Magnetic elastomers with hard or permanent magnetic particulate are able to achieve complex motion not possible from soft magnetic elastomers. Magnetic annealing and fused deposition modeling (FDM) have been used to increase the performance of magnetic composites. This research explores how the magnetoactive properties of hard magnetic elastomers are influenced by magnetic annealing and the addition of the soft magnetic particulate. Three compositions of the thermoplastic magnetic elastomer composite are explored: 15 vol. % SrFe 12 O 19 , 10 vol. % SrFe 12 O 19 /5 vol. % carbonyl iron, and 5 vol. % SrFe 12 O 19 /10 vol. % carbonyl iron. The material is then extruded into FDM filaments. During the extrusion process, some filament is magnetically annealed in an axial applied field. Magnetic hysteresis loops show that the saturation magnetization and coercivity change based on the relative amount of hard and soft magnetic particulate. The presence of only one coercive field indicates magnetic coupling between the hard and soft components. Magnetoactive testing measures each sample’s mechanical deflection angle as a function of transverse applied magnetic field strength. Qualitative and quantitative results reveal that magnetic annealing is critical to the magnetoactive performance of the hard magnetic elastomers. The results also demonstrate that magnetic annealing and increased carbonyl iron both improve the magnetoactive deflection angle for a given applied field. Scanning electron microscopy shows a stratification effect in a range of the filaments. Understanding these hard magnetic elastomers provides insight into how performance can be controlled and optimized by magnetic annealing and combining hard and soft magnetic particulate.
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Near Degeneracy of Magnetic Phases in Two-Dimensional Chromium Telluride with Enhanced Perpendicular Magnetic Anisotropy
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The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.more » « less
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Long time ago, Brochard and de Gennes predicted the possibility of significantly decreasing the critical magnetic feld of the Fredericksz transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid crystals and ferromagnetic particles, the so-called ferronematics. This phenomenon has rarely been measured, usually due to soft homeotropic anchoring induced at the nanoparticle surface. Here we present an optical study of the magnetic Fredericksz transition combined with a light scattering study of the classical nematic liquid crystal, 5CB, doped with 6 nm diameter magnetic and non-magnetic nanoparticles. Surprisingly, for both nanoparticles, we observe at room temperature a net decrease of the threshold field of the Fredericksz transition at low nanoparti cle concentrations, which appears associated with a coating of the nanoparticles by a brush of polydimethylsiloxane copolymer chains inducing planar anchoring of the director on the nanoparticle surface. Moreover the magnetic Fredericksz threshold exhibits non-monotonic behaviour as a function of the nanoparticle concentration for both types of nanoparticles, first decreasing down to a value from 23% to 31% below that of pure 5CB, then increasing with a further increase of nanoparticle concentration. This is interpreted as an aggregation starting at around 0.02 weight fraction that consumes more isolated nanoparticles than those introduced when the concentration is increased above c = 0:05 weight fraction (volume fraction 3:5 x 10^-2). This shows the larger effect of isolated nanoparticles on the threshold with respect to aggregates. From dynamic light scattering measurements we deduced that, if the decrease of the magnetic threshold when the nanoparticle concentration increases is similar for both kinds of nanoparticles, the origin of this decrease is different for magnetic and non-magnetic nanoparticles. For non-magnetic nanoparticles, the behavior may be associated with a decrease of the elastic constant due to weak planar anchoring. For magnetic nanoparticles there are non-negligible local magnetic interactions between liquid crystal molecules and magnetic nanoparticles, leading to an increase of the average order parameter. This magnetic interaction thus favors an easier liquid crystal director rotation in the presence of external magnetic field, able to reorient the magnetic moments of the nanoparticles along with the molecules.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsnano.0c05534
