1,401 research outputs found
An ab-initio theoretical investigation of the soft-magnetic properties of permalloys
We study Ni80Fe20-based permalloys with the relativistic spin-polarized
Korringa-Kohn-Rostoker electronic structure method. Treating the compositional
disorder with the coherent potential approximation, we investigate how the
magnetocrystalline anisotropy, K, and magnetostriction, lambda, of Ni-rich
Ni-Fe alloys vary with the addition of small amounts of non-magnetic transition
metals, Cu and Mo. From our calculations we follow the trends in K and lambda
and find the compositions of Ni-Fe-Cu and Ni-Fe-Mo where both are near zero.
These high permeability compositions of Ni-Fe-Cu and Ni-Fe-Mo match well with
those discovered experimentally. We monitor the connection of the magnetic
anisotropy with the number of minority spin electrons, Nmin. By raising Nmin
via artificially increasing the band-filling of Ni80Fe20, we are able to
reproduce the key features that underpin the magnetic softening we find in the
ternary alloys. The effect of band-filling on the dependence of
magnetocrystalline anisotropy on atomic short-range order in Ni80Fe20 is also
studied. Our calculations, based on a static concentration wave theory,
indicate that the susceptibility of the high permeability of the Ni-Fe-Cu and
Ni-Fe-Mo alloys to their annealing conditions is also strongly dependent on the
alloys' compositions. An ideal soft magnet appears from these calculations.Comment: 20 pages, 6 figure
Effects of short-range order on the electronic structure of disordered metallic systems
For many years the Korringa-Kohn-Rostoker coherent-potential approximation
(KKR-CPA) has been widely used to describe the electronic structure of
disordered systems based upon a first-principles description of the crystal
potential. However, as a single-site theory the KKR-CPA is unable to account
for important environmental effects such as short-range order (SRO) in alloys
and spin fluctuations in magnets, amongst others. Using the recently devised
KKR-NLCPA (where NL stands for nonlocal), we show how to remedy this by
presenting explicit calculations for the effects of SRO on the electronic
structure of the bcc Cu_{50}Zn_{50} solid solution.Comment: 8 pages, 6 figures, Revised versio
Results of the simulations of the petal/lens as part of the LAUE project
In the context of the LAUE project for focusing hard X-/gamma rays, a petal
of the complete lens is being assembled at the LARIX facility in the Department
of Physics and Earth Science of the University of Ferrara. The lens petal
structure is composed of bent Germanium and Gallium Arsenide crystals in
transmission geometry. We present the expectations derived from a mathematical
model of the lens petal. The extension of the model for the complete LAUE
project in the 90 -- 600 keV energy range will be discussed as well. A
quantitative analysis of the results of these simulations is also presented.Comment: 12 pages, 26 figures, SPIE optics + Photonics conference 2013, Vol:
886
Effects of compost age on the release of nutrients
Composted organic materials are applied to help restore disturbed soils, speed revegetation, and control erosion; these changes are generally beneficial for stormwater quality. Ensuring that nutrient release from compost is adequate for plant needs without degrading stormwater quality is important since composts release nitrogen at variable rates (1–3% of total N/yr) and the leaching process can extend for many years. The aim of this work was to understand the effect of compost age on the extent and rates of nitrogen release by conducting detailed rainfall simulation studies of one compost type at three different ages. Models describing temporal changes in nitrogen release to runoff during a single storm and across multiple storms were developed and applied to the runoff data. Nitrogen content (%) and bulk density of compost increased with the increase in compost age and total nitrogen release decreased with increasing compost age. The three rain simulations (storms) performed on each of the three compost ages show that nitrogen release declined each day of the repeated daily storms. A first-order kinetic model was used to estimate the amount of nitrogen remaining on compost after several storms
Temperature dependent magnetic anisotropy in metallic magnets from an ab-initio electronic structure theory: L1_0-ordered FePt
On the basis of a first-principles, relativistic electronic structure theory
of finite temperature metallic magnetism, we investigate the variation of
magnetic anisotropy, K, with magnetisation, M, in metallic ferromagnets. We
apply the theory to the high magnetic anisotropy material, L1_0-ordered FePt,
and find its uniaxial K consistent with a magnetic easy axis perpendicular to
the Fe/Pt layering for all M and to be proportional to M^2 for a broad range of
values of M. For small M, near the Curie temperature, the calculations pick out
the easy axis for the onset of magnetic order. Our results are in good
agreement with recent experimental measurements on this important magnetic
material.Comment: 4 pages, 2 figure
Coarse Grained Density Functional Theories for Metallic Alloys: Generalized Coherent Potential Approximations and Charge Excess Functional Theory
The class of the Generalized Coherent Potential Approximations (GCPA) to the
Density Functional Theory (DFT) is introduced within the Multiple Scattering
Theory formalism for dealing with, ordered or disordered, metallic alloys. All
GCPA theories are based on a common ansatz for the kinetic part of the
Hohenberg-Kohn functional and each theory of the class is specified by an
external model concerning the potential reconstruction. The GCPA density
functional consists of marginally coupled local contributions, does not depend
on the details of the charge density and can be exactly rewritten as a function
of the appropriate charge multipole moments associated with each lattice site.
A general procedure based on the integration of the 'qV' laws is described that
allows for the explicit construction the same function. The coarse grained
nature of the GCPA density functional implies great computational advantages
and is connected with the O(N) scalability of GCPA algorithms. Moreover, it is
shown that a convenient truncated series expansion of the GCPA functional leads
to the Charge Excess Functional (CEF) theory [E. Bruno, L. Zingales and Y.
Wang, Phys. Rev. Lett. {\bf 91}, 166401 (2003)] which here is offered in a
generalized version that includes multipolar interactions. CEF and the GCPA
numerical results are compared with status of art LAPW full-potential density
functional calculations for 62, bcc- and fcc-based, ordered CuZn alloys, in all
the range of concentrations. These extensive tests show that the discrepancies
between GCPA and CEF are always within the numerical accuracy of the
calculations, both for the site charges and the total energies. Furthermore,
GCPA and CEF very carefully reproduce the LAPW site charges and the total
energy trends.Comment: 19 pages, 11 figure
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