The upward-continued SQUID images were then subjected to Convolution using specific matrices (e.g., [0.5 1 1 1 0.5] for the 357 ”m sensor) to simulate the spatial integration effect caused by the elongated, serially connected TMR elements.
- Geological and Planetary Science:
- Submillimeter- to submicrometer-scale paleomagnetism and rock magnetism studies (e.g., volcanic rocks, meteorites).
- Magnetostratigraphic dating of marine ferromanganese crusts and sediments, requiring high-resolution imaging of magnetic reversal boundaries.
- Magnetic moment sensitivity measurements for single zircon crystals in paleointensity estimates.
- Non-Destructive Evaluation (NDE) and Materials Science:
- High-resolution mapping of magnetization and current source distributions in room-temperature samples.
- Quality control and defect detection in magnetic materials and thin films without the complexity of cryogenic cooling.
- Biomedical Sensing (Original Application):
- The TMR sensors were originally developed for ultra-weak magnetic field detection, such as Magnetoencephalography (MEG), demonstrating their suitability for high-sensitivity applications.
- Instrumentation and Microscopy:
- Development of cost-effective, high-performance SMM systems that are simpler to operate and maintain than traditional SQUID microscopes.
View Original Abstract
Abstract Scanning magnetic microscopes enable high-sensitivity mapping of magnetic fields in thin geological sections, facilitating submillimeter- to submicrometer-scale studies of paleomagnetism and rock magnetism. Magnetic fields of geological samples have been mapped using various sensors, including Hall-effect devices, magneto-impedance devices, superconducting quantum interference devices (SQUIDs), quantum diamond devices, and tunnel magneto-resistance (TMR) devices. This study proposes magnetic microscopy using high-sensitivity room-temperature TMR sensors developed for biomagnetic applications. The goal was to create high-performance magnetic microscopes that do not require labor-intensive techniques, such as cryogenic technology. An XYZ stage developed for a scanning SQUID microscope (SSM) was used to demonstrate and evaluate magnetic microscopy with TMR sensors. The original TMR sensors developed for biomagnetic sensing composed of serially connected TMR elements with a total length of 2684 ÎŒm were shortened to 1073 ÎŒm (Sensor #1) and 357 ÎŒm length (Sensor #2). Background measurements at 50 Hz show magnetic field sensitivities better than 200 nT/âHz and 600 nT/âHz at 1 Hz for Sensor #1 and Sensor #2, respectively. By averaging 10 points of the original 50 Hz sampling, magnetic field sensitivities are better than 30 nT/âHz and 90 nT/âHz at 1 Hz for Sensor #1 and Sensor #2, respectively. To demonstrate TMR sensors as magnetic microscopes, a vertically magnetized Hawaii basalt thin section was measured and compared with a SQUID-acquired magnetic field map. Magnetic scanning images obtained with TMR sensors on a 0.1-mm grid were compared with those of SSM after adjusting the lift-off by upward continuation and integrated along the length of the sensors. The results demonstrated that magnetic images for 1073-ÎŒm-long (357 ÎŒm-long) TMR sensors aligned along the y-axis and x-axis are consistent with those after upward continuation to 0.3 mm (0.25 mm) and 0.4 mm (0.25 mm) and convolution by 1 Ă 10 (1 Ă 4) and 10 Ă 1 (4 Ă 1) matrix, respectively. Overall, the high-sensitivity TMR sensors exhibited promising performance. Further improvements can be made by optimizing the sensors, preamplifiers, and measurement systems for magnetic microscopy to achieve an optimum target resolution. Graphical Abstract
- 1996 - Potential theory in gravity and magnetic applications