42 resultados para Orbital magnetism


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The field of molecule-based magnets is a relatively new branch of chemistry, which involves the design and study of molecular compounds that exhibit a spontaneous magnetic ordering below a critical temperature, Tc. One major goal involves the design of materials with tuneable Tc's for specific applications in memory storage devices. Molecule-based magnets with high magnetic ordering temperatures have recently been obtained from bimetallic and mixed-valence transition metal μ-cyanide complexes of the Prussian blue family. Since the μ-cyanide linkages permit an interaction between paramagnetic metal ions, cyanometalate building blocks have found useful applications in the field of molecule-based magnets. Our work involves the use of octacyanometalate building blocks for the self-assembly of two new classes of magnetic materials namely, high-spin molecular clusters which exhibit both ferromagnetic intra- and intercluster coupling, and specific extended network topologies which show long-range ferromagnetic ordering.

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Millennial to orbital-scale rainfall changes in the Mediterranean region and corresponding variations in vegetation patterns were the result of large-scale atmospheric reorganizations. In spite of recent efforts to reconstruct this variability using a range of proxy archives, the underlying physical mechanisms have remained elusive. Through the analysis of a new high-resolution sedimentary section from Lake Van (Turkey) along with climate modeling experiments, we identify massive droughts in the Eastern Med- iterranean for the past four glacial cycles, which have a pervasive link with known intervals of enhanced North Atlantic glacial iceberg calving, weaker Atlantic Meridional Overturning Circulation and Dansgaard-Oeschger cold conditions. On orbital timescales, the topographic effect of large Northern Hemisphere ice sheets and periods with minimum insolation seasonality further exacerbated drought intensities by suppressing both summer and winter precipitation.

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The use of magnetic measurements in the detection of fire signals has been neglected since the work of Rummery et al., (1979), yet considerable developments have been made in the interpretation of magnetic measurements over the last 16 years. This paper presents a study of the fire history of Lago di Origlio in the southern Swiss Alps. The study utilises the technique of mineral magnetism alongside the stratigraphic pollen, spore and charcoal records. Correlation between the various proxy records indicates that a magnetic ‘fire’ record is present within the sediments for the last 4 ka. The magnetic fire record has a distinct mineralogical and magnetic grain size signature that can be recognised against the background sedimentary signal. The results suggest that magnetic measurements may be usefully employed in the reconstruction of fire history. Their application is rapid and non-destructive and the results may provide additional information in relation to the links between catchment fire events and the sedimentary record.

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Bimetallic, oxalate-bridged compounds with bi- and trivalent transition metals comprise a class of layered materials which express a large variety in their molecular-based magnetic behavior. Because of this, the availability of the corresponding single-crystal structural data is essential to the successful interpretation of the experimental magnetic results. We report in this paper the crystal structure and magnetic properties of the ferromagnetic compound {[N(n-C3H7)4][MnIICrIII(C2O4)3]}n (1), the crystal structure of the antiferromagnetic compound {[N(n-C4H9)4][MnIIFeIII(C2O4)3]}n (2), and the results of a neutron diffraction study of a polycrystalline sample of the ferromagnetic compound {[P(C6D5)4][MnIICrIII(C2O4)3]}n (3). Crystal data:  1, rhombohedral, R3c, a = 9.363(3) Å, c = 49.207(27) Å, Z = 6; 2, hexagonal, P63, a = 9.482(2) Å, c = 17.827(8) Å, Z = 2. The structures consist of anionic, two-dimensional, honeycomb networks formed by the oxalate-bridged metal ions, interleaved by the templating cations. Single-crystal field dependent magnetization measurements as well as elastic neutron scattering experiments on the manganese(II)−chromium(III) samples show the existence of long-range ferromagnetic ordering behavior below Tc = 6 K. The magnetic structure corresponds to an alignment of the spins perpendicular to the network layers. In contrast, the manganese(II)−iron(III) compound expresses a two-dimensional antiferromagnetic ordering.

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Context. We investigate the dust coma within the Hill sphere of comet 67P/Churyumov-Gerasimenko. Aims. We aim to determine osculating orbital elements for individual distinguishable but unresolved slow-moving grains in the vicinity of the nucleus. In addition, we perform photometry and constrain grain sizes. Methods. We performed astrometry and photometry using images acquired by the OSIRIS Wide Angle Camera on the European Space Agency spacecraft Rosetta. Based on these measurements, we employed standard orbit determination and orbit improvement techniques. Results. Orbital elements and effective diameters of four grains were constrained, but we were unable to uniquely determine them. Two of the grains have light curves that indicate grain rotation. Conclusions. The four grains have diameters nominally in the range 0.14-0.50 m. For three of the grains, we found elliptic orbits, which is consistent with a cloud of bound particles around the nucleus. However, hyperbolic escape trajectories cannot be excluded for any of the grains, and for one grain this is the only known option. One grain may have originated from the surface shortly before observation. These results have possible implications for the understanding of the dispersal of the cloud of bound debris around comet nuclei, as well as for understanding the ejection of large grains far from the Sun.

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Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

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Orbital tuning is central for ice core chronologies beyond annual layer counting, available back to 60 ka (i.e. thousands of years before 1950) for Greenland ice cores. While several complementary orbital tuning tools have recently been developed using δ¹⁸Oatm, δO₂⁄N₂ and air content with different orbital targets, quantifying their uncertainties remains a challenge. Indeed, the exact processes linking variations of these parameters, measured in the air trapped in ice, to their orbital targets are not yet fully understood. Here, we provide new series of δO₂∕N₂ and δ¹⁸Oatm data encompassing Marine Isotopic Stage (MIS) 5 (between 100 and 160 ka) and the oldest part (340–800 ka) of the East Antarctic EPICA Dome C (EDC) ice core. For the first time, the measurements over MIS 5 allow an inter-comparison of δO₂∕N₂ and δ¹⁸Oatm records from three East Antarctic ice core sites (EDC, Vostok and Dome F). This comparison highlights some site-specific δO₂∕N₂ variations. Such an observation, the evidence of a 100 ka periodicity in the δO₂∕N₂ signal and the difficulty to identify extrema and mid-slopes in δO2∕N2 increase the uncertainty associated with the use of δO₂∕N₂ as an orbital tuning tool, now calculated to be 3–4 ka. When combining records of δ¹⁸Oatm and δO₂∕N₂ from Vostok and EDC, we find a loss of orbital signature for these two parameters during periods of minimum eccentricity (∼ 400 ka, ∼ 720–800 ka). Our data set reveals a time-varying offset between δO₂∕N₂ and δ¹⁸Oatm records over the last 800 ka that we interpret as variations in the lagged response of δ¹⁸Oatm to precession. The largest offsets are identified during Terminations II, MIS 8 and MIS 16, corresponding to periods of destabilization of the Northern polar ice sheets. We therefore suggest that the occurrence of Heinrich–like events influences the response of δ¹⁸Oatm to precession.