Coercivity Grain Research Articles

Reliability of the palaeomagnetic signal recorded in a lava flow erupted on 4 December 2021 in La Palma (Canary Islands, Spain)

SUMMARY A basaltic lava flow erupted from the Tajogaite volcano on 4 December 2021, in La Palma (Canary Islands, Spain) was sampled to find out to what extent reliable and correct information on both intensity and direction of the Earth's magnetic field can be obtained from the palaeomagnetic signal recorded in a lava flow which erupted under known conditions. Samples were taken every few centimetres across a flow up to a total of 27 oriented cores. Palaeomagnetic experiments showed a strong viscous overprint in many samples. Nevertheless, the mean palaeomagnetic direction obtained agrees well with the actual value from IGRF-13. Rock magnetic experiments were performed to obtain additional information about the quality and reliability of the results and the reasons for unsuccessful determinations. Analysis of mostly irreversible thermomagnetic curves showed that the carriers of remanence were magnetite and titanomagnetite of low and/or intermediate Curie-temperature. Hysteresis parameter ratios showed a pronounced variability across the flow. Analyses of frequency dependent susceptibility, IRM acquisition coercivity spectra and FORCs showed a noticeably presence of very low coercivity grains (multidomain and superparamagnetic-single domain boundary). Multimethod palaeointensity experiments were performed with the Thellier-Coe, multispecimen and Tsunakawa-Shaw methods. Only three of 25 cores from the flow yielded successful Thellier-Coe determinations, in agreement with the expected field value of 38.7 μT (IGRF-13). However, palaeointensities of 60 per cent of the specimens agree with the expected value performing an informal analysis without considering criteria thresholds. Four of six Tsunakawa-Shaw determinations performed on samples from the flow yielded correct results, but three multispecimen determinations providing apparently successful determinations largely underestimate the expected field intensity. Combination of three Thellier-Coe and four Tsunakawa-Shaw successful determinations yields a multimethod palaeointensity result B = (36.9 ± 2.0) μT in good agreement with the expected field intensity.

Paleomagnetism and age of the Leucite Hills Volcanic complex, Wyoming: Implications for eruptive history, landscape evolution, and the geomagnetic instability timescale (GITS)

The Leucite Hills Volcanic Field, southwest Wyoming comprises two dozen volcanic features including necks, flows, dikes, and plugs. It has been the focus of many petrologic studies as its volcanic and shallow intrusive rocks are one of the only surficial manifestations of ultrapotassic lamproite. We build on paleomagnetic findings of Sheriff and Shive (1980) by providing further paleomagnetic data from the Boars Tusk dike and Black Rock flows. We also characterize the magnetic mineral assemblage of these lamproites. Principal component analysis of alternating field (AF) and thermal demagnetization data indicate that the dike and breccias of Boars Tusk record a reversed magnetic polarity and the Black Rock lava records a normal polarity, both consistent with previous findings. This recording is typically carried by minerals with coercivities >15 mT and susceptibility measurements indicate magnetite, maghemite, and titanomagnetite as likely magnetic carriers. AF and thermal demagnetization experiments evince secondary magnetizations held by lower coercivity grains, likely caused by lightning strikes. 40Ar/39Ar incremental heating experiments from Boars Tusk and Black Rock give plateau ages of ∼2500 ka and ∼900 ka, respectively. Recent advances in the chronology of geomagnetic field reversals and excursions during the Quaternary permit integration of the Boars Tusk dike into the lower Matuyama chron, whereas the Black Rock lavas most probably record the Kamikatsura excursion. Notably, Black Rock records high inclinations that suggest the short-lived excursion achieved a full geomagnetic reversal, something not observed at other localities recording the Kamikatsura excursion. The Leucite Hills offer further opportunities to refine the Quaternary geomagnetic instability time scale (GITS), and to improve understanding of the eruptive and geomorphic evolution of this unusual volcanism.

Consequences of switching field angular dependence for applications of anhysteretic remanent magnetization

The measurement of anhysteretic remanent magnetization (ARM) is an easy nondestructive means of evaluating many important rock properties. This involves applying an alternating magnetic field (AF) along with a direct biasing field (DF) to magnetize rock samples. Various techniques employing ARM have been widely used with notable success to help solve problems in tectonics, volcanology, sedimentology, environmental and paleoclimate studies and to better understand geomagnetic field behavior. Often, however, too little attention is paid to the effects of switching field angular dependence, which has important influence on the coercivity and angular distributions of grains that actually carry the ARM. As a result, commonplace methods of ARM measurement are not ideally designed for their intended purposes. For example, differentiation of a progressive ARM acquisition or turning on the direct field within a narrow AF window (i.e. partial ARM) are unable to isolate the ARM contribution from a specified coercivity grain fraction. Instead, the optimal method of measuring ARM in a targeted coercivity grain fraction is to differentiate a progressive tumble demagnetization of a total ARM generated with a peak AF high enough to fully activate the coercivity range of interest. Ignoring the activation property can lead to overestimation of relative paleointensity, underestimation of high coercivity grain concentrations, amplification of ARM anisotropy (e.g. error in natural remanence corrections), and unwanted mixing of anisotropy fabric components held in separate coercivity grain fractions.

Optimization of the Spin-Torque Oscillator Response for Microwave-Assisted Magnetic Recording

Microwave assisted magnetic recording (MAMR) is one of the promising next generation recording technologies that is expected to carry the magnetic recording industry beyond today's perpendicular magnetic recording (PMR) limit. MAMR involves the introduction of a high frequency oscillating magnetic field in the microwave frequency (GHz) range at the time of writing. This assisting energy at a specific frequency helps to make the magnetic grains more malleable and thereby reverse more easily which permits the usage of smaller, higher coercivity grains in the media, supporting a higher recording density. The generation of a stable oscillating magnetic field is expected to come from a spin-torque oscillator (STO) that has a rotating field-generating layer (FGL) when a current is injected. The STO must be properly designed in order to generate the stable oscillating magnetic field of the desired frequency and with sufficient amplitude to assist in the media reversal. In this work, we optimize the STO to produce a large and stable oscillating field with the desired parameters. Micromagnetic simulations of the STO are used to determine the STO's behavior and we propose various metrics to be used as a measure of the performance of the STO's response. Subsequently the response surface methodology (RSM) is employed to optimize our selected performance metric, producing the highest performing STO.

Stages in the life of a magnetic grain: Sediment source discrimination, particle size effects and spatial variability in the South African Karoo

Mineral magnetic properties are commonly used to trace the sources of river and lake sediments with the aim of identifying areas of a catchment experiencing the high rates of soil loss. In some areas, however, there is poor understanding of how magnetic properties are able to discriminate between potential sediment sources. The basis for mineral magnetic source discrimination between topsoils and subsurface material, over different rock parent materials, was investigated in the South African Karoo. The effects of particle size, soil development/alteration and diagenesis processes on the source discrimination were investigated. Good discrimination (up to 100% correct discrimination) was found between the sedimentary sources (soils and subsurface) and dolerite soils due to much higher concentrations of magnetic minerals in the dolerite. There was little evidence of alterations to magnetic properties in subsurface sediments, resulting in only a small ability of properties to differentiate between surface and subsurface sources. It was found that during the development of topsoils there was a loss of high coercivity weakly magnetic grains and an enrichment in small superparamagnetic and stable single domain grains. Particle size was strongly related to magnetic properties with almost all of the secondary superparamagnetic and stable single domain grains created during soil formation being concentrated in the <32μm fraction and the loss of high coercivity grains taking place primarily in the >125μm fractions. The small ability of sediment to discriminate between surface and subsurface sources was based upon the dissolution of primarily χfd and χARM carrying grains in the <32μm fractions and to a lesser extent the 63–32μm fraction. As a result it was recommended that source tracing with the <32μm fraction of soils and sediment should be treated separately from the 63–32μm and larger particle size fractions. The relationships between particle size and magnetic properties were fairly consistent in all soil samples in each source group, raising the possibility that reliable correction factors could be developed for each source group.

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew–Wiluna Greenstone Belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulfide ore, serpentinised and olivine-bearing ultramafic host-rocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However, the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirm that massive sulfide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulfide content imposes the strongest control on densities, but can only be identified when comprising >10 vol% of the rock. The pyrrhotite-rich Ni-sulfide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain <10 vol% sulfides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water. Serpentinisation dramatically lowered densities and increased susceptibilities, but had limited influence on the intensity of remanent magnetisation. All ultramafic rocks contain multidomain magnetite, and most contain low coercivity grains prone to overprinting by in situ viscous remanent magnetisation or drilling-induced isothermal remanent magnetisation during extraction. Despite the low coercivities, Koenigsberger ratios of 1–20 are observed indicating that viscous remanent magnetisation aligned parallel to the present Earth field must be considered in any magnetic modelling. It is also noted that coarser-grained intrusive varieties of all rock types (e.g. granite, gabbro) show remanent magnetisation intensities 1–2 orders of magnitude greater than their extrusive equivalents (felsic and basaltic volcanics).

Simulations of the magnetic properties of thin film media with low coercivity grains

This article presents an extensive investigation of the effect of low coercivity (LC) grains on the remanence process in exchange coupled longitudinal thin film. In particular, it is shown that the inclusion of LC grains causes an increase in the slope of the magnetization curve in small negative fields leading essentially to a decrease in the stability of the written information. Switching field distributions and δI plots are used to illustrate the effect of LC grains on intergranular interactions.

Origin of switching field distribution in M-P pigments

Measurements on recording media pigment samples have shown the samples to have a bulk coercivity of circa 2 kOe with a switching field distribution commencing at 1 kOe and the process not completing until a field of 5 kOe is applied. Such a wide switching field distribution can result in thermal loss of data for the low coercivity grains and lead to significant overwrite problems caused by the high coercivity grains. In this paper we report on an investigation of the origin of this distribution of switching fields. Magnetic measurements, Transmission Electron Microscopy (TEM), X-ray Diffraction and Mössbauer spectroscopy have been used to study crystallite size, particle morphology and chemical homogeneity. We believe that at least in part, the crystallites in the metal particle pigments may account for the resulting switching field distribution, in conjunction with effects such as particle size distribution and angular dependence of coercivity.

Microstructural effects in computer simulations of the magnetic and recording properties of thin film media

The effects of microstructural disorder on the micromagnetic simulations of the magnetic properties of recording media are described. Techniques for the generation of random structures are given in detail. We also outline some of the techniques used for the current calculation of magnetostatic fields, which is especially difficult for disordered systems. Finally, we describe the effects of microstructural disorder on the bulk magnetic properties and also introduce an overview of the effects of low coercivity grains which is an extreme form of disorder in the microstructure.

Simulations of MFM images of longitudinal thin films with low coercivity grains

The recording density of thin film media is affected by percolation within the transitions; such percolation has been shown to occur in areas where clusters of low coercivity grains lie. In this paper we present simulated Magnetic Force Microscopy (MFM) images which show it is common for inter-penetration between adjacent written bits to occur close to such clusters.

The effect of fcc grains on the magnetic and recording properties of thin film media

The recording density of thin film media is affected by percolation within the transitions. There is some evidence that the presence of low coercivity grains within the films might provide 'weak links' around which percolation nucleates. We have simulated such a system and found that the bulk parameters of films are affected by such grains and more importantly, a dramatic effect on the magnetic microstructure within transitions occurs.

Compaction-induced inclination shallowing of the post-depositional remanent magnetization in a synthetic sediment

Summary. A synthetic sediment comprised of kaolinite, distilled water and either equidimensional or acicular magnetite was given a post-depositional remanent magnetization (PDRM) by being stirred in a magnetic field. This sediment was compacted under pressures which varied continuously from 0 to 0.14 MPa in a water-tank consolidometer and to higher pressure steps (~2.53 MPa) in a standard soil consolidometer. Compaction took place in the same magnetic field in which the sample was given its PDRM. The compaction caused shallowing of the sample's magnetic inclination. This shallowing was found to be a function of the sample's initial magnetic inclination and the degree of sample compaction; tan (I,) = (1 - aAv) tan (lo), where IR is the remanent inclination after compaction, AV is the volume change, I. is the initial magnetic inclination, and a is an empirically derived constant. The data show a maximum inclination shallowing of 12 for an initial inclination of 54, in good agreement with the maximum inclination shallowing predicted by the above equation. We propose an electrostatic mechanism to be the cause of the inclination shallowing. In this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain's surface. As the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression. Alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post-compaction NRMs, implying that the smaller, higher coercivity grains are most affected by the compaction process. These data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected.