Advancement in scanning magnetic microscopy utilizing high-sensitivity room-temperature TMR sensors for geological applications

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 μm were shortened to 1073 μm (Sensor #1) and 357 μm length (Sensor #2). Background measurements at 50 Hz show magnetic field sensitivities better than 200 nT/√Hz and 600 nT/√Hz 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/√Hz and 90 nT/√Hz 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-μm-long (357 μm-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 × 10 (1 × 4) and 10 × 1 (4 × 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