Ballistic (precessional) contribution to the conventional magnetic switching

We consider a magnetic moment with an easy axis anisotropy energy, switched by an external field applied along this axis. Additional small, time-independent bias field is applied perpendicular to the axis. It is found that the magnet's switching time is a non-monotonic function of the rate at which the field is swept from "up" to "down". Switching time exhibits a minimum at a particular optimal sweep time. This unusual behavior is explained by the admixture of a ballistic (precessional) rotation of the moment caused by the perpendicular bias field in the presence of a variable switching field. We derive analytic expressions for the optimal switching time, and for the entire dependence of the switching time on the field sweep time. The existence of the optimal field sweep time has important implications for the optimization of magnetic memory devices

Comment on "Ferromagnetic film on a superconducting substrate"

A superconducting substrate is not able to shrink drastically domains in a ferromagnetic film, contrary to the prediction of Bulaevskii and Chudnovsky [Phys. Rev. B, 63, issue1 (2001)]. This is shown on the basis of the exact solution for the stripe domain structure.Comment: 8 pages, 1 figure, the version published in Phys. Rev.

Analytical prediction with multidimensional computer programs and experimental verification of the performance, at a variety of operating conditions, of two traveling wave tubes with depressed collectors

Experimental and analytical results are compared for two high performance, octave bandwidth TWT's that use depressed collectors (MDC's) to improve the efficiency. The computations were carried out with advanced, multidimensional computer programs that are described here in detail. These programs model the electron beam as a series of either disks or rings of charge and follow their multidimensional trajectories from the RF input of the ideal TWT, through the slow wave structure, through the magnetic refocusing system, to their points of impact in the depressed collector. Traveling wave tube performance, collector efficiency, and collector current distribution were computed and the results compared with measurements for a number of TWT-MDC systems. Power conservation and correct accounting of TWT and collector losses were observed. For the TWT's operating at saturation, very good agreement was obtained between the computed and measured collector efficiencies. For a TWT operating 3 and 6 dB below saturation, excellent agreement between computed and measured collector efficiencies was obtained in some cases but only fair agreement in others. However, deviations can largely be explained by small differences in the computed and actual spent beam energy distributions. The analytical tools used here appear to be sufficiently refined to design efficient collectors for this class of TWT. However, for maximum efficiency, some experimental optimization (e.g., collector voltages and aperture sizes) will most likely be required

Pressure effects on charge, spin, and metal-insulator transitions in narrow bandwidth manganite Pr1−x_{1-x}Cax_{x}MnO3_{3}

Pressure effects on the charge and spin states and the relation between the ferromagnetic and metallic states were explored on the small bandwidth manganite Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (x = 0.25, 0.3, 0.35). Under pressure, the charge ordering state is suppressed and a ferromagnetic metallic state is induced in all three samples. The metal-insulator transition temperature (T$_{MI}$) increases with pressure below a critical point P*, above which T$_{MI}$ decreases and the material becomes insulating as at the ambient pressure. The e$_{g}$ electron bandwidth and/or band-filling mediate the pressure effects on the metal-insulator transition and the magnetic transition. In the small bandwidth and low doping concentration compound (x = 0.25), the T$_{MI}$ and Curie temperature (T$_{C}$) change with pressure in a reverse way and do not couple under pressure. In the x = 0.3 compound, the relation of T$_{MI}$ and T$_{C}$ shows a critical behavior: They are coupled in the range of $\sim$0.8-5 GPa and decoupled outside of this range. In the x = 0.35 compound, T$_{MI}$ and T$_{C}$ are coupled in the measured pressure range where a ferromagnetic state is present

Serravallian Shales in the Monte dei Corvi Pelagic Sequence (Ancona, Italy): An Organic Geochemical Perspective

In addition to the predominant marly lithologies, the Serravallian-Tortonian sequence at Monte dei Corvi (MDC), south of Ancona, Italy, contains at least 85 thin, dark calcareous shales. Such shales, averaging 14 cm in thickness, comprise 9% of the total Serravallian sequence. Sixteen of them were sampled for a preliminary organic geochemical evaluation. All the MDC shales appear to have been deposited during periodic anoxic events, as demonstrated by the presence of significant quantities of organic matter and authigenic pyrite. The degree of anoxicity (and thus the amount of organic matter preserved) appears to have differed from one event to the next. The quantity of organic matter preserved is highly variable, with Rock Eval S2 ranging from 0.4 to 27.6 mg/g. However, the type is remarkably consistent from sample to sample, as evidenced both by analytical pyrolysis-gas chromatography/mass spectrometry (GC/MS) of the solid organic matter and by the GC/MS analysis of the extractable material. This implies a regeneration of similar microfloral/microbial assemblages and depositional conditions during each anoxic event. The dominant organic matter types are marine, including several types of fossil algae and amorphous material (largely the product of bacterial reworking of organic matter). There is evidence of minor terrestrial input. The presence of isoprenoid hydrocarbons (prist-1-ene) in the pyrolyzates and 17β(H),21β(H) hopanes in the extractable organic matter attest to the low level of thermal alteration of the MDC shales. The MDC organic matter appears to be partially oxidized, probably from weathering at the outcrop, which partly obscures its original nature. To avoid this problem in any future organic geochemical study of the MDC sequence, the authors recommend that fresh, unweathered samples be obtained, preferably by coring

Geochemical Characterization of Maceral Concentrates from Herrin No. 6 Coal (Illinois Basin) and Lower Toarcian Shale Kerogen (Paris Basin)

Density gradient centrifugation (DGC) is a physical method for the separation of sedimentary organic matter into its constituents. Using DGC, it is possible to prepare maceral concentrates from a single sample, which are amenable to microanalysis. DGC fractions from a coal sample from the Illinois Basin (Herrin No. 6, Upper Carboniferous) and from the kerogen of a marine shale sample from the Paris Basin (Lower Toarcian) were analyzed by flash pyrolysis-gas chromatography-mass spectrometry, after extraction by CH2Cl2. Chemical differences between the coal DGC fractions are the easiest to recognize, indicating very distinctive biological precursors. For example, the liptinite fraction ( = 1.12 g ml-1) is dominated by long-chain aliphatic compounds (n-alkanes and n-alkenes) along with alkylbenzenes and alkylphenols. Vitrinite (1.29 g ml-1) shows a predominance of alkylbenzenes and phenolic compounds. Polyaromatic hydrocarbons (especially naphthalene, phenanthrene, anthracene and their pseudohomologues) are major compounds in the pyrolyzate of fusinite (1.45 g ml-1). In contrast, there is less variety of organic compounds in the Toarcian sample. Petrographically, this kerogen is primarily amorphous. However, a main DGC peak ( = 1.18 g ml-1) with two shoulders (1.15 and 1.23 g ml-1) is resolved using multi-step centrifugation. The chemical differences between these fractions are subtle but significant. Concentrations of alkylbenzenes, alkylthiophenes, alkylpyrroles and phenolic compounds increase with density, relative to the aliphatics. This indicates that this kerogen, probably of algal and bacterial origin, is partially separable by DGC

Synchrotron radiation photoionization mass spectrometry of laser ablated species

The present paper describes an experimental apparatus suitable to create and study free clusters by combining laser ablation and synchrotron radiation. First tests on sulfur samples, S, showed the production, through laser ablation, of neutral Sn clusters (n = 1–8). These clusters were ionized using synchrotron radiation at photon energies from 160 eV to 175 eV, across the S 2p core edge. The feasibility of such combined ablation–synchrotron radiation experiments is demonstrated, opening new possibilities on the investigation of free clusters and radical

Utrzymanie central telefonicznych. Przegląd Zagadnień Łączności, 1968, nr 2 (77)

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Pomiary ruchu telefonicznego międzymiastowego. Przegląd Zagadnień Łączności, 1969, nr 3 (90)

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