203 research outputs found
Out- versus in-plane magnetic anisotropy of free Fe and Co nanocrystals: tight-binding and first-principles studies
We report tight-binding (TB) and Density Function Theory (DFT) calculations
of magnetocrystalline anisotropy energy (MAE) of free Fe (body centerd cubic)
and Co (face centered cubic) slabs and nanocrystals. The nanocrystals are
truncated square pyramids which can be obtained experimentally by deposition of
metal on a SrTiO(001) substrate. For both elements our local analysis shows
that the total MAE of the nanocrystals is largely dominated by the contribution
of (001) facets. However, while the easy axis of Fe(001) is out-of-plane, it is
in-plane for Co(001). This has direct consequences on the magnetic reversal
mechanism of the nanocrystals. Indeed, the very high uniaxial anisotropy of Fe
nanocrystals makes them a much better potential candidate for magnetic storage
devices.Comment: 8 pages, 7 figure
How agency models inspire large scale participatory planning and its evaluation
International audienceWe describe how three models, for sustainable change, human agency in collective resource management, and socio-environmental systems, have been used to design a protocol and the tools for a large scale (1500 participants, 35 villages) multi-level participatory process held in Africa for Integrated Natural Resource Management, through the European Project Afromaison. The process especially combines a common action model to support proposals by stakeholders, an integration matrix to build coherent plans, a role playing game design process, and a method to combine planning and playing to engage into the plans. It has also inspired the design of the attached monitoring and evaluation process. We describe the process in two countries, Ethiopia and Uganda, present the theoretical bases of the evaluation framework using the ENCORE paradigm and the implemented methodology transferred to local evaluators. We introduce some results and propose comments on potential learning back to the modelling community
Integrated Thermal Protection Systems and Heat Resistant Structures
In the early stages of NASA's Exploration Initiative, Snecma Propulsion Solide was funded under the Exploration Systems Research & Technology program to develop a CMC heatshield, a deployable decelerator, and an ablative heat shield for reentry vehicles. Due to changes within NASA's Exploration Initiative, this task was cancelled in early FY06. This paper will give an overview of the work that was accomplished prior to cancellation. The Snecma team consisted of MT Aerospace, Germany, and Materials Research & Design (MR&D), NASA Langley, NASA Dryden, and NASA Ames in the United States. An Apollo-type capsule was chosen as the reference vehicle for the work. NASA Langley generated the trajectory and aerothermal loads. Snecma and MT Aerospace began the design of a ceramic aft heatshield (CAS) utilizing C/SiC panels as the capsule heatshield. MR&D led the design of a C/SiC deployable decelerator, NASA Ames led the characterization of several ablators, NASA Dryden led the development of a heath management system and the high temperature structures testing, and NASA Langley led the insulation characterization. Though the task was pre-maturely cancelled, a significant quantity of work was accomplished
Orbital contribution to the magnetic properties of iron as a function of dimensionality
The orbital contribution to the magnetic properties of Fe in systems of
decreasing dimensionality (bulk, surfaces, wire and free clusters) is
investigated using a tight-binding hamiltonian in an and atomic
orbital basis set including spin-orbit coupling and intra-atomic electronic
interactions in the full Hartree-Fock (HF) scheme, i.e., involving all the
matrix elements of the Coulomb interaction with their exact orbital dependence.
Spin and orbital magnetic moments and the magnetocrystalline anisotropy energy
(MAE) are calculated for several orientations of the magnetization. The results
are systematically compared with those of simplified hamiltonians which give
results close to those obtained from the local spin density approximation. The
full HF decoupling leads to much larger orbital moments and MAE which can reach
values as large as 1 and several tens of meV, respectively, in the
monatomic wire at the equilibrium distance. The reliability of the results
obtained by adding the so-called Orbital Polarization Ansatz (OPA) to the
simplified hamiltonians is also discussed. It is found that when the spin
magnetization is saturated the OPA results for the orbital moment are in
qualitative agreement with those of the full HF model. However there are large
discrepancies for the MAE, especially in clusters. Thus the full HF scheme must
be used to investigate the orbital magnetism and MAE of low dimensional
systems
Giant Anisotropic Magneto-Resistance in ferromagnetic atomic contacts
Magneto-resistance is a physical effect of great fundamental and industrial
interest since it is the basis for the magnetic field sensors used in computer
read-heads and Magnetic Random Access Memories. As device dimensions are
reduced, some important physical length scales for magnetism and electrical
transport will soon be attained. Ultimately, there is a strong need to know if
the physical phenomena responsible for magneto-resistance still hold at the
atomic scale. Here, we show that the anisotropy of magneto-resistance is
greatly enhanced in atomic size constrictions. We explain this physical effect
by a change in the electronic density of states in the junction when the
magnetization is rotated, as supported by our ab-initio calculations. This
stems from the "spin-orbit coupling" mechanism linking the shape of the
orbitals with the spin direction. This sensitively affects the conductance of
atomic contacts which is determined by the overlap of the valence orbitals.Comment: latex AAMR.tex, 6 files, 5 figures, 4 pages
(http://www-drecam.cea.fr/spec/articles/S06/011
Direct Amidation of Esters via Ball Milling
The direct mechanochemical amidation of esters by ball milling is described. The operationally simple procedure requires an ester, an amine, and substoichiometric KOtBu and was used to prepare a large and diverse library of 78 amide structures with modest to excellent efficiency. Heteroaromatic and heterocyclic components are specifically shown to be amenable to this mechanochemical protocol. This direct synthesis platform has been applied to the synthesis of active pharmaceutical ingredients (APIs) and agrochemicals as well as the gram-scale synthesis of an active pharmaceutical, all in the absence of a reaction solvent
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