Franz Heider

Magnetic Properties of Hydrothermally Recrystallized Magnetite Crystals

The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution (<0.3 micrometer) and of crushed sifted grains (>0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 1010 meter -2) and natural (1 x 1010 meter-2) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains.

Magnetic susceptibility and remanent coercive force in grown magnetite crystals from 0.1 μm to 6 mm

Abstract Initial susceptibility is frequently used as a palaeoclimatic indicator in sediments, but its grain size dependence is not well established. We measured initial magnetic susceptibility χ0 in grown and natural magnetite crystals ranging from 0.09 μm to 6 mm in grain size. Over these five decades of grain diameter, the presented initial susceptibilities are essentially independent of grain size with a mean value of 3.1 SI and a standard deviation of ±0.4 SI. Numerical results of micromagnetic calculations for cylindrical particles in the size range 0.06 μm N ≈ 1 3 and large intrinsic susceptibility (χi > 200) using the relation χ0 = χi(1 + Nχi). The observed number of magnetic domains in magnetite grains between 50 μm and 1000 μm is too low for the required demagnetizing factor of about 0.33. In a lamellar domain model one needs a higher number of domains than those observed, to obtain a demagnetizing factor of 0.33. A simple lamellar stripe domain model without closure domains is therefore not a good approximation for large magnetite grains. Remanent coercive force of grown magnetite grains shows a weak dependence on grain diameter. The remanent coercive force Hcr decreases gradually from about 35 mT to 10 mT between 0.09 μm and 6 mm. A noticeable drop in Hcr occurs at a grain size of about 110 μm, which is interpreted as the transition from pseudo-single-domain to multidomain grains. The remanent coercive force of magnetite grains is not a sensitive indicator of grain size, unlike coercive force or saturation remanent magnetization.

Low-temperature and alternating field demagnetization of saturation remanence and thermoremanence in magnetite grains (0.037 μm to 5 mm)

Low-temperature demagnetization (LTD), consisting of zero-field cycling from room temperature to 77 K and back to room temperature, was performed on magnetites from different sources having grain sizes between 0.037 μm and 5 mm. The memory fractions, RI and RT of saturation isothermal remanent magnetization (IRMs) and 0.1 mT thermoremanent magnetization (TRM), respectively, have a strong intrinsic grain-size dependence below ≈ 10 μm, but are almost size independent in the multidomain (MD) range above 10 μm. The size dependence of RI and RT in the MD range reported by Parry [1979, 1980] probably results from stress introduced during crushing to obtain finer size fractions. We demonstrate that the method of preparation of magnetites, irregularities in the crystals, and increase or decrease of the state of internal stress by quenching or annealing all have a direct effect on the amount of magnetic memory. Magnetic memories (the fractions of TRM and IRM5 surviving after LTD) are much more resistant against alternating-field (AF) demagnetization than the original TRM and IRMs before LTD. Even large magnetite crystals (≫20 μm) reveal a hard, single-domain-like component of magnetization when the softer magnetization is removed by LTD. A comparison of AF demagnetization properties of IRMs and TRM in these MD magnetites shows that the Lowrie and Fuller [1971] test must be reinterpreted.

Magnetic blocking temperatures of magnetite calculated with a three‐dimensional micromagnetic model

We present an analysis of thermal stability of magnetic remanence in fine grains of magnetite (grain size d = 15–120 nm). In order to model incoherent transitions between single-domain (SD) and pseudo-single-domain (PSD) magnetization configurations, we employ a three-dimensional constrained minimization method proposed by Enkin and Williams [1994]. Using this approach, one can track in detail the transition from one local energy minimum state into another by constraining the magnetization vectors of appropriate cells in a discrete model. For each particle, we obtain the energy barriers EB(T) from 25° to 578°C. Magnetic blocking temperatures (TB) are calculated by integrating EB(T) for two extreme cooling schedules representing laboratory and geological timescales. The computed blocking temperatures for laboratory timescales are in excellent agreement with the experimentally determined blocking temperatures for magnetite by Dunlop [1973b]. The results of our computations are summarized as relaxation time versus blocking temperature curves, which deviate from the curves of Pullaiah et al. [1975] for particles with grain sizes in the SD-PSD transition region. A consequence of the dependence of TB on timescale is that some PSD size particles are blocked in vortex states on geologic timescales but are blocked in the SD state on laboratory timescales. Paleointensity determinations with the Thellier method on such samples can therefore underestimate the paleofield. The superparamagnetic to SD threshold size dS is determined as 50 nm for cubic grains, whereas a small aspect ratio of q=1.1 is sufficient to depress dS to 27 nm. SD particles of magnetite with small shape anisotropy and cubic grains with 58 nm≤d≤72 nm are reliable carriers of paleomagnetic information.

Two types of chemical remanent magnetization during the oxidation of magnetite

Abstract The chemical remanent magnetization (CRM), M CRM , resulting from oxidation of equidimensional magnetite to a mixture of 90% hematite and 10% cation-deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization, ( M ARM ), of the parent magnetite and the field H CRM applied during oxidation. M CRM lies in the plane defined by H CRM and M ARM (which were perpendicular in our experiments) in an intermediate orientation determined by the relative strengths of H CRM (0.05, 0.1 and 0.2 mT) and M ARM (0.002–0.12 × saturation remanence). The CRMs are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization. Similar experiments in which acicular single-domain cation-deficient magnetite was oxidized to maghemite gave quite different results. M CRM was always parallel to M ARM . Its direction was unaffected by field strengths as high as H CRM = 1.5 mT. The essential factors controlling CRM direction seem to be the number and crystallographic structure of oxidation products. For single-phase oxidation without change in crystal lattice, CRM remains parallel to the initial remanence of the parent mineral. For multiphase oxidation to a mixture of rhombohedral and spinel phases, both initial remanence and the field acting during oxidation influence the CRM direction, which therefore has an intermediate orientation that parallels neither M ARM nor H CRM .

Hydrothermal growth of magnetite crystals (1 μm to 1 mm)

Abstract Hydrothermal recrystallization of submicron (0.5 μm) magnetite at temperatures from 416 to 800 °C using 0.1m HI or 2m NH 4 Cl solutions as mineralizers gave euhedral magnetite crystals from 0.8 μm to 1 mm in size. The method produced samples with narrow size distributions or samples that could be easily separated into uniform size fractions. These hydrothermally grown stoichiometric magnetite crystals are ideally suited for studying size dependent magnetic properties.

Pliocene carbonate accumulation along the California Margin

Recent modeling studies call on increased ocean heat transport to explain high-latitude warming observed for intervals throughout the middle Pliocene. Possible vehicles for ocean heat transport are the poleward arms of the subtropical gyres. Sites from the California margin (Ocean Drilling Program Leg 167) provide monitors of wind field within the eastern arm of the gyre which may be an indication of basin-wide subtropical gyral strength. At most sites (water depths from 1106 to 4212 m) CaCO3 mass accumulation rate (MAR) was highest in the middle Pliocene (3.5–2.0 Ma). This high CaCO3 MAR “event” is attributed primarily to higher CaCO3 production due to higher offshore upwelling associated with the zone of the greatest wind stress curl. Thus, in the middle Pliocene, there was enhanced wind stress curl along the California margin, and possibly enhanced North Pacific sub-tropical gyral circulation and meridional ocean heat advection.

Magneto-optical Kerr effect on magnetite crystals with externally applied magnetic fields

Abstract Magnetic domain structures of a multidomain magnetite crystal were observed as a function of applied magnetic field with the magneto-optical Kerr effect. Normal and reverse magnetic fields were applied from zero to 165 mT until the grain was nearly saturated. The magnetic hysteresis, determined from the domain patterns on the surface of a single crystal, was inconsistent with bulk hysteresis measurements on magnetite of equivalent grain size. A nearly single-domain configuration at zero field for the remanence state was interpreted as a surface phenomenon concealing magnetic domains in the interior of the grain. A model is presented in which magnetization lies in the observed plane and domain walls are dipping at a shallow angle.

Volcanic ash particles as carriers of remanent magnetization in deep-sea sediments from the Kerguelen Plateau

Abstract Carbonate sediments from the Kerguelen Plateau (ODP Leg 120) of Eocene to Pliocene age were investigated with rock magnetic, petrographic and geochemical methods to determine the carriers of remanent magnetization. Magnetic methods showed that the major magnetic minerals were titanomagnetites slightly larger than single domain particles. Submicrometre to micrometre-size grains of titanomagnetite were identified as inclusions in volcanic glass particles or as crystals in lithic clasts. Volcanic fallout ash particles formed the major fraction of the magnetic extract from each sediment sample. Three groups of volcanic ashes were identified: trachytic ashes, basaltic ashes with sideromelane and tachylite shards, and palagonitic ashes. These three groups could be equally well defined based on their magnetic hysteresis properties and alternating field demagnetization curves. The highest coercivities of all samples were found for the tachylite, due to the submicrometre-size titanomagnetite inclusions in the matrix. Trachytic ashes had intermediate magnetic properties between the single-domain-type tachylites and the palagonitic (altered) basaltic ashes with low coercivities. Samples which contained mixtures of these different volcanic ashes could be distinguished from the three types of ashes based on their magnetic characteristics. There was neither evidence of biogenic magnetofossils in the transmission electron micrographs nor did we find magnetic particles derived from continental Antarctica. The presence of dispersed volcanic fallout ashes between visible ash layers suggests continuous explosive volcanic activity on the Kerguelen Plateau in the South Indian Ocean since the early Eocene. The continuous fallout of volcanic ash from explosive volcanism on the Kerguelen Archipelago is the source of the magnetic particles and thus responsible for the magnetostratigraphy of the nannofossil oozes drilled during Leg 120.

Alternating field demagnetization, single‐domain‐like memory, and the Lowrie‐Fuller test of multidomain magnetite grains (0.6–356 μm)

[1]xa0A fundamental question in paleomagnetism is how magnetite grains much larger than single-domain size preserve stable remanence over millions of years. In an effort to answer this question we measured alternating field (AF) demagnetization of thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization (SIRM) before and after low-temperature demagnetization (LTD). LTD (zero-field cycling through 120 K) unpins or nucleates domain walls, reducing the remanence of multidomain grains. We used two sets of sized crushed magnetites (0.6, 1, 3, 6, 9, 14, 20, 110, and 135 μm), one set unannealed and the other annealed, and a set of hydrothermally grown magnetites (0.8, 6.3, 25, 64, 94, and 356 μm). For all sizes, TRM and SIRM memories after LTD are much harder to AF demagnetize than the original remanences. AF demagnetization curves after LTD are flat for the first ∼10 mT. Such initial plateaus are one hallmark of single-domain behavior. In high-stress unannealed grains, after-LTD response depends on grain size, larger grains demagnetizing more easily than smaller ones. In low-stress annealed and hydrothermal magnetites, after-LTD response is almost independent of grain size over nearly 3 decades in grain size. This size-independent behavior could be due to grains in metastable single-domain states in the smaller-sized samples, but in >100 μm grains, there must be a source of single-domain-like AF behavior within the multidomain grains themselves. We propose an ad hoc model in which LTD triggers domain wall unpinning and nucleation events up to a coercivity threshold, producing the observed initial plateaus in AF demagnetization curves of TRM and SIRM memories. The size-independent demagnetization behavior of memory in hydrothermal and annealed magnetites is ascribed to nucleation events above the threshold level, and the additional size-dependent AF demagnetization of memory in high-stress unannealed grains is explained by wall unpinning from strong stress centers. In both cases the AF properties merely mimic single-domain behavior. Although LTD memory has single-domain-like AF curves and erased remanence has multidomain-like curves in grains of all sizes, the Lowrie and Fuller [1971] test still works for the annealed samples. For memory, erased fraction and pre-LTD remanence alike, TRM is more stable than SIRM in fine grains (1 μm), less stable in large grains (135 μm), and of comparable stability in medium-size grains (9 and 20 μm).