Ann M. Hirt

Modeling Magnetic Torque and Force for Controlled Manipulation of Soft-Magnetic Bodies

We calculate the torque and force generated by an arbitrary magnetic field on an axially symmetric soft-magnetic body. We consider the magnetization of the body as a function of the applied field, using a continuous model that unifies two disparate magnetic models. The continuous torque and force follow. The model is verified experimentally, and captures the often neglected region between weak and saturating fields, where interesting behavior is observed. We provide the field direction to maximize torque for a given field magnitude. We also find an absolute maximum torque, for a given body geometry and material, which can be generated with relatively weak applied fields. This paper is aimed at those interested in systems-level analysis, simulation, and real-time control of soft-magnetic bodies.

The anisotropy of magnetic susceptibility in biotite, muscovite and chlorite single crystals

Abstract The anisotropy of magnetic susceptibility (AMS) of single crystals of biotite, muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock-forming minerals. The low-field AMS and the high-field paramagnetic susceptibility are defined. For the high-field values, it is necessary to combine the paramagnetic deviatoric tensor obtained from the high-field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals. This leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice. The ellipsoid of paramagnetic susceptibility, which was obtained for the three phyllosilicates, is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c-axes. The anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements. The degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite the smaller than previously published values. The shape of the chlorite AMS ellipsoid for all the samples is near-perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies. Reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks.

MAGNETIC ANISOTROPY, ROCK FABRICS AND FINITE STRAIN IN DEFORMED SEDIMENTS OF SW SARDINIA (ITALY)

Abstract Although some empirical relationships have been established between the anisotropy of magnetic susceptibility (AMS) and the state of finite strain in studies from specific geologic areas, the mechanisms governing correlations are not well understood. A comparative study has been made in order to elucidate the AMS-strain relationship by investigating mineral preferred orientations and the responsible microstructural deformation processes. Detailed analyses are presented from low-grade metamorphic slates of a Palaeozoic sequence exposed in the Variscan of SW Sardinia. Rock magnetic experiments suggest that paramagnetic minerals dominate the AMS in these rocks. Excellent agreement was found between magnetic fabrics and lattice preferred orientations (001) of mica and chlorite, determined from X-ray texture goniometry measurements. Depending on the oblate or prolate ellipsoidal shape of anisotropy, the minimum principal axes of AMS and X-ray textures can coincide with the poles to slaty cleavage or the maximum principal axes can coincide with the bedding/cleavage intersection, respectively. The measured AMS and X-ray textures only partially reflect the finite strain determined from deformed micro-pebbles and reduction spots. Axial orientations of magnetic fabrics, mica and chlorite preferred orientations, and finite strain ellipsoids are generally in good agreement depending on development of oblate or prolate fabric ellipsoidal shapes. Linear correlations can be established for magnitudes of minimum and intermediate axes of the AMS and finite strain ellipsoids but shape comparisons are complex. Microstructural studies by scanning electron microscopy (SEM) indicate that the preferred orientation of the phyllosilicates is caused by different grain-scale deformation mechanisms, such as kinking, micro-folding and preferential growth. Thus, magnetic fabrics and mineral preferred orientations evidently result from different deformation mechanisms than the finite strain. They reflect the heterogeneous deformation due to micro-mechanical reorientation processes of mineral grains while finite strain reflects the homogeneous deformation accommodated cumulatively by the strain markers.

The age and timing of folding in the central Appalachians from paleomagnetic results

Comparison of the paleomagnetic pole positions of late Paleozoic secondary magnetizations with a time-averaged reference apparent polar wander path for North America shows that sedimentary rocks within the central Appalachians were remagnetized in the later half of the Permian, between ca. 255 and 275 Ma. Ages of remagnetization do not vary for sites distributed along or across strike of the central Appalachian fold and thrust belt or across the Appalachian Plateau in New York, nor do they vary with stratigraphic position of the host rocks. However, the pattern of folding relative to remagnetization across the Valley and Ridge and Great Valley in Pennsylvania, Maryland, and West Virginia shows a distinct temporal variation. Individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt, synfolding magnetizations in the central part of the belt, and prefolding magnetizations near the foreland. Because of the similar age of the late Paleozoic magnetization, we conclude that rocks across the central Appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland. We interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization, folds in the central region grew during remagnetization, and folds near the foreland grew after remagnetization. Rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized (synfolding) folds. In such folds, rocks were apparently remagnetized during an increment of fold growth, in which there was partial and possibly negligible fold-limb rotation. Collectively, these results may constrain the mechanism for basinal fluid mobilization during deformation. Current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism. If realistic, our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets, as has been suggested for other fold and thrust belts.

Separation of ferrimagnetic and paramagnetic anisotropies using a high-field torsion magnetometer

Abstract The analysis of magnetic fabric in magnetic fields that are strong enough to saturate ferrimagnetic phases permits the separation of paramagnetic and ferrimagnetic fabrics. Anisotropy measurements in two different fields are necessary to separate the components, but are insufficient to define them precisely. A method of analysis using several high fields has been developed and applied to three different lithologies from the Betic Cordillera in Southern Spain, in which the anisotropies are controlled by different mineral fractions. The magnetic anisotropy of granites was found to be dominated by paramagnetic minerals. Peridotites possessed a mixed magnetic fabric in which the measured anisotropy is due to both magnetic fractions. The magnetic anisotropy of serpentinites was dominated by the ferrimagnetic minerals.

Formation of magnetite nanoparticles at low temperature: from superparamagnetic to stable single domain particles.

The room temperature co-precipitation of ferrous and ferric iron under alkaline conditions typically yields superparamagnetic magnetite nanoparticles below a size of 20 nm. We show that at pH  =  9 this method can be tuned to grow larger particles with single stable domain magnetic (> 20–30 nm) or even multi-domain behavior (> 80 nm). The crystal growth kinetics resembles surprisingly observations of magnetite crystal formation in magnetotactic bacteria. The physicochemical parameters required for mineralization in these organisms are unknown, therefore this study provides insight into which conditions could possibly prevail in the biomineralizing vesicle compartments (magnetosomes) of these bacteria.

An AMS, structural and paleomagnetic study of quaternary deformation in eastern Sicily

Abstract An integrated structural, anisotropy of magnetic susceptibility (AMS) and paleomagnetic study was carried out on Plio-Pleistocene sedimentary basins in eastern Sicily. These basins belong to three main tectonic domains: the Tyrrhenian hinterland domain, the Catania foredeep domain, and the Hyblean foreland domain. We sampled 329 oriented samples from 25 sites in selected areas from the different tectonic domains. The AMS is typical for weakly deformed sediments, with a magnetic foliation sub-parallel to the bedding plane, and a well-defined magnetic lineation. The orientation of the magnetic lineation is strongly controlled by the main tectonic deformation recorded in the basins. Structural and AMS data define a transition from NW–SE extension in the Tyrrhenian hinterland domain, to E–W compression in the Catania foredeep domain, to E–W extension in the Hyblean foreland domain, respectively. Reliable paleomagnetic results have been obtained in 12 out of 25 sampled sites. Data show that no significant rotations occurred in any of the studied basins at least since the middle Pleistocene. These results allow us to define an upper limit to the large rotations about vertical axes that have been previously found in the Calabria and Sicily regions.

Paleomagnetic results in support of a model for the origin of the Asturian arc

Abstract Paleomagnetic directions have been determined at 17 Cambrian and 28 Carboniferous limestone sites distributed along the Asturian arc of the Hercynian fold belt in northwestern Spain. The stable vectors are carried by hematite in all cases, and also by magnetite in some grey Carboniferous limestones. A secondary component of magnetization is usually removed well below 400°C, and the higher-temperature component is taken to be the characteristic direction. The structure at each site was carefully evaluated in order to make optimum local tectonic corrections. In addition the data must be corrected for rotations about a vertical axis to allow for a phase of radial folding superimposed on earlier structures. The corrected paleomagnetic declinations are found to vary systematically along the arc of the fold belt. Consequently, paleomagnetic data from the Asturian arc should not be included in compilations used for the construction of a Paleozoic apparent polar wander path. The paleomagnetic data allow us to distinguish between different tectonic models for the evolution of the Asturian arc. A two-stage model for the development of the present curvature is favored. Part of the curvature appears to be primary, preceding the Hercynian deformation. The first stage of the model involved rotations of thrust sheets during their emplacement, producing a more tightly curved arc than the original form. In a second stage, the development of radial folds further tightened the curvature of the arc. Both stages result in clockwise rotations in the north and anticlockwise rotations in the south.

Paleomagnetic evidence for a Neogene two-phase counterclockwise tectonic rotation in the Northern Apennines (Italy)

Paleomagnetic directions have been determined for a new collection of Early Oligocene and Late Miocene‐Pliocene Epiligurian clastic sediments from the frontal portions of the northern Apennines. These results are combined with Cenozoic data from the literature to evaluate whether rotations of units in this region are related to the OligoMiocene Corsica‐Sardinia rotation and/or to younger phases of deformation of the Apennine chain. When Corsica/Sardinia moved counterclockwise oV the coast of France, the Ligurian units located at its front were presumably pushed eastward and rotated counterclockwise above a main boundary thrust onto the Adria/Africa margin. We propose that about 24 out of a total of 52° of rotation observed in the Epiligurian units can be related to the Oligo-Miocene motion of the Corsica‐Sardinia block, in partial agreement with previous conclusions, and the remaining 28° to the Pliocene tectonic phase at the Apennine chain front, which may have (re)activated thrust planes in the Adria/Africa succession below the Ligurian wedge.

Hybrid Wood Materials with Magnetic Anisotropy Dictated by the Hierarchical Cell Structure

Anisotropic and hierarchical structures are bound in nature and highly desired in engineered materials, due to their outstanding functions and performance. Mimicking such natural features with synthetic materials and methods has been a highly active area of research in the last decades. Unlike these methods, we use the native biomaterial wood, with its intrinsic anisotropy and hierarchy as a directional scaffold for the incorporation of magnetic nanoparticles inside the wood material. Nanocrystalline iron oxide particles were synthesized in situ via coprecipitation of ferric and ferrous ions within the interconnected pore network of bulk wood. Imaging with low-vacuum and cryogenic electron microscopy as well as spectral Raman mapping revealed layered nanosize particles firmly attached to the inner surface of the wood cell walls. The mineralogy of iron oxide was identified by XRD powder diffraction and Raman spectroscopy as a mixture of the spinel phases magnetite and maghemite. The intrinsic structural architecture of native wood entails a three-dimensional assembly of the colloidal iron oxide which results in direction-dependent magnetic features of the wood-mineral hybrid material. This superinduced magnetic anisotropy, as quantified by direction-dependent magnetic hysteresis loops and low-field susceptibility tensors, allows for directional lift, drag, alignment, (re)orientation, and actuation, and opens up novel applications of the natural resource wood.