High Remanent Magnetization Research Articles

One-step synthesis and properties of urchin-likePS/α-Fe2O3 composite hollow microspheres

A new progressive hard template method has been developed to synthesize uniform polystyrene(PS)/α-Fe2O3 composite hollow microspheres under hydrothermal conditions. Sulfonated polystyrene (PS)microspheres were used as templates as well as the endothecia of the composite microspheres;Fe2O3 shell was obtainedthrough the reaction of FeSO4 and KClO3 under hydrothermal conditions. Field-emission scanning electron microscopy (FESEM) andtransmission electron microscopy (TEM) studies showed that the as-obtained productsconsist of large scale monodisperse urchin-like hollow microspheres. In comparison with theFe2O3 of other structures, the composite hollow microspheres have good photocatalytic activity,large surface area and a high remanent magnetization. A possible mechanism for theformation of composite hollow microspheres is also proposed in the paper.

Heat capacity and magnetic properties of aEuVO3single crystal

We have studied the heat capacity and magnetic properties of a $\mathrm{Eu}\mathrm{V}{\mathrm{O}}_{3}$ single crystal. The compound undergoes an orbital ordering transition at ${T}_{\mathrm{O}\mathrm{O}}=204\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is then followed by a spin ordering transition at ${T}_{\mathrm{S}\mathrm{O}}=131\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the spin ordered region, we have found that the compound is very sensitive to small trapped fields present in the superconducting solenoid of a magnetometer during cooling. Large negative Weiss temperatures are seen along all principal crystallographic axes indicating that strong antiferromagnetic interactions are present in this compound. Nevertheless, at low temperatures, we observed open hysteresis loops with a relatively high remanent magnetization and coercivity. We discuss the results for $\mathrm{Eu}\mathrm{V}{\mathrm{O}}_{3}$ in terms of an inhomogeneity with random fields.

Magnetic/non-magnetic nanoparticles films with peculiar properties produced by ultrashort pulsed laser deposition

Films of magnetic nanoparticles uniformly mixed with non-magnetic nanoparticles have been produced by ultrashort pulsed laser deposition. These films present innovative characteristics with respect to their counterparts produced by standard techniques, as for example nanosecond laser ablation or sputtering, due to the peculiar shape and preferential distribution of their constituent nanoparticles. In the present investigation, the difficult coalescence among the deposited nanoparticles, specific characteristic of the ultrashort pulsed laser deposition, is particularly stressed for what concerns its effect on the collective magnetic behaviour. In particular, we observed that, even for a significant fraction of magnetic particles, the films exhibit an unusual high remanent magnetization, together with relatively low values of saturation and coercive fields, showing a strong squareness of the hysteresis loops. In perspective, these nanogranular films appear very promising for potential application as permanent magnets and in magnetic recording.

The Stardalur magnetic anomaly revisited—New insights into a complex cooling and alteration history

This study provides new rock magnetic and magneto-mineralogical data including Mössbauer spectroscopy of basaltic drill cores from the Stardalur volcanic complex, Iceland, in order to better understand the strong magnetic anomaly, which is caused by an extraordinary high natural remanent magnetization (NRM). NRM and magnetic susceptibility ( χ) display a positive linear correlation ( R 2 = 0.81) and reach very high values up to 121 A/m and 148 × 10 −3 SI. Although a Curie temperature of 580 °C and a Verwey transition at about −160 °C is indicative of magnetite, χ– T heating experiments in argon and air atmosphere and thermal demagnetization measurements of NRM revealed a slight cation-deficiency. According to induced remanent magnetization experiments the remanence is carried solely by this low coercive phase. Minor titanomaghemite with a T C at about 340 °C only occurs in samples with larger oxide grains (20–80 μm). High vesicle abundances and the exsolution texture of Fe–Ti oxides suggest subaerial extrusion of the lava. A high oxygen fugacity (probably above the NNO buffer) and a low Ti/(Ti + Fe) ratio of the basaltic melt are suggested as a precondition for high concentration of magnetic minerals and therefore high primary TRM. During high temperature oxidation, ilmenite exsolution-lamellae, developed in titanomagnetite, and symplectic magnetite (+ pyroxene) formed by the breakdown of olivine. This secondary magnetite, grown at temperatures above the Curie temperature, increases the primary TRM. Early stage hydrothermal alteration (below about 375 °C) led to maghemitization of (titano)magnetite, clearly indicated by shrinkage cracks and irreversible χ– T curves. During later stage hydrothermal alteration, NRM intensity increased slightly due to the growth of secondary magnetite at lower temperatures (about 250–300 °C). This hydrothermally formed magnetite acquired only a low CRM but increased magnetic susceptibility significantly. According to our results it is suggested, that hydrothermal alteration does not necessarily lower remanent magnetization, but contributes to an increase in magnetization. The interplay of the three factors melt composition, small grain sizes of secondary magnetite due to decomposition of silicates and new formation under hydrothermal conditions caused the strong magnetic anomaly at the surface.

Locating geologic contacts with magnitude transforms of magnetic data

Locating geologic contacts by mapping lateral magnetization changes provides a variety of useful geologic information. Magnetic contacts will often correspond to lithologic boundaries, while the shapes and textures of contact maps can also indicate rock type and the style of deformation present. Well-established methods for contact mapping are the horizontal gradient magnitude of the pseudogravity (PSG-hgm) and the total field (TF-hgm). More recent mappers include the analytic signal (AS), the 3-D local wavenumber (LW) and the horizontal gradient magnitude of the tilt (TI-hgm). PSG-hgm and TF-hgm generally produce coherent contact trends but the locations are dependent on the contact dip and its magnetization direction. The AS, LW and TI-hgm methods produce contact locations that are independent of dip and magnetization. Magnitude transforms of the anomalous magnetic field offer an alternative approach to contact mapping since only the field components or the field component derivatives are used, in contrast with other methods that require total field derivatives, possibly up to second order. The magnitude transforms are also dip- and magnetization-independent. Contact maps determined for the magnitude transforms are compared with those from the PSG-hgm and TF-hgm methods for a recently-flown survey over a poorly-exposed Precambrian Shield area in Manitoba, Canada. The T transform map (magnitude of the anomalous magnetic field) is similar to that from PSG-hgm. The other magnitude transforms perform less well because of sensitivity to noise and gridding artefacts, resulting in less coherent trends. None of the transforms are able to resolve the same detail that TF-hgm does. Contact maps from magnitude transforms are therefore only to be preferred over PSG-hgm and TF-hgm maps in areas of high remanent magnetization and gently dipping geology.

Electric currents along earthquake faults and the magnetization of pseudotachylite veins

Pseudotachylites occur in the form of thin glassy veins quenched from frictional melts along the fault planes of major earthquakes. They contain finely grained magnetite and often exhibit a high natural remanent magnetization (NRM). High NRM values imply strong local electric currents. These currents must persist for some time, while the pseudotachylite veins cool through the Curie temperature of magnetite around 580 °C. There is no generally accepted theory explaining how such powerful, persistent currents may be generated along the fault plane. Data presented here suggest the activation of electronic charge carriers, which are present in igneous rocks in a dormant, inactive form. These charge carriers can be “awakened” by the application of stress. They are electrons and defect electrons, also known as positive holes or p-holes for short. While p-holes are capable of spreading out of the stressed rock volume into adjacent p-type conductive unstressed rocks, electrons require a connection to the hot, n-type conductive lower crust. However, as long as the (downward) electron flow is not connected, the circuit is not closed. Hence, with the outflow of p-holes impeded, no current can be sustained. This situation is comparable to that of a charged battery where one pole remains unconnected. The friction melt that forms coseismically during rupture, provides a conductive path downward, which closes the circuit. This allows a current to flow along the fault plane. Extrapolating from laboratory data, every km 3 of stressed igneous rocks adjacent to the fault plane can deliver 10 3–10 5 A. Hence, the current along the fault plane will not be limited by the number of charge carriers but more likely by the (electronic) conductivity of the cooling pseudotachylite vein. The sheet current will produce a magnetic field, whose vectors will lie in the fault plane and perpendicular to the flow direction.

Intrinsic ferromagnetic properties of Ti0.94Fe0.06O2∕Ti0.94Mn0.06O2 superlattice films for dilute magnetic semiconductor applications

Superlattice films have been proposed to get dilute magnetic semiconductor (DMS) with an intrinsic room-temperature ferromagnetism. For a TiO2-based DMS superlattice structure, each layer was alternately doped by two different transition metals (Fe and Mn) and deposited to a thickness of approximately 2.7Å on r-Al2O3(1102) substrates by pulsed laser deposition. Samples of Ti0.94Fe0.06O2 (TiFeO), Ti0.94Mn0.06O2 (TiMnO), and Ti0.94(Fe0.03Mn0.03)O2 show a low remanent magnetization and coercive field, as well as superparamagnetic features at room temperature. On the other hand, superlattice films (TiFeO∕TiMnO) show a high remanent magnetization and coercive field, resulting in intrinsic ferromagnetic properties. The superlattice films composed of alternating layers of Ti0.94Fe0.06O2 and Ti0.94Mn0.06O2 exhibit intrinsic ferromagnetic properties for dilute magnetic semiconductor applications.

Magnetic MEMS Reconfigurable Frequency-Selective Surfaces

A reconfigurable frequency-selective electromagnetic filter implemented by integrating hard magnetic materials with microelectromechanical systems (MEMS) provides a new variation of reconfigurable frequency-selective surfaces (FSS). By incorporating magnetically actuated dipole elements that are capable of being tilted away from the supporting surface, we can tune the FSSs operating frequency without having to physically alter the dimensions of the dipole elements. The 25/spl times/25 array of microactuators used in this work each consist of a 896/spl times/168/spl times/30 /spl mu/m/sup 3/ ferromagnetic plate made of 40Co-60Ni, layered with a 1-/spl mu/m-thick conductor (Au), attached to a pair of 400/spl times/10/spl times/1 /spl mu/m/sup 3/ polysilicon torsion beams, suspended just above the supporting substrate. The high remanent magnetization of the ferromagnetic material allows for relatively small magnetic fields (/spl sim/2.1 kA/m) to induce significant angular deflections (/spl sim/45/spl deg/). This innovative reconfigurable FSS design has successfully demonstrated electromagnetic-signal diplexing and tuning its resonant frequency over a bandwidth of 2.7 GHz at a frequency of 85 GHz.

Comment on a paper “The origin of high magnetic remanence in fault pseudotachylites: Theoretical considerations and implication for coseismic electrical currents” by E.C. Ferré, M.S. Zechmeister, J.W. Geissman, N. Mathana Sekaran, and K. Kocak

Pseudotachylites occur in the form of thin glassy veins quenched from frictional melts along the fault planes of major earthquakes. They contain finely grained magnetite and often exhibit a high natural remanent magnetization (NRM). High NRM values imply strong local electric currents. These currents must persist for some time, while the pseudotachylite veins cool through the Curie temperature of magnetite around 580 °C. There is no generally accepted theory explaining how such powerful, persistent currents may be generated along the fault plane. Data presented here suggest the activation of electronic charge carriers, which are present in igneous rocks in a dormant, inactive form. These charge carriers can be “awakened” by the application of stress. They are electrons and defect electrons, also known as positive holes or p-holes for short. While p-holes are capable of spreading out of the stressed rock volume into adjacent p-type conductive unstressed rocks, electrons require a connection to the hot, n-type conductive lower crust. However, as long as the (downward) electron flow is not connected, the circuit is not closed. Hence, with the outflow of p-holes impeded, no current can be sustained. This situation is comparable to that of a charged battery where one pole remains unconnected. The friction melt that forms coseismically during rupture, provides a conductive path downward, which closes the circuit. This allows a current to flow along the fault plane. Extrapolating from laboratory data, every km3 of stressed igneous rocks adjacent to the fault plane can deliver 103–105 A. Hence, the current along the fault plane will not be limited by the number of charge carriers but more likely by the (electronic) conductivity of the cooling pseudotachylite vein. The sheet current will produce a magnetic field, whose vectors will lie in the fault plane and perpendicular to the flow direction.

Ba-hexaferrite films for next generation microwave devices (invited)

Next generation magnetic microwave devices require ferrite films to be thick (>300μm), self-biased (high remanent magnetization), and low loss in the microwave and millimeter wave bands. Here we examine recent advances in the processing of thick Ba-hexaferrite (M-type) films using pulsed laser deposition (PLD), liquid-phase epitaxy, and screen printing. These techniques are compared and contrasted as to their suitability for microwave materials processing and industrial production. Recent advances include the PLD growth of BaM on wide-band-gap semiconductor substrates and the development of thick, self-biased, low-loss BaM films by screen printing.

Microwave and magnetic properties of self-biased barium hexaferrite screen printed thick films

The interest in barium hexaferrite thick films, particularly with high remanent magnetization, is driven by the development of small planar ferrite microwave devices. We report here processing and microwave characterization of BaFe12O19 ferrite thick films (100–400μm). The films were deposited on silicon and alumina substrates by screen printing, oriented under a magnetic field of 8kOe, then annealed at 250°C and sintered at temperatures ranging from 850to1300°C. Scanning electron microscopy and x-ray diffraction exhibited strong crystallographic alignment of c-axis crystals perpendicular to the film plane. The magnetization measurement indicated that a typical dense film with 270μm thickness yielded a high squareness (Mr∕Ms) of 0.93. Ferrimagnetic resonance (FMR) measurements were performed in the frequency range of 40–55GHz. From the linear dependence of FMR frequency on the external field, a g factor of 2.03±0.08 was deduced, while the smallest linewidth was obtained to be 1.2kOe at 40GHz. The broadening of the FMR linewidth arises predominantly from magnetic inhomogeneities in the polycrystalline films. The thick films have great potential for use in future microwave and millimeter wave monolithic integrated circuits.

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.

Perpendicularly magnetized, grooved GdTbFeCo microstructures for atom optics

Periodically grooved, micron-scale structures incorporating perpendicularly magnetized Gd10Tb6Fe80Co4 magneto-optical films have been fabricated and characterized. Such structures produce a magnetic field having flat equipotentials and whose magnitude decays exponentially with distance above the surface, making them attractive for manipulating ultracold atoms in atom optics. The GdTbFeCo films have been deposited on a Cr underlayer on a silicon (100) wafer and on a grooved silicon microstructure using DC magnetron sputtering. The films are found to have excellent magnetic properties for magnetic atom optics applications, including high remanent magnetization, high coercivity and excellent homogeneity.

Preparation and characterization of shuttle-like α-Fe 2O 3 nanoparticles by supermolecular template

Shuttle-like α-Fe 2O 3 nanoparticles have been successfully synthesized via a new soft-template route using polyethylene glycol (PEG) as polymer, cetyltrimethylammonium bromide (CTAB) as surfactant and FeCl 3·6H 2O as iron source materials. Meanwhile, spherical α-Fe 2O 3 nanoparticles are also fabricated under the similar conditions without surfactant and polymer. The resultant products are characterized by means of thermalgravimetric analysis (TGA), powder X-ray diffraction (XRD), infrared (IR) spectroscopy, transmission electron micrograph (TEM), X-ray photoelectron spectra (XPS) and magnetization measurements. The homogeneous α-Fe 2O 3 nanoparticles with shuttle-like shape have an average length of 120 nm and a mean diameter of about 50 nm in the middle part (an average aspect ratio of about 2.5) whereas spherical α-Fe 2O 3 nanoparticles have a mean particle diameter of about 35 nm. Magnetic hysteresis measurements reveal that shuttle-like α-Fe 2O 3 nanoparticles display normal ferromagnetic behaviors while spherical α-Fe 2O 3 nanoparticles exhibit weak ferromagnetic behaviors at room temperature. The two types of α-Fe 2O 3 exhibit hysteretic features with the remanence and coercivity of 0.156 emu/g and 664 Oe, 0.048 emu/g and 110 Oe, respectively. The higher remanent magnetization and coercivity of shuttle-like α-Fe 2O 3 nanoparticles may be associated with the aspect ratio of α-Fe 2O 3 since shape anisotropy would exert a tremendous influence on their magnetic properties.

AIN-based dilute magnetic semiconductors

AlMnN and AlCrN have been synthesized by gas-source molecular beam epitaxy (GSMBE). Using optimized growth conditions and compositions, sccm films as determined by x-ray diffraction (XRD) and transmission electron microscopy, which also show room-temperature magnetic behavior were obtained for both materials. Chromium was found to produce greater magnetic ordering as evidenced by a higher technical saturation. The AlCrN also exhibited a higher remanent magnetization and a M versus T behavior more typical of ferromagnetism than that observed for AlMnN. These results suggest that Cr is a superior dopant for formation of AlN-based, dilute magnetic semiconductors.

Effects of additive elements (Cu, Zr, Al) on morphological and magnetic properties of NdFeB thin films with perpendicular magnetic anisotropy

Cu, Zr, Al were doped into the NdFeB thin films to improve the morphological and magnetic properties of the films. Distinct results have been observed due to the different nature of these additives (such as atomic radius and chemical affinity, etc.). The morphological and magnetic properties of the films could be improved selectively according to the additive element. Grains in the Cu-doped film significantly grew and became strip-like. Both Zr and Al addition effectively reduced the grain size. But Zr addition led to a large grain size distribution while Al addition produced more uniformly sized grains in the film. The perpendicular coercivities of NdFeB films could be enhanced by doping a certain amount of the additive element. Addition of 1.0 at.% Cu or 3.8 at.% Zr increased the perpendicular coercivity H c⊥ up to 487 or 565 kA/m from 382 kA/m, respectively. On the other hand, addition of 5.0 at.% Al could effectively moderate H c⊥ to 279 kA/m while remained a high perpendicular remanent magnetization ratio S ⊥ around 0.9. Magnetization reversal mechanisms of the doped samples were also discussed according to the angular dependence of the coercivity H c( θ) and M–H loop shapes. It was suggested that magnetization reversal of Cu-doped sample was initiated by domain wall motion and ended by domain wall pinning. Most of the grains in the Zr-doped sample were inversely magnetized by magnetization coherent rotation, but the rest were demagnetized by magnetization incoherent switching. In the Al-doped sample, magnetization reversal was proceeded mainly by magnetization incoherent rotation.

Magnetic Anisotropy as an Aid to Identifying CRM and DRM in Red Sedimentary Rocks

To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal. In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock’s remanence may have been affected by strain and could have originated as a DRM or a CRM. Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.

Earth analog for Martian magnetic anomalies: remanence properties of hemo-ilmenite norites in the Bjerkreim-Sokndal intrusion, Rogaland, Norway

To explain the very large remanent magnetic anomalies on Mars, which no longer has a global magnetic field, it is important to evaluate rocks on Earth with the necessary properties of high natural remanent magnetization (NRM) and coercivity. Here, we describe a possible analog from the 230-km 2 930 Ma Bjerkreim-Sokndal layered intrusion (BKS) in Rogaland, Norway. In the layered series of the BKS, fractional crystallization of jotunitic magma was punctuated by influx and mixing of more primitive magmas, producing six megacyclic units, each typically with early plagioclase-rich norites, intermediate hemo-ilmenite-rich norites and late magnetite norites with subordinate near end-member ilmenite. Following each influx, the magma resumed normal crystallization and, following the last, near the base of Megacyclic Unit IV, crystallization continued until norites gave way to massive fayalite-magnetite mangerites and quartz mangerites in the upper part of the intrusion. The Megacycles are marked on a regional aeromagnetic map by remanent-controlled negative anomalies over ilmenite norites and induced positive anomalies over magnetite norites and mangerites. A prominent negative anomaly (with amplitude −13,000 nT in a high-resolution helicopter survey, down to −27,000 nT below background in ground magnetic profiles) occurs over the central part of Megacyclic Unit IV. The anomaly is centered on ilmenite norite Unit IVe and is most intense where cumulate layering is near vertical at the southeast edge of the Bjerkreim Lobe of the intrusion at Heskestad. Here, Unit IVe is flanked to the east by magnetite norite of Unit IVc and country-rock gneisses (group E) and to the west by Unit IVf magnetite norite and mangerites (group W). Magnetic properties were measured on 128 oriented samples. Susceptibilities are similar for all three sample groups at ∼8×10 −2, but Koenigsberger ratios are very different, with average values of 7.7 for IVe, and <1 for groups E and W. The IVe samples, with only a few percent of oxides, have the highest NRMs measured from the BKS, up to 74 A/m, with an average of 30.6 A/m, making them prime candidates for consideration as Mars analogs. The mean direction for IVe samples is D=17.6°, I=−79.9, a 95=10°, almost opposite the present field. Evidence on origin of the strong NRM in IVe as compared to groups E and W, include greater abundance of hemo-ilmenite and of orthopyroxene with hemo-ilmenite exsolution, and the strong lattice-preferred orientation of both in a relationship favorable for “lamellar magnetism”. Massive magnetite-free hemo-ilmenite ores in anorthosite from the same district also produce negative magnetic anomalies. They have a substantial but much lower NRM, suggesting that there are special oxide properties in the IVe rocks at Heskestad.

Effects of nanoscale exsolution in hematite–ilmenite on the acquisition of stable natural remanent magnetization

To investigate the acquisition mechanism of high and stable natural remanent magnetization (NRM) in rocks of the Russell Belt, Adirondack Mountains, New York, we examined the exsolution microstructures and compositions of magnetic minerals using three samples with different magnetic properties. The samples contain titanohematite with ilmenite lamellae, end-member hematite without lamellae and rare magnetite as potential carriers for the NRM. Transmission electron microscopy (TEM) observations and element mapping by energy-filtered TEM (EFTEM) of the titanohematite indicated that very fine ilmenite lamellae with a minimum thickness ∼2 nm are abundant between larger ilmenite lamellae a few hundreds of nanometers thick. The ilmenite lamellae and titanohematite hosts, with the compositions of Ilm 90–100Hem 10–0 and Ilm 7–16Hem 93–84, respectively, share (001) planes, and the abundant fine ilmenite lamellae have coherent, sharp structural and compositional interfaces with their titanohematite hosts. Comparison between samples shows that the magnetization is correlated with the amount of fine exsolution lamellae. These results are consistent with the lamellar magnetism hypothesis, suggesting that the acquisition of high and stable NRM is related to the interfacial area between fine ilmenite lamellae and their host titanohematite. End-member hematite with a multi-domain magnetic structure only contributes a minor amount to the NRM in these samples.

Paleomagnetic results from deep-sea sediment of the Korea Deep Ocean Study (KODOS) area (northern equatorial Pacific) and their paleodepositional implications

Paleomagnetic properties of sediment cores were examined to reconstruct paleodepositional conditions in the Korea Deep Ocean Study (KODOS) area, located in the northeastern equatorial Pacific. The studied KODOS sediments have a stable remanent magnetization with both normal and reversed polarities, which are well correlated with the geomagnetic polarity timescale for the late Pliocene and Pleistocene. Average sedimentation rates are 1.56 and 0.88 mm/kiloyear for the Pleistocene and late Pliocene, respectively. Clay mineralogy and scanning electron microscope analyses of the sediments indicate that terrestrial material was transported to the deep-sea floor during these times. The variations of sedimentation rates with age may be explained by the onset of the northern hemisphere glaciation and subsequent climatic deterioration during the Pliocene and Pleistocene. For the Pleistocene, an increasing sedimentation rate implies that input of terrestrial materials was high, and also a high input of biogenic materials was detected as a result of increased primary production in the surface water. The down-core variations in paleomagnetic and rock-magnetic properties of the KODOS sediments were affected by dissolution processes in an oxic depositional regime. As shown by magnetic intensity and hysteresis parameters, the high natural remanent magnetization (NRM) stability in the upper, yellowish brown layers indicates that the magnetic carrier was in pseudo-single domain states. In the lower, dark brown sediments, only coarse magnetic grains survived dissolution and the NRM was carried by more abundant, multi-domain grains of low magnetic stability. The down-core variation of magnetic properties suggests that the KODOS sediments were subjected to dissolution processes resulting in a loss of the more stable components of the magnetic fraction with increasing core depth.