Faculty and Staff Publications, IRM
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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 A combined magnetooptic/magnetic force microscope study of Co/Pd multilayer films(Journal of Applied Physics (American Institute of Physics), 1997) Pokhil, Taras; Proksch, RogerWe 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.Item Compositional control of anisotropy of remanent and induced magnetization in synthetic samples(Geophysical Research Letters (American Geophysical Union), 1991) Johns, Mary; Jackson, MikeItem 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 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 wall displacements in amorphous films and multilayers studied with magnetic force microscope(Journal of Applied Physics (American Institute of Physics), 1997) Pokhil, TarasThe 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.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 hydrostatic pressure on viscous remanent magnetization in magnetite-bearing specimens(Geophyical Research Letters (American Geophysical Union), 1995) Kelso, Paul; Banerjee, SubirRocks at depth in the crust acquire a viscous (i.e., time-dependent) magnetization under the pressure-temperature conditions at which they reside. There have been numerous studies on the effect of temperature on viscous magnetization but little work has been performed on the effect of hydrostatic pressure. We have studied viscous remanent magnetization at 22°C in a 0.1 mT field at 0.1 and 100 MPa for natural and synthetic multidomain magnetite. The viscous remanent magnetization was found to increase nearly linearly with log(time) at both pressures. There was little change in the rate of viscous acquisition between 0.1 and 100 MPa over the multidomain grain size range studied. Thus for rocks buried at depth in the earth the enhancement of magnetic viscosity by thermal fluctuations will dominate over effects due to hydrostatic pressure.Item 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 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 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 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 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.Item Low-temperature magnetic properties of siderite and magnetite in marine sediments(Geophysical Research Letters (American Geophysical Union), 1996) Housen, Bernard; Banerjee, Subir; Moskowitz, BruceLow 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.Item 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, BruceMagnetic 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.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 Magnetic force microscope study of domain wall structures in magnetite(Journal of Applied Physics (American Institute of Physics), 1996) Pokhil, Taras; Moskowitz, BruceDomain 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.