The Experts below are selected from a list of 186 Experts worldwide ranked by ideXlab platform
S J Doh - One of the best experts on this subject based on the ideXlab platform.
-
rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology
Journal of Geophysical Research, 1992Co-Authors: Jan Bloemendal, John W King, F R Hall, S J DohAbstract:We use bivariate scatter plots to illustrate variations in selected rock magnetic properties (low-Field Susceptibility, anhysteretic and isothermal remanence) of late Neogene and Pleistocene deep-sea sediments from 16 sites in the Arctic Ocean, North Atlantic, equatorial Atlantic and North Pacific Ocean, and the Arabian Sea. Our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology, geographical area, and the dominant mode of terrigenous sedimentation, which at these sites is via ice-rafting, via bottom currents, or via eolian processes. Overall, correlations between sediment magnetic properties and gross lithology is poor, and factors such as the source and transport path of terrigenous sediment (and detrital magnetic minerals), together with the action of reductive diagenetic processes, are the major controls on the magnetic properties. On the bivariate scatter plots, sites with major ice-rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations, relatively coarse ferrimagnetic grain-sizes, and scattered sample point distributions; in contrast, sites where we infer significant bottom-current supply of terrigenous material have tightly grouped sample point distributions. Carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration, caused by glacial-interglacial fluctuations in carbonate accumulation and eolian activity. Sediments containing significant volcanogenic material have high concentrations of relatively coarse-grained ferrimagnetic material. Reductive diagenesis is a significant determinant of sediment magnetic properties in high-productivity areas and has the effect of preferentially removing the fine-grained ferrimagnetic fraction, causing a coarsening of the ferrimagnetic grain-size distribution and a rise in the antiferromagnetic:ferrimagnetic ratio.
P C M Van Zijl - One of the best experts on this subject based on the ideXlab platform.
-
lesion heterogeneity on high Field Susceptibility mri is associated with multiple sclerosis severity
American Journal of Neuroradiology, 2016Co-Authors: Daniel M Harrison, H Liu, C Jones, B Caffo, Peter A Calabresi, P C M Van ZijlAbstract:BACKGROUND AND PURPOSE: Susceptibility MR imaging contrast variations reflect alterations in brain iron and myelin content, making this imaging tool relevant to studies of multiple sclerosis lesion heterogeneity. In this study, we aimed to characterize the relationship of high-Field, Susceptibility contrasts in multiple sclerosis lesions to clinical outcomes. MATERIALS AND METHODS: Twenty-four subjects with multiple sclerosis underwent 7T MR imaging of the brain, disability examinations, and a fatigue inventory. The inverse of T2* relaxation time (R2*), frequency, and relative Susceptibility (from quantitative Susceptibility mapping) were analyzed in 306 white matter lesions. RESULTS: Most lesions were hypointense on R2* (88% without a rim, 5% with). Lesions that were hyperintense on quantitative Susceptibility mapping were more frequent in relapsing-remitting than in progressive multiple sclerosis (54% versus 35%, P = .018). Hyperintense lesion rims on quantitative Susceptibility maps were more common in progressive multiple sclerosis and patients with higher levels of disability and fatigue. Mean lesion R2* was inversely related to disability and fatigue and significantly reduced in progressive multiple sclerosis. Relative Susceptibility was lower in lesions in progressive multiple sclerosis (median, −0.018 ppm; range, −0.070 to 0.022) than in relapsing-remitting MS (median, −0.010 ppm; range, −0.062 to 0.052; P = .003). CONCLUSIONS: A progressive clinical phenotype and greater disability and fatigue were associated with lower R2* and relative Susceptibility values (suggestive of low iron due to oligodendrocyte loss) and rimmed lesions (suggestive of chronic inflammation) in this multiple sclerosis cohort. Lesion heterogeneity on Susceptibility MR imaging may help explain disability in multiple sclerosis and provide a window into the processes of demyelination, oligodendrocyte loss, and chronic lesion inflammation.
Andreas U Gehring - One of the best experts on this subject based on the ideXlab platform.
-
ferromagnetic resonance and magnetic characteristics of intact magnetosome chains in magnetospirillum gryphiswaldense
Earth and Planetary Science Letters, 2008Co-Authors: H Fischer, Peter G Weidler, Giovanni Mastrogiacomo, Jorg F Loffler, Rolf Warthmann, Andreas U GehringAbstract:Abstract The magnetic characteristics of intact magnetosome chains in Magnetospirillum gryphiswaldense bacteria were investigated by means of static and dynamic magnetic analyses and ferromagnetic resonance spectroscopy. The nano-sized magnetosomes are generally in a stable single-domain state, but magnetosomes smaller than 30 nm characteristic of superparamagnetic magnetite particles were also found. Alternating current (AC) Susceptibility indicates that all magnetosomes are blocked below 150 K. At room temperature the anisotropy of M. gryphiswaldense is dominated by the shape of the magnetosome chains. Low-temperature ferromagnetic resonance (FMR) spectroscopy indicates that this dominant shape anisotropy can affect the detection of the Verwey transition at 100 K. The static and dynamic magnetic analyses show that the Verwey transition is smeared and that our magnetotactic bacteria fail the Moskowitz test. This failure is explained by the biomineralization of non-stoichiometric magnetosomes. This interpretation is based on the increase in high-Field Susceptibility and the distinct peak in the out-of-phase component of the AC Susceptibility below 50 K. These results are attributed to freezing of spins associated with defect structures in the core and at the surface of nano-sized magnetosomes. The results obtained from M. gryphiswaldense demonstrate that intrinsic properties of nano-sized magnetosomes are significantly influenced by non-stoichiometry and by the anisotropy excited from their arrangement in the bacteria.
-
low temperature magnetic properties of lepidocrocite
Journal of Geophysical Research, 2002Co-Authors: Ann M. Hirt, J Dobson, Peter G Weidler, Luca Lanci, Andreas U GehringAbstract:[1] The magnetic properties of synthetic lepidocrocite (γ-FeOOH) were investigated between 5 and 300 K. The purity of the samples was verified by X-ray diffraction and Mossbauer spectra at room temperature. The lepidocrocite is paramagnetic at room temperature with a low Field Susceptibility of 57.8 × 10−8 m3 kg−1. It undergoes magnetic ordering at low temperature and a peak in in-phase Susceptibility occurs around 52 K, where it is antiferromagnetic. Thermal demagnetization of low-temperature remanence after cooling in zero Field and in a 2.5-T Field display a large loss in remanence from 5 to 30 K. Hysteresis loops are open and symmetric about the origin below 50 K; the remanent magnetization probably arises from a defect magnetic moment. The Field-cooled hysteresis loop at 5 K is shifted along the Field axis, which implies an exchange bias between antiferromagnetic and ferrimagnetic alignment in the lepidocrocite.
R Reyessanchez - One of the best experts on this subject based on the ideXlab platform.
-
calculation of the zero Field Susceptibility and critical temperature of ising magnets on the basis of the principle of limit similarity
Physical Review B, 1993Co-Authors: V B Magalinsky, A L Popov, R ReyessanchezAbstract:A simple analytical approximate method for the calculation of the zero-Field Susceptibility and the critical temperature for the Ising model is proposed. The analysis is based on the rule of limit correspondence. Assuming that the first three terms of the high-temperature series expansion are known, accurate values for the eight types of lattices are obtained. These values (excepting the honeycomb lattice) differ from those obtained through series extrapolation by 0.8% rms, and generally compare favorably to estimates computed through the heuristic Bishop formula. In connection with an associated percolation problem, a more accurate formula for the critical bond percolation probability is proposed
P G Appleby - One of the best experts on this subject based on the ideXlab platform.
-
sediment source variations and lead 210 inventories in recent potomac estuary sediment cores
Journal of Quaternary Science, 2010Co-Authors: Frank Oldfield, Barbara A Maher, P G ApplebyAbstract:We present the results of magnetic measurements (low Field Susceptibility, anhysteretic and isothermal remanences) on samples of sediment from 11 cores from the Potomac River between Washington DC and its mouth. The magnetic properties indicate that at sites in the upstream reaches especially, there has been a shift to surface-soil-derived sediments in recent times. No such shift is indicated at the mouth of the Estuary. Comparison with pollen, 210Pb and trace metal analyses, where available, suggests that the shift to soil-derived material dates from the early to mid-19th century and coincides with a major land use change previously documented by Brush et al. (1982). Inventories of unsupported (excess) 210Pb activity in the cores increase upstream, are positively correlated with total sedimentation rates, and, in the upper reaches where the magnetic properties indicate soil inwash, greatly exceed values expected from direct atmospheric deposition alone. We infer that in these cores, most of the unsupported 210Pb is deposited from the surfaces of the catchment as a result of soil erosion.