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    Millennial-scale climatic change during the last interglacial period: Superparamagnetic sediment proxy from paleosol S1, western Chinese Loess Plateau.
    (Geophysical Research Letters (American Geophysical Union), 1999) Fang, Xiaomin; Li, J.-J.; Banerjee, Subir; Jackson, Mike; Oches, E.A.; Van der Voo, Rob
    Detailed magnetic analyses of samples from the Jiuzhoutai well section on the western Chinese Loess Plateau reveal high-resolution signals of summer monsoon change. The last interglacial period is represented here by the 8-meter thick S1 paleosol complex. We have used low-temperature remanence studies to analyze variations in the concentration of Superparamagnetic (SP) grains, which previous studies have shown to be largely of pedogenic origin. The SP concentration, interpreted as a proxy for the extent of pedogenesis, shows millennial scale variations within S1. We conclude that the last interglacial period in Asia was characterized by rapid climate fluctuations, with at least one brief return to near-glacial conditions in the middle of oxygen-isotope substage 5e.
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    The superparamagnetism of Yucca Mountain Tuff.
    (Journal of Geophysical Research (American Geophysical Union), 1999) Worm, Horst; Jackson, Mike
    Yucca Mountain Tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range [Schlinger et al., 1988]. Magnetic measurements on three samples (comprising hysteresis loops at low and ambient temperatures, acquisition and demagnetization of isothermal remanent magnetization, thermal demagnetization of the saturation remanence and of a low-temperature thermoremanence, and frequency- and temperature-dependent susceptibilities) allow evaluation of the magnetic properties in terms of Néel's [1949] single-domain theory. Precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results. In contrast, an attempt to derive mean particle volumes by fitting a Langevin curve to the room temperature magnetization curves fails, probably because the precondition for the Langevin function, KV/kT≪1, is not met. It is only for the sample with the smallest grains and in weak fields (<20 mT) that a Langevin fit provides a reasonable volume estimate. There is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility, thus verifying that Néel's theory is sufficient for the magnetic description of single-domain assemblages spanning the superparamagnetic/stable single-domain boundary. However, some deviations between modeled and measured susceptibilities exist, and the physical causes may include size-dependent anisotropy, nonuniform magnetizations, and also an uncertain preexponential time “constant” τ0. While τ0 = 10−11 s gives the best fit for the sample with the largest grains, τ0 = 10−9 s is more reasonable for the others. Thus τ0 may indeed be size- and temperature-dependent as predicted by Brown [1959]. The commonly cited parameter χfd (frequency dependence of susceptibility) reaches 30% at room temperature (RT) for one sample with a blocking temperatures just below RT, while χfd = 0 at RT for a superparamagnetic sample with smaller grains. These results thus exemplify that χfd is not limited to 15%, as a number of studies suggest, and that χfd = 0 must not be taken to imply the absence of superparamagnetic grains.
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    A new high-resolution geomagnetic paleointensity record for the North American Holocene: A comparison of sedimentary and absolute intensity data
    (Journal of Geophysical Research (American Geophysical Union), 2000) Brachfeld, Stefanie; Banerjee, Subir
    A new high-resolution paleointensity record for North America has been constructed using Holocene sediments from Lake Pepin, Minnesota. Lake Pepin sediments yield the same Holocene paleosecular variation curve as nearby Lake St. Croix and satisfy all of the criteria recommended for paleointensity studies. Absolute paleointensity data for North America recorded by Holocene volcanic and archeomagnetic samples provide an independent record of geomagnetic field paleointensity against which the relative intensity records from Lake Pepin and Lake St. Croix can be compared. Since the absolute field paleointensity is known a priori, the effects of the magnetic recording assemblage can be isolated. Anhysteretic remanent magnetization (ARM) is the best choice among normalization parameters for the Lake Pepin sediments as the natural remanent magnetization (NRM) normalized by ARM shows no coherence with magnetic grain size proxies and yields a record of relative paleointensity peaks and troughs whose amplitudes are very similar to those in the archeomagnetic (ARCMAG) and Lake St. Croix data sets. Features with a wavelength of 1000 years are correlative between the three paleointensity records. NRM normalized by saturation isothermal remanent magnetization (SIRM) and by susceptibility (χ) shows grain-size dependences that cause errors in the amplitudes of paleointensity features. NRM/SIRM and NRM/χ are not coherent with their normalizers but are both strongly coherent with independent grain size proxies such as the median destructive field of the NRM and HCR. We successfully removed the grain size dependences from NRM/SIRM by applying a correction function based on the linear relationship between normalized intensity and the median destructive field of the NRM.
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    Domain walls in single-crystal magnetite investigated by magnetic force microscopy
    (Journal of Geophysical Research (American Geophysical Union), 1999) Foss, Sheryl; Moskowitz, Bruce; Proksch, Roger; Dahlberg, E.D.
    Domain walls in bulk single-crystal magnetite were studied using a variable magnetic field magnetic force microscope (MFM). Classical configurations of 180°, 109°, and 71° walls were observed on (110) surfaces. Magnetostatic effects on these different walls were compared. Profiles of the MFM response above the walls were measured with the MFM tip magnetized in different directions. The contribution to the profiles from the z component of the sample field was distinguished from the in-plane components. An asymmetry of the z component of the response profiles for all wall types was observed, consistent with the existence of Néel caps which terminate the interior Bloch walls near the surface. The wall profiles of the non-180° walls were more asymmetric than that of the 180° walls. The 180° walls were observed to be subdivided into alternating polarity segments of average length 15 μm. These walls formed a characteristic zig-zag structure in which the Bloch lines separating segments were located at the corners of the zig-zag. Only unusually long 109° walls were observed to contain a single Bloch line, and the 71° walls, although the longest, were never observed to be subdivided. An applied field perpendicular to the sample plane moved the Bloch lines within the walls without translating the walls themselves. Multipolar walls were converted to unipolar in perpendicular applied fields from O to 100 mT. Profiles of opposite polarity segments of a subdivided wall indicated that the Néel cap formation does not alternate sides of the wall from segment to segment. Alignment of opposite polarity segments of parallel subdivided walls provided an example of long range magnetostatic interactions between walls and possibly their Néel caps.
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    Magnetic domains and domain walls in pseudo-single-domain magnetite studied with magnetic force microscopy
    (Journal of Geophysical Research (American Geophysical Union), 1997) Pokhil, Taras; Moskowitz, Bruce
    Magnetic domain and domain wall structures in pseudo-single-domain grains (5–20 μm) of magnetite (Fe3O4) were studied using magnetic force microscopy. Many of the observed micromagnetic features can be explained by the magnetostatic effects of surfaces and grain edges and interactions within and between walls. Domain walls were frequently subdivided into 1–3 opposite polarity segments separated by Bloch lines, although some walls contained no Bloch lines. Subdivided walls display a characteristic zigzag structure along the easy axis direction, where zigzag angles can be as high as 20°–40°. The zigzagging structure, in addition to wall segmentation, further minimizes the magnetostatic energy of the walls. Bloch lines can be (de)nucleated during wall displacement or after repeated alternating field (AF) demagnetization. Within individual walls, the number of Bloch lines and their pinning locations were found to vary after repeated AF demagnetization demonstrating that walls, like individual grains, can exist in several different local energy minima. The number of Bloch lines appears to be independent of domain state, but frequently the polarity of the wall was coupled with the direction of magnetization in the adjoining domains, such that wall polarity alternates in sign between adjacent walls across an entire grain. Even after the domain magnetization is reversed, the same sense of wall chirality is maintained across the grain producing unique grain chiralities. For one particular grain it was possible to reconfigure a likely three-dimensional (3-D) domain structure. The body and surface structures result primarily from a combined volume magnetostatic interaction between all grain surfaces and magnetocrystalline anisotropy. Finally, commonly observed open-flux features within the interior of grains or along grain edges terminating planar domains are inconsistent with the prediction of edge closure domain formation based on recent 2-D micromagnetic models. Our observations suggest that 3-D micromagnetic models are required to model results even for grains larger than 1 μm.
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    Determination of the pre-exponential frequency factor for superparamagnetic maghemite particles in magnetoferritin
    (Journal of Geophysical Research (American Geophysical Union), 1997) Moskowitz, Bruce; Frankel, R.B.; Walton, S.A.; Dickson, D.; Wong, K.K.W.; Douglas, T.; Hann, S.
    Magnetization and Mössbauer measurements on maghemite particles with an average particle diameter of 10 nm have been made in the temperature range from 5 K to 353 K spanning the superparamagnetic (SPM) and stable single domain (SD) regimes. The maghemite particles were produced within the iron-storage protein ferritin, resulting in a narrowly-sized, weakly interacting nanocomposite material called magnetoferritin. Experiments combining hysteresis measurements, low temperature remanence, and Mössbauer spectroscopy were used to characterize magnetoferritin and to provide experimental estimates of (1) the pre-exponential frequency factor ƒ0 in the Néel-Arhennius relaxation equation; (2) the SPM threshold size at room temperature for maghemite; and (3) the SD value of Hr/Hc at 0 K. The frequency factor was determined from the difference in blocking temperatures measured by dc magnetization and Mössbauer spectroscopy, yielding a value of f0≈109 Hz. This agrees well with the standard value and justifies the usually assumed superparamagnetic blocking condition of KV = 25 kT for remanence measurements. The SPM threshold size at room temperature for remanence measurements was estimated to be 20–27 nm and the extrapolated SD value at 0 K for Hr/Hc is 1.32. The latter value is slightly larger than the theoretical value of 1.09 but may be more appropriate for weakly interacting SD particles commonly found in sediments and soils. However, ƒ0 for ferrimagnetic magnetoferritin is a factor of 103 lower than was determined previously for native ferritin, which contains antiferromagnetic ferrihydrite cores. The difference in ƒ0 values between the two varieties of ferritin is probably related to the two different types of magnetic spin ordering of the core minerals and suggests that the higher value of ƒ0 is more appropriate for antiferromagnetic minerals like hematite and goethite, whereas the lower value is more appropriate for ferrimagnetic minerals like maghemite, magnetite, or greigite.
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    Domain wall displacements in amorphous films and multilayers studied with magnetic force microscope
    (Journal of Applied Physics (American Institute of Physics), 1997) Pokhil, Taras
    The magnetic force microscope(MFM) was used to study the displacement of domain walls (DW) in amorphous TbFe alloy films and Co/Pd multilayerfilms with high spatial resolution. The reversible bending of domain wall segments pinned to defects and irreversible, jumplike displacement of domain wall segments were imaged with the MFM in an applied magnetic field. The maximum reversible displacement of domain walls was 50–100 nm and the length of the segments which reversibly curved in the field was about 150 nm. Measurement of the change in radius of curvature of a DW segment in response to an applied field allowed estimation of the DW energy density and self-demagnetizing field of the film acting on the DW. The DW energy density for the TbFe films was about 1 erg/cm2. It was shown that the self-demagnetizing field acting on a domain wall depends on the domain structure surrounding the studied DW segment. For instance, for a film with saturation magnetization 100 G and thickness 80 nm, which exhibited a mazelike domain structure, the demagnetizing field varied from 100 G in the center of a mazelike domain to 400 G near the edge of a domain. The irreversible displacement of a DW was not a continuous process. The 200–400 nm long DW segments exhibited jumplike motion over distances of 100–150 nm.
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    A combined magnetooptic/magnetic force microscope study of Co/Pd multilayer films
    (Journal of Applied Physics (American Institute of Physics), 1997) Pokhil, Taras; Proksch, Roger
    We have combined a magnetic force/atomic force microscope (MFM/AFM) with a magneto-optic (MO) microscope. This instrument combines the high spatial resolution of the MFM/AFM and its capability to correlate magnetic structure with the structure of the sample surface with the real-time imaging capabilities and large field of view of the MO microscope. Our MO/MFM setup is based on the Nanoscope III Multimode™ MFM/AFM (Digital Instruments, Santa Barbara, CA). Currently, the spatial resolution of the MO microscope is about 3 μm and polarization sensitivity is on the order of 0.5°. Using this instrument, we observed domain structures in Co/Pd multilayerfilms. We found that in a film with 20 Co/Pd layer pairs and 16 nm total thickness, nucleation of domains during sample remagnetizations occurs repeatedly in the same points, and that displacement of domain walls is unidirectional. The high topographic resolution of the AFM allowed us to show that domains nucleate at small defects on the sample surface. The depth of the defects is 1–2 nm, they are 20–30 nm wide and up to 500 nm long. The unidirectional displacement of the domain walls was found to correlate with the anisotropic structure of the sample surface.
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    Measurement of the effects of the localized field of a magnetic force microscope tip on a 180° domain wall.
    (Journal of Applied Physics (American Institute of Physics), 1997) Foss, Sheryl; Dahlberg, E.D.; Proksch, Roger; Moskowitz, Bruce
    Opposite polarity magnetic force microscope(MFM) profiles of domain walls (DWs) in magnetite were measured with a commercial MFM tip magnetized in opposite directions perpendicular to the sample surface. The influence of the tip field on a DW resulted in an overall more attractive interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip. The dependence of the measured alteration on tip-sample separation was fit with a power law at different positions across the DW. The rate of decay of the alteration with tip-sample separation, quantified by the exponent of the power law fit, varied across the DW and was much slower than expected from a simple model.
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    Localized micromagnetic perturbation of domain walls in magnetite using a magnetic force microscope
    (Applied Physics Letters (American Institute of Physics), 1996) Foss, Sheryl; Proksch, Roger; Dahlberg, E.D.; Moskowitz, Bruce; Walsh, B.
    Magnetic force microscope(MFM) profiles of domain walls (DWs) in magnetite were measured using commercially available MFM tips. Opposite polarity profiles of a single DW segment were obtained by magnetizing the MFM tip in opposite directions perpendicular to the sample surface. During a measurement, the field of the tip locally magnetized the DW, resulting in a more attractive tip‐sample interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip.
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    Domain structures in single-crystal magnetite below the Verwey transition as observed with a low-temperature magnetic force microscope
    (Geophysical Research Letters (American Geophysical Union), 1996) Moloni, Katerina; Moskowitz, Bruce; Dahlberg, E.D.
    The magnetic domain structures on the {110} plane of magnetite (Fe3O4) below the Verwey transition (Tv=120K) were studied using a Low-Temperature Magnetic Force Microscope (LTMFM). At 298K, domain structures consisted of arrays of 180°, 109° and 71° walls, typical for magnetite with cubic anisotropy. At 77K (below Tv), the cubic style patterns disappeared and transformed into uniaxial patterns consistent with the uniaxial magnetocrystalline symmetry of the low-temperature monoclinic phase of magnetite. We also observed two distinct styles of domain patterns below Tv: (1) wide domains separated by straight 180° walls along the in-plane [100] easy axis; and (2) intricate wavy walls with reverse spike domains characteristic of out-of-plane easy axes. This intimate mixture of domain styles within adjacent areas of the crystal reflects variations in the direction of the magnetic easy axes in different regions produced by c-axis twinning of the crystal below Tv The thermal dependence of planar and wavy-wall patterns show little change from 77K until 110K, where patterns disappear. Upon cooling back to 77K, domain structures are different from the initial 77K states, indicating that renucleation of different domain states occurs by cycling near Tv.
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    Soil magnetism, an approach for examining archeological landscapes
    (Geophysical Research Letters (American Geophysical Union), 1996) Dalan, Rinita
    To investigate archaeological landscapes and the extent to which humans have influenced them, we have developed an innovative approach that employs complementary geophysical methods drawn from the fields of exploration geophysics and soil magnetism. Integrating soil magnetic techniques with exploration surveys and more standard archaeological procedures (i.e., topographic mapping, excavation, coring, and soil-chemical studies) at the Cahokia Mounds site has helped us to better understand the formation of this premier North American mound center and has also demonstrated a scientific methodology of great potential for subsurface investigations of archaeological terrains.
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    Low-temperature magnetic properties of siderite and magnetite in marine sediments
    (Geophysical Research Letters (American Geophysical Union), 1996) Housen, Bernard; Banerjee, Subir; Moskowitz, Bruce
    Low temperature magnetic techniques provide useful tools to detect the presence of magnetite and pyrrhotite in sediments through identification of their low temperature transitions, to determine the amount of ultrafine-grained (superparamagnetic) material in sediments, and can potentially detect the presence of certain types of magnetotactic bacteria. Application of these types of experiments to nannofossil chalks from beneath the Barbados accretionary prism led to some unusual results, which are attributed to the presence of siderite. Thermal demagnetization of low-temperature remanence after cooling in zero field and in a 2.5 T field both displayed large remanence losses from 20 K to 40 K. Below 40 K, the magnetization of the chalks was much higher in the field-cooled experiments than in the zero-field-cooled experiments. Low temperature hysteresis experiments, made after cooling in a 2.5 T field, displayed offsets in magnetization parallel to the direction of the initial applied field, when measured below 40 K. The offset loops can be due to either an exchange anisotropy between siderite and magnetite phases in the sediments, a defect moment in the siderites, or a canted moment in the siderites. Apparent similarity between the low-temperature thermal demagnetization results from these siderite-bearing sediments, pure siderite, and pure rhodochrosite samples and the well-known 34 K transition in pyrrhotite should lead to caution in identification of pyrrhotite in marine sediments based on low-temperature remanence studies alone.
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    Sediment reveals Early Holocene climate change in China
    (EOS (American Geophysical Union), 1996) Banerjee, Subir
    A major question regarding paleorecords is whether the easy-to-acquire oceanic record can be validated by continental proxy records that are available only from the Arctic and Antarctic ice cores. The ice core records may not provide a complete record of the changes that have occurred in the temperate and tropical regions of the continents. Thick (100–300 m) deposits of wind-borne dust in China are providing new insight into paleoclimate. These deposits vie with the ice cores in providing a record of continuous accumulation over the last 2.6 m.y., but they are poorer in temporal resolution than the ice cores.
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    Magnetic force microscope study of domain wall structures in magnetite
    (Journal of Applied Physics (American Institute of Physics), 1996) Pokhil, Taras; Moskowitz, Bruce
    Domain walls (DW) in a small multidomain grain (≊20 μm) of magnetite (Fe3O4) exhibiting a planar domain pattern were studied using a magnetic force microscope(MFM). Most walls were subdivided with one or two Bloch lines and all walls displayed asymmetric MFM responses. Domain walls were observed to have small offsets either at the location of Bloch lines or at other locations without Bloch lines. The experimental data were described by a model in which (1) the easy axis of magnetization is not exactly parallel to the grain surface but is slightly inclined, and (2) there is also some plane dividing the grain in two parts with slightly different inclined easy axis directions. The inclined easy axis produces asymmetric spin distributions across the DW and wall offsets occur to reduce the surface magnetostatic energy of the wall.
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    Thermal dependence of magnetocrystalline anisotropy and magnetostriction constants of single crystal Fe2.4Ti0.61O4
    (Geophysical Research Letters (American Geophysical Union), 1995) Sahu, Sanghamitra; Moskowitz, Bruce
    The temperature dependence of the magnetocrystalline anisotropy (K′) and magnetostriction (λ) constants of single crystal Fe2.4Ti0.61O4 (TM61) is measured using torque magnetometry and strain gage techniques in the temperature range between 300 K and Tc. At all temperatures, K′ is positive making the <100> instead of <111> crystal directions the easy axes of magnetization in TM61. The thermal dependence of K′(T) varies as (1-T/Tc)2.2. In terms of the saturation magnetization, K′(T)∝Ms(T)n where n≈6, a much weaker dependence than that found for magnetite (n=8–10). We also observed that K′∝λ1–2, consistent with K′∝Ms6. These results show that the magnetoelastic contribution to the magnetocrystalline anisotropy dominates the anisotropy constant of TM61 at elevated temperature and that the effects of stress cannot be neglected in theories of thermoremanence and micromagnetic models for intermediate titanomagnetites commonly found in oceanic basalts.
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    Magnetic stratigraphy of Chinese loess as a record of natural fires
    (Geophysical Research letters (American Geophysical Union), 1995) Kletetschka, Gunther; Banerjee, Subir
    Magnetic susceptibility records of paleosols and loess show high correlation with oxygen-isotope stratigraphy of ocean sediments [Kukla, 1987], providing a global paleoclimatic record. Different models have been put forth to explain the nature and cause of susceptibility variations, but consensus has not yet been achieved. Our low-temperature studies reveal a secondary magnetite component in paleosols that is characterized by a higher Verwey transition (115K) than that for the magnetite (100 K) in unaltered loess. The same shift in the Verwey transition can be achieved by heating and cooling loess samples. This is consistent with a new hypothesis that the magnetic signal from paleosols may be produced by natural fires in the past. Natural fire intensity is sensitive to the amount of annual precipitation, so that increased fire-induced susceptibilities should reflect an increase in the humidity of regional climate.
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    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.
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    Biomineralization of magnetic minerals
    (Reviews of Geophysics (American Geophysical Union), 1995) Moskowitz, Bruce
    New developments and discoveries in biomineralization have occurred almost continuously in the intervening decade since the previous IUGG quadrennial report on biomineralization and biomagnetism was published [Kirschvink, 1983]. Biomineralization is widespread in the biosphere and over 60 different inorganic minerals are produced by a variety of organisms from bacteria to humans [Lowenstam and Weiner, 1989]. The literature on biomineralization is interdisplinary, combining research in microbiology, biotechnology, physics, geology, and paleomagnetism. For paleomagnetism and rock magnetism, iron biomineralization of magnetic minerals is of prime importance. From a paleomagnetism perspective, biogenic magnetic minerals can be deposited in sediments and acquire a natural remanent magnetization that preserves a record of the ancient geomagnetic field. From a rock magnetism perspective, biogenic magnetic minerals provide novel sources of magnetic material for experimental studies in fine particle magnetism. Both perspectives are interrelated through a common goal of developing magnetic techniques to detect biogenic magnetic minerals in sediments and soils. For example, the extent to which iron biominerals contribute to the fine-grained magnetic mineral assemblages in freshwater and marine sediments is important for identifying and interpreting the magnetic record of environmental change [Oldfield, 1992; Reynolds and King, this issue].
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    Magnetic force microscopy of the submicron magnetic assembly in a magnetotactic bacterium
    (Applied Physics Letters (American Institute of Physics), 1995) Proksch, Roger; Schäffer, T.E; Moskowitz, Bruce; Dahlberg, E.D.; Bazylinski, Dennis; Frankel, R.B.
    A magnetic force microscope(MFM) was used to image topography and magnetic forces from a chain of submicron single magnetic domain particles produced by and contained in isolated magnetotactic bacteria. The noncontact magnetic force microscope data were used to determine a value for the magnetic moment of an individual bacterial cell, of order 10−13 emu, consistent with the average magnetic moment of bacteria from the same sample, obtained by superconducting quantum interference device magnetometry. The results represent the most sensitive quantification of a magnetic force microscope image to date.