Faculty and Staff Publications, IRM
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Item Compositional control of anisotropy of remanent and induced magnetization in synthetic samples(Geophysical Research Letters (American Geophysical Union), 1991) Johns, Mary; Jackson, MikeItem The effect of low‐temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites: A case for acquisition of chemical remanent magnetization(Journal of Geophysical Research (American Geophysical Union), 1991) Kelso, Paul; Banerjee, Subir; Worm, HorstItem Magnetostriction in aluminum-substituted titanomagnetites(Geophysical Research Letters (American Geophysical Union), 1992) Özdemir, Özden; Moskowitz, BruceItem Rock magnetism and the interpretation of anisotropy of magnetic susceptibility(Reviews of Geophysics (American Geophysical Union), 1992) Rochette, Pierre; Jackson, Mike; Aubourg, CharlesItem The rock magnetic fingerprint of chemical remagnetization in midcontinental Paleozoic carbonates(Geophysical Research Letters (American Geophysical Union), 1992) Jackson, Mike; Sun, Wei-wei; Craddock, JohnResults of a paleomagnetic and rock magnetic survey of Paleozoic carbonates from 39 sites in the midcontinental U.S. show that many of these sites carry a stable remanence of apparent Permian age. Many of these remagnetized sites also have relatively high anhysteretic susceptibilities, and higher saturation remanence than most of the sites where the late Paleozoic remanence is absent. However the correlation between late Paleozoic remanence and high anhysteretic susceptibility or high saturation remanence is imperfect. The most diagnostic rock magnetic parameter for recognizing remagnetized sites is a ratio of anhysteretic remanence/saturation remanence exceeding 10%. We have found high ratios in almost all remagnetized sites, but in very few sites where the Late Paleozoic remanence is absent. The high ratios reflect the presence of a significant fraction of extremely fine-grained magnetite (a few tens of nanometers), spanning the superparamagnetic-single domain threshhold.Item High-temperature magnetostriction of magnetite and titanomagnetites(Journal of Geophysical Research (American Geophysical Union), 1993) Moskowitz, BruceItem The effect of oxidation on the Verwey transition in magnetite(Geophysical Research Letters (American Geophysical Union), 1993) Özdemir, Özden; Dunlop, David; Moskowitz, BruceAt the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 10^12 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.Item Micromagnetic study of the influence of crystal defects on coercivity in magnetite(Journal of Geophysical Research (American Geophysical Union), 1993) Moskowitz, BruceA one-dimensional micromagnetic model is used to calculate the thermal dependence of microcoercivity (hc) produced by the unpinning of a domain wall (DW) from various types of defects in magnetite. Equilibrium solutions are found that minimize the magnetoelastic, anisotropy, exchange, magnetostatic, and external field energies with respect to the wall width (w) and position of the wall relative to the defect. The defect may be a single dislocation, dislocation dipole, planar defect, or planar defect bounded by two parallel dislocations. Wall pinning is produced by (1) microstress fields of dislocations, (2) local changes in exchange and anisotropy constants within a planar defect region, or (3) a combination of both effects. The calculations, using temperature-dependent parameters, predict the thermal dependence of hc (T) as a function of grain size, domain wall width, defect spacing, and type of defect. Results show that, for grain sizes between 1 and 100 μm, hc(T) is usually a function of the wall width raised to some power n. The particular value of n is found to be a function of the DW-defect interaction spacing (d/w), type of defect, and grain size. Also, within this size range, the wall width expands with temperature more gradually than classical theory predicts. The microcoercivity results are used with the theory of Xu and Merrill (1990) to predict the thermal dependence of the macroscopic coercivity Hc in magnetite. For grains with low defect densities, such as recrystallized magnetites, negative dislocation dipoles with d/w≈0.1–1 produce a thermal dependence of coercivity that agrees with experimental results. In the high defect density limit, a population of positive and negative dislocation dipoles with a distribution of dipole widths produce an Hc (T) dependence consistent with experimental data from crushed and glass ceramic magnetites.Item Rock magnetism of remagnetized Paleozoic carbonates: low-temperature behavior and susceptibility characteristics(Journal of Geophysical Research (American Geophysical Union), 1993) Jackson, Mike; Rochette, Pierre; Fillion, Gerard; Banerjee, Subir; Marvin, JamesItem Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long-wavelength magnetic anomalies(Journal of Geophysical Research (American Geophysical Union), 1993) Kelso, Paul; Banerjee, Subir; Teyssier, ChristianRock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudo-single-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to thermal demagnetization, with nearly 50% of the magnetization remaining after 400°C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanent magnetization along the present-day field is likely to dominate. The magnetic susceptibilities of the samples are only capable of producing a magnetization of less than 1 A/m in the 0.05 mT present-day field of Central Australia. Susceptibility is nearly constant with temperature to within 30°C of the Curie temperature where it decreases rapidly, i.e., there is no significant Hopkinson peak. The granulite samples from this study have magnetizations, both remanent and induced components, that are large enough to account for most long-wavelength magnetic anomalies if they are juxtaposed with relatively nonmagnetic rocks, similar to the high-grade rocks in the Canadian Shield.Item Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties, and magnetic anisotropy(Journal of Geophysical Research (American Geophysical Union), 1993) Jackson, Mike; Borradaile, Graham; Hudleston, Peter; Banerjee, SubirItem Separation of local signals from the regional paleomonsoon record of the Chinese Loess Plateau: A rock-magnetic approach(Geophysical Research Letters (American Geophysical Union), 1993) Banerjee, Subir; Hunt, Christopher; Liu, Xiu-MingWe propose a method based on thermal unblocking of low-temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic [Cullity, 1972] fraction (size, d < 30 nm) of magnetite produced by pedogenesis in the Chinese loess plateau [Liu, 1988]. We applied this method to the proxy climatic records of the last 130 ka from two sites 250 km apart, but separated by the mountain range Liupan-shan. Xifeng to the east (35.7°N, 107.6°E) and Baicaoyuan to the west (36.2°N, 105.0°E) currently have humid and arid microclimates, respectively. As expected, the superparamagnetic fraction increases during known warm temperature intervals at each site. Furthermore, the more humid site clearly has higher overall superparamagnetic fractions during most of the last 130 ka. However, during the period 5 to 10 ka ago, the relative humidity at both sites was the same within experimental errors. Bulk grain size evidence confirms the magnetic data, and we suggest that the present easterly summer monsoon in China came from a more southerly direction during this time to flow parallel to Liupan-shan, resulting in very similar summer humidity at Xifeng and Baicaoyuan.Item Magnetic force microscopy of single crystal magnetite (Fe3O4)(Journal of Applied Physics (American Institute of Physics), 1994) Proksch, Roger; Foss, Sherry; Orme, C.; Sahu, Sanghamitra; Moskowitz, BruceItem Effect of nonstoichiometry on the magnetic and electrical properties of synthetic single crystal Fe2.4Ti0.6O4(Geophyical Research Letters (American Geophysical Union), 1994) Wannamaker, B.J.; Moskowitz, BruceA single crystal of titanomagnetite Fe2.4Ti0.6O4 (TM60) was synthesized using the floating zone technique. The Curie temperature, saturation magnetization at 5 K, and thermopower were measured for several pieces of the crystal as grown and following high temperature annealing at different oxygen fugacities within the stability field of the TM60. The magnetic and electrical data indicate that long-range ordering in TM60 is a function of nonstoichiometry with higher cation vacancy concentrations producing a more random cation distribution. This effect may explain the differences among cation distribution models for TM developed previously by other workers.Item Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri(Journal of Geophysical Research (American Geophysical Union), 1994) Sun, Wei-wei; Jackson, MikeMagnetic carriers in remagnetized early Paleozoic carbonates of the North American midcontinent were studied in extracts and in situ, using rock magnetism and scanning electron microscopy (SEM). SEM observation of extracts and in situ samples shows that the dominant magnetic particles are large irregular grains and framboids (10–20 μm in diameter) and individual spherical and euhedral particles (0.2–1 μm). Comparing the saturation remanence of the bulk rock and extracts indicates that the extracts only account for ∼10% of the saturation remanence of the bulk rock. Most of the remanence carriers were lost during the dissolution and separation procedure, especially the finer-grained carriers. The framboids and the individual submicron euhedral magnetic particles observed by SEM thus make a fairly minor contribution to the magnetic remanence of the remagnetized carbonates. Hysteresis loops of the extracts are very similar to those reported by Suk et al (1993), with saturation remanence to saturation magnetization (Jr/Js) ratios from 0.1 to 0.13 and remanent to bulk coercivity (Hcr/Hc) ratios ∼4. However, the loops for the “nonmagnetic” residue are strongly “wasp-waisted”, and Hcr/Hc ratios are very high, similar to whole rock specimens of remagnetized carbonates (Jackson, 1990). This suggests that bulk rock and “nonmagnetic” residue contain very fine (superparamagnetic (SP) and single domain (SD)) magnetic particles which were not recovered by the magnetic separation procedure. Low-temperature experiments support the conclusion that the principal remanence carriers in the remagnetized carbonates are SD magnetite and that SP magnetite plays an important part in the unusual bulk magnetic properties of these rocks. Owing to their dissolution, the actual natural remanent magnetization bearing particles cannot be observed petrographically.Item Magnetite dissolution in deep sediments and its hydrologic implication: a detailed study of sediments from site 808, leg 131(Journal of Geophysical Research (American Geophysical Union), 1994) Lu, Ran; Banerjee, SubirOur previous study of deep sea sediments from site 808, leg 131 of the Ocean Drilling Program has demonstrated that the two anomalously low-intensity zones of natural remanent magnetization (NRM), 675 to 925 and 1080 to 1243 m below seafloor, were caused by unusually low magnetite content in the sediment sections and that this was not related to the variations in sediment sources, calcite dilution, or magnetite destruction occurring in the top sediment layer during early diagenesis. For an explanation, we now suggest that these low INRM intensity zones were produced by catagenesis at great depth by organic matter decomposition in sediments, which in turn causes magnetite dissolution and hence a lower magnetite content. A similar process also applies to manganese ions; as a consequence, iron and manganese concentration, grain size and content of magnetite, and NRM intensity all decrease, whereas sulfur content increases in these sediments. Our interpretation is also supported by other studies of organic geochemistry and sedimentology of these same sediments. We suggest, therefore, that detailed rock magnetic and geochemical tests should be carried out before geomagnetic field variations are studied using ocean sediments from such great depth. Additionally, we suspect that catagenesis may have been retarded and magnetic degradation was prevented near a décollement due to cold water percolation.Item Interactions between single domain particles(Journal of Applied Physics (American Institute of Physics), 1994) Proksch, Roger; Moskowitz, BruceWe present a variation of the Wohlfarth–Henkel technique for studying interactions in single domain particles (SDPs) in which samples are prepared in different remanent states before the remanent magnetization curves are measured. By analyzing the resulting series of switching field distributions (SFDs), it is possible to separate the effects of positive (magnetizing) and negative (demagnetizing) interactions, even when one type dominates the other. The method is applied to two types of samples consisting of uniform SPDs of magnetite produced by magnetotactic bacteria: (1) whole bacterial cells containing single, linear chains of SDPs; and (2) SDPs extracted from the cells and allowed to aggregate into clumps.Item Contributions of fine‐particle magnetism to reading the global paleoclimate record (invited)(Journal of Applied Physics (American Institute of Physics), 1994) Banerjee, SubirPaleoclimate changes are recorded by proxy as variations in concentration, composition, and grain size of magnetic minerals, principally magnetite (Fe3O4), in the sediments deposited in lakes,oceans, and continental eolian deposits. Cross‐validated multiple‐parameter magnetic measurements of such sediment cores provide global change data of high temporal resolution, useful for constructing a base‐line record against which anthropogenic modifications may be discerned. Theories of superparamagnetism and magnetic domains are used to explain the physical basis of magnetic proxy recording. Examples of applications to validation of Milankovitch theory of climate change and delineation of the glacial and interglacial stages of the last 1 000 000 years are provided.Item Magnetic fine structure of domain walls in iron films observed with a magnetic force microscope(Journal of Applied Physics (American Institute of Physics), 1994) Proksch, Roger; Foss, Sherry; Dahlberg, E.D.; Prinz, G.The submicron magnetic structure of domain walls in a single‐crystal ironfilm has been studied using a magnetic force microscope(MFM). The MFM tip was sensitized to the component of the field perpendicular to the film plane. The sample examined was a 500‐nm‐thick single‐crystal film of iron,grown by molecular‐beam epitaxy(MBE). Before it was imaged, the film was magnetized along its (in‐plane) easy axis in a 2000‐Oe field. Studies of the domain structure at numerous locations on the filmsurface revealed a rich variety of micromagnetic phenomena. Parallel domain walls, determined to be Bloch walls with a width of 70–100 nm, were seen along the easy axis, spaced roughly 30 μm apart. These appeared to be Bloch walls. Bloch lines were also observed in the walls with an average periodicity of 1.5 μm. This is a value smaller than that predicted for Bloch wall‐line structures. In addition, a pronounced zig–zag structure was observed, as expected from previous Fe whisker observations.Item Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China(Geophysical Journal International (Oxford University Press), 1995) Hunt, Christopher; Banerjee, Subir; Han, J.=M.; Solheid, Peter; Oches, E.A.; Sun, Wei-wei; Liu, T.-S.Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining—one of the few palaeoclimate sites in the currently arid western Loess Plateau of China—illustrate the importance of making many types of rock-magnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction. The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. The resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. The data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites. In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.