4,350 research outputs found
Training Induced Positive Exchange Bias in NiFe/IrMn Bilayers
Positive exchange bias has been observed in the
NiFe/IrMn bilayer system via soft x-ray resonant
magnetic scattering. After field cooling of the system through the blocking
temperature of the antiferromagnet, an initial conventional negative exchange
bias is removed after training i. e. successive magnetization reversals,
resulting in a positive exchange bias for a temperature range down to 30 K
below the blocking temperature (450 K). This new manifestation of magnetic
training is discussed in terms of metastable magnetic disorder at the
magnetically frustrated interface during magnetization reversal.Comment: 4 pages, 3 figure
Cryogenic fluid management experiment
The cryogenic fluid management experiment (CFME), designed to characterize subcritical liquid hydrogen storage and expulsion in the low-q space environment, is discussed. The experiment utilizes a fine mesh screen fluid management device to accomplish gas-free liquid expulsion and a thermodynamic vent system to intercept heat leak and control tank pressure. The experiment design evolved from a single flight prototype to provision for a multimission (up to 7) capability. A detailed design of the CFME, a dynamic test article, and dedicated ground support equipment were generated. All materials and parts were identified, and components were selected and specifications prepared. Long lead titanium pressurant spheres and the flight tape recorder and ground reproduce unit were procured. Experiment integration with the shuttle orbiter, Spacelab, and KSC ground operations was coordinated with the appropriate NASA centers, and experiment interfaces were defined. Phase 1 ground and flight safety reviews were conducted. Costs were estimated for fabrication and assembly of the CFME, which will become the storage and supply tank for a cryogenic fluid management facility to investigate fluid management in space
Conceptual design and analysis of orbital cryogenic liquid storage and supply systems
A wide variety of orbital cryogenic liquid storage and supply systems are defined in NASA and DOD long-range plans. These systems include small cooling applications, large chemical and electrical orbit transfer vehicles and supply tankers. All have the common requirements of low-g fluid management to accomplish gas-free liquid expulsion and efficient thermal control to manage heat leak and tank pressure. A preliminary design study was performed to evaluate tanks ranging from 0.6 to 37.4 cu m (22 to 1320 cu ft). Liquids of interest were hydrogen, oxygen, methane, argon and helium. Conceptual designs were generated for each tank system and fluid dynamic, thermal and structural analyses were performed for Shuttle compatible operations. Design trades considered the paradox of conservative support structure and minimum thermal input. Orbital performance and weight data were developed, and a technology evaluation was completed
Behavior of fluids in a weightless environment
Fluid behavior in a low-g environment is controlled primarily by surface tension forces. Certain fluid and system characteristics determine the magnitude of these forces for both a free liquid surface and liquid in contact with a solid. These characteristics, including surface tension, wettability or contact angle, system geometry, and the relationships governing their interaction, are discussed. Various aspects of fluid behavior in a low-g environment are then presented. This includes the formation of static interface shapes, oscillation and rotation of drops, coalescence, the formation of foams, tendency for cavitation, and diffusion in liquids which were observed during the Skylab fluid mechanics science demonstrations. Liquid reorientation and capillary pumping to establish equilibrium configurations for various system geometries, observed during various free-fall (drop-tower) low-g tests, are also presented. Several passive low-g fluid storage and transfer systems are discussed. These systems use surface tension forces to control the liquid/vapor interface and provide gas-free liquid transfer and liquid-free vapor venting
Candidate locations for SPS rectifying antennas
The feasibility of placing 120 Satellite Power System (SPS) rectifying antenna (rectenna) sites across the U.S. was studied. An initial attempt is made to put two land sites in each state using several land site selection criteria. When only 69 land sites are located, it is decided to put the remaining sites in the sea and sea site selection criteria are identified. An estimated projection of electrical demand distribution for the year 2000 is then used to determine the distribution of these sites along the Pacific, Atlantic, and Gulf Coasts. A methodology for distributing rectenna sites across the country and for fine-tuning exact locations is developed, and recommendations on rectenna design and operations are made
Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers
We present a comprehensive study of the exchange bias effect in a model
system. Through numerical analysis of the exchange bias and coercive fields as
a function of the antiferromagnetic layer thickness we deduce the absolute
value of the averaged anisotropy constant of the antiferromagnet. We show that
the anisotropy of IrMn exhibits a finite size effect as a function of
thickness. The interfacial spin disorder involved in the data analysis is
further supported by the observation of the dual behavior of the interfacial
uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we
have observed that the antiferromagnetic uncompensated spins are dominantly
frozen with nearly no rotating spins due to the chemical intermixing, which
correlates to the inferred mechanism for the exchange bias.Comment: 4 pages, 3 figure
decays to from new data on
Data on the reaction with ,
taken with unpolarized or polarized beams in combination with an unpolarized or
polarized proton-target, were analyzed within the Bonn-Gatchina (BnGa) partial
wave analysis. Differential cross sections, several spin density matrix
elements, the beam asymmetry , the normalized helicity difference ,
and the correlation between linear photon and longitudinal target
polarization were included in a large data base on pion and photo-induced
reactions. The data on photoproduction are used to determine twelve
branching ratios; most of these are determined for the first
time.Comment: 6 pages, 4 figures, 2 table
Democracy Doesn't Always Happen Over Night: Regime Change in Stages and Economic Growth
How substantial are the economic benefits from democratic regime change? We argue that democratisation is often not a discrete event but a two-stage process: autocracies enter into 'episodes' of political liberalisation which eventually culminate in regime change or not. To account for this chronology and the implicit counterfactual groups, we introduce a repeated-treatment difference-in-difference implementation capturing non-parallel trends and selection into treatment. We find that modelling regime change in two stages rather than a single event yields stronger long-run growth effects. Among democratizers, experiencing repeated episodes without regime change reduces growth in democracy whereas length of episode does not
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