Browsing by Subject "rock magnetism, mineral magnetism"
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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 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 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.Item Magnetostriction in aluminum-substituted titanomagnetites(Geophysical Research Letters (American Geophysical Union), 1992) Özdemir, Özden; Moskowitz, BruceItem 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 The superparamagnetism of Yucca Mountain Tuff.(Journal of Geophysical Research (American Geophysical Union), 1999) Worm, Horst; Jackson, MikeYucca 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.Item Thermal dependence of magnetocrystalline anisotropy and magnetostriction constants of single crystal Fe2.4Ti0.61O4(Geophysical Research Letters (American Geophysical Union), 1995) Sahu, Sanghamitra; Moskowitz, BruceThe 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.