Browsing by Author "Moskowitz, Bruce"
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Item Biomineralization of magnetic minerals(Reviews of Geophysics (American Geophysical Union), 1995) Moskowitz, BruceNew 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].Item 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.Item 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.Item 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.Item 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 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 High-temperature magnetostriction of magnetite and titanomagnetites(Journal of Geophysical Research (American Geophysical Union), 1993) Moskowitz, BruceItem 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 IRM Quarterly, Volume 03, Number 2 (Summer 1993). Cover article: Performing Tasks at the IRM Takes Time(University of Minnesota. Institute for Rock Magnetism, 1993) Brachfeld, Stefanie; Foss, Sherry; Hunt, Chris; Kletetschka, Gin; Marvin, Jim; Moskowitz, Bruce; Rubin, Scott; Sahu, Sanghamitra; Solheid, PeatItem IRM Quarterly, Volume 03, Number 3 (Fall 1993). Cover article: Tasks at the IRM Still Take Time(University of Minnesota. Institute for Rock Magnetism, 1993) Brachfeld, Stefanie; Foss, Sherry; Hunt, Chris; Kletetschka, Gin; Marvin, Jim; Moskowitz, Bruce; Rubin, Scott; Sahu, Sanghamitra; Solheid, PeatItem IRM Quarterly, Volume 03, Number 4 (Winter 1993-1994). Cover article: Tasks at the IRM Take Time, Part III(University of Minnesota. Institute for Rock Magnetism, 1994) Brachfeld, Stefanie; Foss, Sherry; Hunt, Chris; Kletetschka, Gin; Marvin, Jim; Moskowitz, Bruce; Rubin, Scott; Sahu, Sanghamitra; Solheid, PeatItem IRM Quarterly, Volume 15, Number 3 (Fall 2005). Cover article: A FORC in the Road?(University of Minnesota. Institute for Rock Magnetism, 2005) Chen, Amy; Egli, Ramon; Moskowitz, BruceItem IRM Quarterly, Volume 15, Number 4 (Winter 2005-2006). Cover article: Fe3O4 Smoked at 80K(University of Minnesota. Institute for Rock Magnetism, 2006) Carter-Stiglitz, Brian; Moskowitz, Bruce; Solheid, PeatItem IRM Quarterly, Volume 16, Number 2 (Summer 2006). Cover article: Santa Fe VII(University of Minnesota. Institute for Rock Magnetism, 2006) Banerjee, Subir; Moskowitz, BruceItem IRM Quarterly, Volume 19, Number 4 (Winter 2009-2010). Cover article: New Low-Temp Probe for 3-Axis Magnetic Measurements(University of Minnesota. Institute for Rock Magnetism, 2010) Bowles, Julie; Solheid, Peat; Jackson, Mike; Feinberg, Joshua; Moskowitz, BruceItem IRM Quarterly, Volume 20, Number 4 (Winter 2011). Cover article: Interpretation of Low-Temperature Data Part III: The Magnetite Verwey Transition (Part A)(University of Minnesota. Institute for Rock Magnetism, 2011) Jackson, Mike; Moskowitz, Bruce; Bowles, JulieItem IRM Quarterly, Volume 22, Number 1 (Spring 2012). Cover article: "Blowin' in the Wind" and "Beneath Still Waters": A Report on the 9th Santa Fe Conference on Rock Magnetism(University of Minnesota. Institute for Rock Magnetism, 2012) Bowles, Julie; Jackson, Mike; Moskowitz, BruceItem IRM Quarterly, Volume 24, Number 4 (Winter 2014-2015). Cover article: A new basis for the SI system of units: occasion to reconsider the presentation and teaching of magnetism(University of Minnesota. Institute for Rock Magnetism, 2014) Stacey, Frank; Moskowitz, Bruce; Jackson, Mike; Dunlop, David; Ӧzdemir, Ӧzden; Banerjee, Subir;Item IRM Quarterly, Volume 25, Number 3 (Fall 2015). Cover article: The IRM at 25: A Quarter Century of Community-Based Research and Education(University of Minnesota. Institute for Rock Magnetism, 2016-03) Moskowitz, Bruce; Banerjee, Subir; Feinberg, Joshua; Jackson, Mike; Solheid, Peat; Bilardello, DarioItem 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.