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 La$_2$CuO$_4$ 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 $3d$ orbitals of $E_g$ and $T_{2g}$ 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 $T_{2g}$ 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 $E_g$ 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 $E_g$ 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