Spinner Magnetometer Research Articles

Analysis of inhomogeneous magnetization using a spinner magnetometer

The assumption for measuring a discrete sample by spinner magnetometer in paleomagnetism is that the sample under study is a magnetic dipole so that the magnetization vector can be calculated by successive measurements of components along three orthogonal axes. This assumption may not hold for samples with inhomogeneous magnetization, irregular shape, or both, where non-dipole components are no longer ignorable. This study draws attention to their effect on the measurement of remanent magnetization based on an offset dipole model using multipole expansion of magnetic potential. Results from this model are compared with the experimental data measured by a new type of spinner magnetometer featured by high spatial resolution and the capability of measuring the fundamental wave component as well as the harmonic wave components. By using the relative amplitude of the harmonic waves to the fundamental wave, the contribution of non-dipole components can be quantified, leading to assessing how dipolar a sample is. The new analyses and instrumentation will be useful as a new diagnostic tool to obtain more reliable data from the samples collected from tectonically active regions with complicated geological histories.

Corrigendum to “Simple method of the complete result measurement of an inhomogeneously magnetised palaeomagnetic sample on a spinner magnetometer with a coil sensor” [Measurement 127 (2018) 501–511

Corrigendum to “Simple method of the complete result measurement of an inhomogeneously magnetised palaeomagnetic sample on a spinner magnetometer with a coil sensor” [Measurement 127 (2018) 501–511

Simple method of the complete result measurement of an inhomogeneously magnetised palaeomagnetic sample on a spinner magnetometer with a coil sensor

Nearly all spinner magnetometers have hitherto been designed and manufactured for measuring the (dipole) magnetism of palaeo- and rockmagnetic samples of relatively weak magnetisation. In the present work, it will be shown that the spinner magnetometer with a Helmholtz-coil sensor in two different connections could be applied also to the complete result measurement of inhomogeneously distributed magnetisation modelled by magnetism due to an offset dipole or equivalently a dipole plus quadrupole.

Paleomagnetic evidence for counterclockwise rotation of the Dofan magmatic segment, Main Ethiopian Rift

Twenty-six paleomagnetic sites in basalt and trachyte flows and ignimbrite deposits sampled in the Dofan magmatic segment, Main Ethiopian Rift (MER). From each site, 6 to 8 core samples were collected. The samples were then cut into 200 standard specimens and their Natural Remanent Magnetization (NRM) directions were measured using a JR6A spinner magnetometer. Most specimens were subjected to stepwise alternating field (AF) and at least one specimen per site to thermal (TH) demagnetization. The directional analysis of these individual specimens revealed either one or two components of NRM. Where two components are present, the first is isolated below a temperature of 300 °C or AF field below 20 mT; the second is isolated above those steps and mostly defined straight lines directed towards the origin and are interpreted as the Characteristic Remanent Magnetization (ChRM) acquired during cooling. Rock magnetic experiments on representative specimens indicate that the dominant magnetic minerals are titanium poor titanomagnetite and in few cases titanohematites. The overall mean directions calculated for the 23 sites of Dofan is Dec = 354.1°, Inc. = +11.6° (N = 23, K = 35.1, α95 = 5.2°). When these values are compared with the 1.5 Ma expected mean geomagnetic dipole reference field directions Dec = 1.0°, Inc. = +16.4° (N = 32, K = 105.6, α95 = 2.3°), obtained from African Apparent Polar Wander Path Curve; a difference in declination ΔD = −6.9° ± 4.7° and inclination ΔI = +4.8° ± 5.5° are determined. The declination difference is interpreted as a very slight counterclockwise rotation about vertical axis of the Dofan magmatic segment and the result is consistent with previous paleomagnetic reports and analogue modeling in Fentale magmatic segment.

A spinner magnetometer for large Apollo lunar samples.

We developed a spinner magnetometer to measure the natural remanent magnetization of large Apollo lunar rocks in the storage vault of the Lunar Sample Laboratory Facility (LSLF) of NASA. The magnetometer mainly consists of a commercially available three-axial fluxgate sensor and a hand-rotating sample table with an optical encoder recording the rotation angles. The distance between the sample and the sensor is adjustable according to the sample size and magnetization intensity. The sensor and the sample are placed in a two-layer mu-metal shield to measure the sample natural remanent magnetization. The magnetic signals are acquired together with the rotation angle to obtain stacking of the measured signals over multiple revolutions. The developed magnetometer has a sensitivity of 5 × 10-7 Am2 at the standard sensor-to-sample distance of 15 cm. This sensitivity is sufficient to measure the natural remanent magnetization of almost all the lunar basalt and breccia samples with mass above 10 g in the LSLF vault.

High‐sensitivity multifunctional spinner magnetometer using a magneto‐impedance sensor

AbstractA novel spinner magnetometer was developed with a wide dynamic range from 10−10 to 10−4 Am2 and a resolution of 10−11 Am2. High sensitivity was achieved with the use of a magneto‐impedance (MI) sensor, which is a compact, sensitive magnetic sensor used industrially. Its slow‐spinning rate (5 Hz) and the incorporation of a unique mechanism for adjusting the spacing between the sensing unit and the spinning axis allows the measurement of fragile samples sized 10–50 mm. The sensor configuration, in which a pair of MI sensors is connected in opposite serial, along with an amplification circuit with a programmable low‐pass filter, reduces the problems of external noise and sensor drift. The signal, with reference to the spinning frequency, is detected with a lock‐in amplifier. The MI spinner has two selectable measurement modes: the fundamental mode (F mode) and the harmonic mode (H mode). Measurements in the F mode detect signals of the fundamental frequency (5 Hz), in the same way as conventional spinner magnetometers. In the H mode, the second (10 Hz) and the third (15 Hz) harmonic components are measured, in addition to the fundamental component. Tests in the H mode were performed using a small coil and a natural sample to simulate dipoles with various degrees of offset. The results revealed that the magnitude of the fundamental component of the offset dipole was systematically larger (by several percent) than that of the nonoffset dipole. These findings suggest that this novel MI spinner will be useful in estimating the inhomogeneity of the magnetization of a sample that can equivalently be described by an offset dipole.

Remanent magnetization measurements of polymeric materials containing fullerol C60(OH)24 before and after exposure to external magnetic field

ABSTRACTThe problem of low solubility of fullerene is surpassed by its functionalization with –OH groups making a new molecule called fullerol C60(OH)24. Materials used in this study are synthesized by incorporating fullerol into commercial soft contact lens material Definitive74 made by Contamac, UK. Three different mass concentrations of fullerol are used: 0.003%, 0.01% and 0.015%. Hence, three materials are produced and the fourth material, the standard one, was used as a reference sample. The purpose of incorporating fullerol into polymer was to develop new materials for soft contact lenses with improved optical and mechanical characteristics. The magnetic properties of polymers change due to fullerol presence therefore the aim was to show the differences in magnetic properties of the materials before and after the exposure to four different external influences. The measurements of a remanent magnetization of these materials are done by spinner magnetometer, JR-6A.

A LabVIEW software for Thellier paleointensity measurements with an automated three-component spinner magnetometer TSpin

The Thellier method is classical but is still regarded as the most reliable method for paleointensity determination. Recently, many types of additional protocols have been advocated to ensure reliability and make laboratory work more laborious and time-consuming. An automated system coupling a magnetometer and an electric furnace is now of prime importance to cope with the increasing demand. Here, we describe a graphics-based program for controlling a fully automatic system combining a spinner magnetometer and a thermal demagnetizer, and for acquiring and processing the magnetization data. A single fluxgate sensor, which can measure the vector magnetization by spinning and translating a standard-sized 1-in. specimen, was calibrated with a rotatable reference specimen that can make the magnetization parallel or perpendicular to the spinning axis. By placing a cooling chamber between the furnace and the sensor for the updated system, the specimen can be heated up more efficiently to ensure an identical thermal history for the double heatings of the Thellier method. The direction of the vector magnetization was precisely obtained as well as the intensity, the results being comparable with those from an ordinary spinner magnetometer. We present an application of the fully automatic system for a Thellier measurement on a recent lava flow, which took about 24 h for approximately ten-step double heatings without manual operation.

Corrigendum to “The principles of the complete measurement of an off-centred elementary magnet on a spinner magnetometer with a coil sensor” [J. Magn. Magn. Mater. 372 (2014) 236–246

Corrigendum to “The principles of the complete measurement of an off-centred elementary magnet on a spinner magnetometer with a coil sensor” [J. Magn. Magn. Mater. 372 (2014) 236–246

The principles of the complete measurement of an off-centred elementary magnet on a spinner magnetometer with a coil sensor

The aim of the present treatise is to show that, albeit a precise measurement of a magnetically inhomogeneous sample may not be obtained on a traditional spinner magnetometer, it is still possible to get one on a modified spinner provided that magnetic inhomogeneity can be correctly modelled by an elementary magnet of moment m which is offset (l) relative to the geometrical centre of the sample. Following the deduction of the equations of the response of the pick-up coils to such a magnet rotated within the coils (forward problem), a proposal is presented as to how to modify the pick-up unit in order to make the magnetometer adaptable to the complete measurement of both magnetic moment (m) and eccentricity (l) (inverse problem). The proposed modification, pick-up coils that can be set both vertically as well as about the vertical axis, makes it theoretically possible first to measure |l|, then to eliminate the effect of it from the response of the pick-up coils leaving a record from which m can be extracted unequivocally. In practical realisation, the measurements will have to be carried out with the pick-up unit tuned in resonance with the frequency of the sample rotation, and optionally with the treble of it if it is deemed necessary for checking or improving upon the precision of measurement.

Counterclockwise block rotation linked to southward propagation and overlap of sub-aerial Red Sea Rift segments, Afar Depression: Insight from paleomagnetism

Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR–6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100°C–300°C or by AF of 10–30mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination ΔD=−6.5°±4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta–Dabbahu magmatic segments. The difference in inclination ΔI=7.8°±3.9° is believed to be related to the long-standing non-dipole field in Afar (Courtillot et al., 1984; Kidane et al., 2003).

A new type of a three-component spinner magnetometer to measure the remanence of rocks at elevated temperature

Abstract A new instrument to continuously determine the remanent magnetisation of standard paleomagnetic (inchsized) rock samples during heating and cooling (continuous thermal demagnetisation) was developed. The design as an off-axis spinner magnetometer (i.e. the samples rotate on a circular path in a radial distance to the spinning axis) allows the simultaneous measurement of several samples and offers a way to determine the full vector of magnetisation. Six fluxgate sensors are used to record three gradients of the magnetic fields caused by the samples and the vector of remanence is determined by regression analysis of the gradient signals. The sensitivity of the instrument is 2·10−7 Am2. Hot air is circulating through copper pipes which heat the samples by thermal radiation to avoid magnetic fields which would arise from direct electrical heating. Currently, the measurements are restricted to a maximum of 350°C. The instrument was tested with oceanic basalt samples that were given an artificial remanence. Some continuous thermal demagnetisation experiments of this artificial remanence and of the natural remanent magnetisations are shown for oceanic basalts.

Palaeomagnetic errors related to sample shape and inhomogeneity

When determining the remanent magnetization (RM) of palaeomagnetic rock samples, it is assumed that the samples can be approximated by a magnetic dipole. This assumption greatly simplifies the inverse problem of determining the RM from the measured magnetic field of the sample. The magnetic field of the normally used cylindrical rock samples is however not identical to that of a dipole and care must be taken not to introduce any systematic errors. A numerical test of the effect of cylindrical sample shape on the determination of RM for a spinner-type magnetometer is presented. We find that for a spinner magnetometer the non-ideal sample shape has an insignificant effect (less than 0.3° error), for even the smallest possible sample-sensor distance. Comparing static and spinner-type magnetometers, it is clear that spinner-type magnetometers are less affected by the sample shape. Inhomogeneously magnetized samples are an obvious source of error when determining the RM. We numerically test the error in the determination of RM for the simple case of a laminated cylindrical sample with constant direction, but varying intensity of magnetization between the laminae. For strongly inhomogeneous samples we find an error of ∼4° for typical spinner-type magnetometer and ∼10° for static-type magnetometers.

Three component spinner magnetometer featuring rapid measurement times

We describe the fabrication and performance of a spinning rock magnetometer for measuring the intensity and direction of the remanent magnetisation of rock specimens collected in palaeomagnetic surveys. The strength of these fields is measured using a high temperature superconductor (HTS) rf SQUID. Samples are rotated around two orthogonal axes to facilitate the calculation of the three-component remanence vector with a minimum of operator intervention. Measurements of remanence values ranging from 10/sup 1/ to 10/sup -5/ A/m, with errors of 1/spl times/10/sup -5/ A/m, have been achieved in measurement times of 20 s, while operating the instrument in a typical geophysical laboratory environment. In this paper we discuss the design requirements and data processing necessary to achieve a user-friendly system.

An Archaeomagnetic Study of a Wattle and Daub Building Collapse

AbstractArchaeomagnetic methods, originally developed to assist in the reconstruction of pottery vessels, are applied to the study of sintered remains of Neolithic wattle and daub buildings in Calabria, Italy in order better to understand the circumstances of burning and structural collapse.These Stentinello-culture buildings from Piana di Curinga comprise one of the densest areas of Neolithic settlement known in the western Mediterranean. A spinner magnetometer was used to analyze both the orientations of thermoremanent magnetism vectors in one structure's daub fragments as well as the intersection angles they make with timber imprints and wall surface planes. It was found, in conjunction with experimentation, that the building was probably intentionally burned and that those who set fire to it knew the structure's walls would at least partially collapse. Several lines of evidence indicate how old, abandoned daub buildings were probably set afire so as to harden and hence preserve the pieces of now-fired...

Natural remanent magnetization of Portland cement concrete

Previous research has demonstrated that both Portland cement paste and mortar become magnetized in earth's magnetic field. In this work, Portland cement concrete was studied to determine the degree to which this composite material becomes magnetized during the casting process, also due to earth's magnetic field. Concrete prisms consisting of ordinary Portland cement, Ottawa sand, and crushed gravel were constructed. Twenty-five mm diameter cores were extracted from the blocks and were tested in a spinner magnetometer to ascertain the direction and intensity of acquired magnetization. The results indicated that ordinary concrete becomes magnetized parallel to the ambient magnetic field due to the alignment of magnetic constituents found in the Portland cement. The magnetization was significantly enhanced when fly ash was present, due to highly magnetic grains typically present in fly ash.

The Thellier Palaeointensity Technique: Minisamples versus Standard Size.

Palaeointensities using 3×3mm mini-cylinders and a cryogenic magnetometer have been compared with those using 23×25mm standard cylinders and a spinner magnetometer. When samples satisfy the reliability criteria of both approaches there is good agreement but the proportion of samples rejected is higher in the case of minisamples. One possible reason is that the minisamples are more vulnerable to physico-chemical alteration during laboratory heating, because of their much higher surface to volume ratio.

An Automatic Spinner Magnetometer with Thermal Demagnetization Equipment.

We have developed a fully automated spinner magnetometer which can operate, under the control of a personal computer, the measurement of remanence as well as thermal demagnetization of paleomagnetic samples. A fluxgate sensor and an electric furnace is housed within a three-layer magnetic shield equipped with a solenoid coil. The measurement of the three components of magnetization is made possible by sampling the magnetic field generated by the sample which is both rotating around and moving along an axis parallel to the axis of the magnetic shield. Thermal demagnetization is carried out in an electric furnace placed further away along the axis of translation but still inside the magnetic shield. The rotation and translation of the sample, the measurement of the magnetic field by a ring-core fluxgate sensor, heating and cooling of the electric furnace, and switching on and off of the solenoid magnetic field are all controlled by a personal computer. This instrument is ideally suited for performing paleointensity experiments by the Thellier method because a series of measurements after heating cycles with or without an applied field can be programmed and carried out automatically by computer control.

山陰西部の鮮新―更新統都野津層：山地磁気層序と堆積年代

Paleomagnetism was measured with a cryogenic magnetometer and a spinner magnetometer for 458 semi-consolidated mud samples from eight sections of the Plio-Pleistocene Tsunozu Formation at its type locality, the Tsunozu area, San'in district. The results, revealing a magnetostratigraphic sequence with repeated magnetically normal and reversed intervals, are discussed with regard to the chronology of the formation. Based on previous data on magnetostratigraphy and the fission track age of the Oe-Takayama Volcano Group northeast of the Tsunozu area, a correlation with the standard paleomagnetic polarity time scale leads to the conclusion that the entire Tsunozu Formation ranges from just before the Cochiti normal event (about 4.1Ma) to just before the Jaramillo normal event (about 1Ma), and that the Plio-Pleistocene boundary (1.64Ma) is situated at the horizon just above the M3 marine clay bed in the Middle Member. An approximate chronology of four marine clay interbeds within the Middle Member is: M1 bed, 3.8-4.0 Ma; M2 bed, 2.5-2.7Ma; M3 bed, 1.7-1.8Ma; M4 bed, 1.1-1.5Ma, in ascending order. The apparent sedimentary rate of the Middle Member is about 2cm/103y. Our estimation of the stratigraphic horizon of the Plio-Pleistocene boundary is quite consistent with the previous view based on pollen stratigraphy.

Comparative measurements of magnetic susceptibility anisotropy in red sandstones

The anisotropy of magnetic susceptibility of Triassic red sandstones from northern Bavaria (Plattensandstein Formation) is very weak within the maximum susceptibility plane. The anisotropy resulting from the shape and size of cylindrical red bed samples can exceed and obscure the lineation of the sedimentary fabric when measured with a spinner magnetometer. This frequently observed error can be eliminated when the cylindrical specimens are drilled perpendicular to bedding. Alternatively, cubic samples which are measured on a susceptibility bridge, are not affected by this error.