324 research outputs found
First-principles modelling of magnetic excitations in Mn12
We have developed a fully microscopic theory of magnetic properties of the
prototype molecular magnet Mn12. First, the intra-molecular magnetic properties
have been studied by means of first-principles density functional-based
methods, with local correlation effects being taken into account within the
local density approximation plus U (LDA+U) approach. Using the magnetic force
theorem, we have calculated the interatomic isotropic and anisotropic exchange
interactions and full tensors of single-ion anisotropy for each Mn ion.
Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually
large, reflecting a low symmetry of magnetic pairs in molecules, in comparison
with bulk crystals. Based on these results we predict a distortion of
ferrimagnetic ordering due to DM interactions. Further, we use an exact
diagonalization approach allowing to work with as large Hilbert space dimension
as 10^8 without any particular symmetry (the case of the constructed magnetic
model). Based on the computational results for the excitation spectrum, we
propose a distinct interpretation of the experimental inelastic neutron
scattering spectra.Comment: 8 pages, 2 figures. To appear in Physical Review
Correlated band theory of spin and orbital contributions to Dzyaloshinskii-Moriya interactions
A new approach for calculations of Dzyaloshinskii-Moriya interactions in
molecules and crystals is proposed. It is based on the exact perturbation
expansion of total energy of weak ferromagnets in the canting angle with the
only assumption of local Hubbard-type interactions. This scheme leads to a
simple and transparent analytical expression for Dzyaloshinskii-Moriya vector
with a natural separation into spin and orbital contributions. The main problem
was transferred to calculations of effective tight-binding parameters in the
properly chosen basis including spin-orbit coupling. Test calculations for
LaCuO give the value of canting angle in a good agreement with
experimental data.Comment: 4 pages, 1 figur
Microscopic origin of Heisenberg and non-Heisenberg exchange interactions in ferromagnetic bcc Fe
By means of first principles calculations we investigate the nature of
exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the
basic electronic structure reveals a drastic difference between the
orbitals of and symmetries. The latter ones define the shape of
the Fermi surface, while the former ones form weakly-interacting impurity
levels. We demonstrate that, as a result of this, in Fe the orbitals
participate in exchange interactions, which are only weakly dependent on the
configuration of the spin moments and thus can be classified as
Heisenberg-like. These couplings are shown to be driven by Fermi surface
nesting. In contrast, for the states the Heisenberg picture breaks down,
since the corresponding contribution to the exchange interactions is shown to
strongly depend on the reference state they are extracted from. Our analysis of
the nearest-neighbour coupling indicates that the interactions among
states are mainly proportional to the corresponding hopping integral and thus
can be attributed to be of double-exchange origin.Comment: 5 pages, 4 figure
Открытось образовательного пространства как элемент эффективной среды подготовки современного педагога
The article discusses the issues of constructing additional professional education for teachers, reflects the main contradictions between the needs of the modern teacher and the proposals of the current system of professional development, formulates proposals for the modernization of the system, including the introduction of informal online education in the model of continuous education of a modern teacher.В статье рассмотрены вопросы построения дополнительного профессионального образования педагогов, отражены основные противоречия между потребностями современного учителя и предложениями действующей системы повышения квалификации, сформулированы предложения по модернизации системы, включая внедрение неформального онлайн-образования в модель непрерывного образования современного педагога
Layer-resolved magnetic exchange interactions of surfaces of late 3d elements: effects of electronic correlations
We present the results of an ab initio study of magnetic properties of Fe, Co
and Ni surfaces. In particular, we discuss their electronic structure and
magnetic exchange interactions (Jij), as obtained by means of a combination of
density functional theory and dynamical mean-field theory. All studied systems
have a pronounced tendency to ferromagnetism both for bulk and surface atoms.
The presence of narrow-band surface states is shown to enhance the magnetic
moment as well as the exchange couplings. The most interesting results were
obtained for the Fe surface where the atoms have a tendency to couple
antiferromagnetically with each other. This interaction is relatively small,
when compared to interlayer ferromagnetic interaction, and strongly depends on
the lattice parameter. Local correlation effects are shown to lead to strong
changes of the overall shape of the spectral functions. However, they seem to
not play a decisive role on the overall picture of the magnetic couplings
studied here. We have also investigated the influence of correlations on the
spin and orbital moments of the bulk-like and surface atoms. We found that
dynamical correlations in general lead to enhanced values of the orbital
moment.Comment: 13 pages, 12 figure
Finite element modeling of multiple electrode submerged arc welding of large diameter pipes
For the process of multiple-electrode submerged arc welding (MESAW) of large- diameter pipes (LDP) a mathematical model has been developed with a numerical solution in the software product "SYSWELD". The paper defines the parameters of double ellipsoid model of heat sources by J. Goldak. Using regression analysis of experimental data empirical formulas for calculating geometrical parameters of heat sources model by J. Goldak and deposition rate were developed for welding with direct current on positive polarity and alternating current. The model allows to predict the shape of weld and to estimate analytically the cooling rate in the heat affected zone (HAZ). The developed model and its numerical solutions with sufficient accuracy for practical calculations reflect the process of longitudinal welding of LDP. © 2019 IOP Publishing Ltd
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