Hirokuni Oda

First‐order reversal curve (FORC) diagrams are an important tool for assessing the domain state and anisotropy type of magnetic particles, as well as for detecting magnetostatic interactions among particles, in paleoclimatic, geomagnetic, tectonic, and planetary studies. We present here FORC diagrams for diverse natural monoclinic 4C pyrrhotite (Fe ₇ S ₈ ) samples, including sieved particle size fractions. Experimental FORC distributions for natural pyrrhotite have a dominant asymmetric “kidney shaped” feature for fine particles in the stable single domain (SD) size range. As particles coarsen to the multi‐domain state, FORC distributions diverge vertically at low coercivities. Vertical spreading in FORC distributions is also an intrinsic response of magnetic particle systems with multiaxial anisotropy even for magnetostatically noninteracting SD particles. The observed kidney shaped FORC distributions are attributed to twin domain and magnetic moment pinning along easy directions that follow the hexagonal parent structure to produce an apparent triaxial magnetocrystalline anisotropy signal. Similar FORC distributions have been reported for pyrrhotite, hematite, and smythite, which all have basal plane triaxial anisotropy. Central ridge‐type signatures, which are typical of noninteracting uniaxial SD particle assemblages, have been reported for some pyrrhotite samples, but are not documented here. Our results explain FORC features for pyrrhotite and provide a catalog for comparison with other studies.

This paper reports magnetic microscopy using high-sensitivity room-temperature tunnel magnetoresistance (TMR) devices for thin geological sections. The sensitivity region of the TMR sensor has dimensions of 178 µm (L) × 0.1 µm (W) × 100 µm (H), consisting of two TMR devices. Magnetic images were obtained for a vertically magnetized Hawaii basalt thin section in two sensor configurations, with the sensor length aligned parallel to the X- (lift-off = 174 μm) and Y-axes (lift-off = 200 μm), without introducing anisotropic distortion in the magnetic images. Although the magnetic images obtained with a scanning SQUID microscope (SSM) were similar, slight discrepancies were observed in the high-spatial-resolution region. A magnetic point source (50 μm × 50 μm) with a perpendicular magnetization film was prepared for evaluation. The SSM measurements showed a clear magnetic dipole at an angle of approximately 1° from the vertical direction. The FWHMs for both the SSM and TMR sensors increased linearly with lift-off. However, the peak magnetic fields, magnetic moments, and dipole tilts of the TMR sensor were significantly larger than those of the SSM sensor. This discrepancy may be due to the vertical extent of the active region of the TMR sensor, as well as due to sensor noise and drift.

We developed a scanning superconducting quantum interference device (SQUID) microscope for imaging the magnetic field of geological samples in 2015 and have been continuously improving it. Since its introduction, the scanning SQUID microscope system has been used to measure various geological samples, including marine ferromanganese crusts, ferromanganese nodules, single-grain zircon crystals with magnetite inclusions, fault gauge samples, stalagmites with ultra-low magnetizations, ultramafic rocks containing strong magnetic minerals. In this paper, we detail the improvements made to the system and developments of measurement and post-processing software. We improved the implementation and the electrical connection of a SQUID chip on a sapphire rod. A new electrical connection combines aluminum wire bonding and silver paste, ensuring stable and low contact resistance across cooling cycles leading to constant sensitivity and stability of measurements. For the improved system, the shortest sensor-to-sample distance was measured at approximately 125 µm using a precision line current. Electromagnetic shielding and improvements on grounding were made, which contributed to improved signal-to-noise ratios. The introduction of the reference sensor allows subtraction of environmental magnetic field. The new feature of stacking at each measurement grid is effective in reducing noise for weakly magnetized samples. Additionally, the post-processing software has been developed. Various drift correction algorithms provide flexibility to the operator to deal with the situation including a narrow marginal area or a dirty marginal area contaminated with magnetic materials. Spike noises could be removed by median filter, and flux jumps could be corrected by another independent algorithm. Dipole fitting allows the operator to calculate the position, intensity, and direction of a dipole one by one. A list of dipoles is made, which could be subtracted from the magnetic images to clean up for further analyses. Upward continuation enables leveling of the magnetic images measured with different sensor-to-sample distances allowing meaningful comparison and subtraction between different demagnetization steps. Calculation of X- and Y-components from Z-component allows to obtain total magnetic field image, which could be used to locate magnetized materials easily.

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

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 magnetoresistance (TMR) devices. This study proposes magnetic microscopy using high-sensitivity room-temperature TMR sensors developed for magnetocardiography. The goal was to create high-performance magnetic microscopes that do not require laborious 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 3 mm were shortened to 1 mm (Sensor #1) and 0.4 mm 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 scanning SQUID microscope (SSM) after adjusting the lift-off by upward continuation and integrated along the length of the sensors. The results demonstrated that magnetic images for 1 mm-long (0.4 mm-long) 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.

Ferromanganese nodules are cm‐sized, authigenic, abyssal manganese‐iron‐hydroxide concretions. They grow very slowly at rates of a few mm per million years. Although their ages are older than millions of years, they are often found half buried on the modern sediment surface. The mechanisms for the nodules' persistence at the surface without complete burial could be linked to their occasional motion or agitation. Here, we report evidence for the rotation of a nodule from the Penrhyn Basin, South Pacific detected by paleomagnetism. The paleomagnetic inclinations of specimens from the nodules' surface are consistent with the recent geomagnetic fields. The paleomagnetic directions from the surface to the core show successive changes and form a great circle with a pole at (azimuth = 53.9°, dip = 32.1°). This suggests that the nodule rotated along its pole while successively recording magnetizations. As the nodule was found on a gentle slope at the foot of an abyssal hill, it may have moved downslope due to bottom current underwashing. Rock magnetic analyses of the nodule suggest the presence of magnetite in single domain and vortex states. Low temperature magnetometry revealed that magnetite grains were heavily oxidized to maghemite, especially close to the core of the nodule. The rotation may have exposed the rising part of the nodule to oxidative pore water. Oxygenated Antarctic Bottom Water might have caused remagnetization due to low temperature oxidation of magnetite. The rotation would also facilitate the omnidirectional growth of the nodules' mixed layer of diagenetic buserite and hydrogenetic vernadite.

Pigmentary hematite carries important signals in paleomagnetic and paleoenvironmental studies. However, weak magnetism and the assumption that it has high magnetic coercivity prevents prevents routine identification of the size distribution of pigmentary hematite, especially for fine particle sizes. We present a strategy for estimating joint hematite particle volume and microcoercivity (f (V, Hk0)) distributions from low-temperature demagnetization curves and thermal fluctuation tomography (TFT) of pigmentary hematite in bulk samples of Triassic-Jurassic Inuyama red chert, Japan. The coercivity of the pigmentary hematite increases exponentially with decreasing temperature, following a modified Kneller’s law, where microcoercivity has a wide but approximately symmetric distribution in logarithmic space from ~1 tesla to tens of tesla. All of the red chert samples contain stable single domain (SSD) hematite with 35 - 160 nm diameter; a significant superparamagnetic (SP) hematite population with sizes down to several nanometers also occurs in Jurassic samples. The SP/SSD threshold size is estimated to be 8 - 18 nm in these samples. The fine particle size of the pigmentary hematite is evident in its low median unblocking temperature (194 °C to 529 °C) and, thus, this hematite may contribute to all four paleomagnetic components identified in published thermal magnetization studies of the Inuyama red chert. In this work, uniaxial anisotropy and magnetization switching via coherent rotation are assumed. Uniaxial anisotropy is often dominant in fine-grained hematite, although the dominant anisotropy type should be evaluated before using TFT. This approach is applicable to studies that require knowledge of coercivity and size distributions of hematite pigments.

We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka. Graphical Abstract