Boundary Energies and the Geometry of Phase Separation in Double--Exchange Magnets

We calculate the energy of a boundary between ferro- and antiferromagnetic regions in a phase separated double-exchange magnet in two and three dimensions. The orientation dependence of this energy can significantly affect the geometry of the phase-separated state in two dimensions, changing the droplet shape and possibly stabilizing a striped arrangement within a certain range of the model parameters. A similar effect, albeit weaker, is also present in three dimensions. As a result, a phase-separated system near the percolation threshold is expected to possess intrinsic hysteretic transport properties, relevant in the context of recent experimental findings.Comment: 6 pages, including 4 figures; expanded versio

Reciprocal space study of Heisenberg exchange interactions in ferromagnetic metals

The modern quantum theory of magnetism in solids is getting commonly derived using Green's functions formalism. The popularity draws itself from remarkable opportunities to capture the microscopic landscape of exchange interactions, starting from a tight-binding representation of the electronic structure. Indeed, the conventional method of infinitesimal spin rotations, considered in terms of local force theorem, opens vast prospects of investigations regarding the magnetic environment, as well as pairwise atomic couplings. However, this theoretical concept practically does not devoid of intrinsic inconsistencies. In particular, naturally expected correspondence between single and pairwise infinitesimal spin rotations is being numerically revealed to diverge. In this work, we elaborate this question on the model example and canonical case of bcc iron. Our analytical derivations discovered the principal preference of on-site magnetic precursors if the compositions of individual atomic interactions are in focus. The problem of extremely slow or even absent spatial convergence while considering metallic compounds was solved by suggesting the original technique, based on reciprocal space framework. Using fundamental Fourier transform-inspired interconnection between suggested technique and traditional spatial representation, we shed light on symmetry breaking in bcc Fe on the level of orbitally decomposed total exchange surrounding

Mechanisms and origins of half-metallic ferromagnetism in CrO2

Using a realistic low-energy model, derived from the first-principles electronic structure calculations, we investigate the behavior of interatomic exchange interactions in CrO2, which is regarded to be one of the canonical half-metallic (HM) ferromagnetics. For these purposes we employ the dynamical mean-field theory (DMFT), based on the exact diagonalization of the effective Anderson impurity Hamiltonian, which was further supplemented with the theory of infinitesimal spin rotations for the exchange interactions. In order to elucidate the relative roles played by static and dynamic electron correlations, we compare the obtained results with several static techniques, including the unrestricted Hartree-Fock (HF) approximation, static DMFT (corresponding to the infinite frequency limit for the self-energy), and optimized effective potential method for treating the correlation interactions in the random-phase approximation. Our results demonstrate that the origin of the HM ferromagnetism in CrO2 is highly nontrivial. As far as the interactions in the neighboring coordination spheres are concerned, HF and DMFT methods produce very similar results, due to the partial cancellation of ferromagnetic (FM) double-exchange and antiferromagnetic (AFM) superexchange contributions, which represent two leading terms in the (ΔΣ)-1 expansion for the exchange interactions (ΔΣ being the intra-atomic spin splitting). Both contributions are weaker in the HF approximation due to, respectively, additional orbital polarization of the t2g states and neglect of dynamic correlations. The role of higher-order terms in the (ΔΣ)-1 expansion is twofold. On the one hand, they give rise to additional FM contributions to the neighboring exchange interactions, which tend to stabilize the FM state. On the other hand, they produce AFM long-range interactions, which make the FM state unstable in the single-site DMFT calculations for the minimal model, consisting of the t2g bands. Thus, the robust ferromagnetism in the minimal model, which can be easily obtained using static approximations, is fortuitous and this picture is largely revised at the level of more rigorous DMFT approach. We argue that the main ingredients, which are missing in the minimal model, are the direct exchange interactions and the magnetic polarization of the oxygen 2p band. We evaluate these contributions in the local-spin-density approximation and argue that they play a very important role in stability of the FM ground state in CrO2. © 2015 American Physical Society. ©2015 American Physical Society

FAST PREDETERMINED EQUILIBRIUM DYNAMICS APPLIED TO MAGNETIC SYSTEMS

In this paper, a fast algorithm for implementing the method [1] is proposed for consider-ation. Its application to the problem of modeling microscopic magnetic dynamics is also shown

Band filling dependence of the Curie temperature in CrO2

Rutile CrO2 is an important half-metallic ferromagnetic material, which is also widely used in magnetic recording. In an attempt to find the conditions, which lead to the increase of the Curie temperature (T C), we study theoretically the band-filling dependence of interatomic exchange interactions in the rutile compounds. For these purposes, we use the effective low-energy model for the magnetic t 2g bands, derived from the first-principles electronic structure calculations in the Wannier basis, which is solved by means of dynamical mean-field theory. After the solution, we calculate the interatomic exchange interactions, by using the theory of infinitesimal spin rotations, and evaluate T C. We argue that, as far as the Curie temperature is concerned, the band filling realized in CrO2 is far from being the optimal one and much higher T C can be obtained by decreasing the number of t 2g electrons (n) via the hole doping. We find that the optimal n is close to 1, which should correspond to the case of VO2, provided that it is crystallized in the rutile structure. This finding was confirmed by using the experimental rutile structure for both CrO2 and VO2 and reflects the general tendency towards ferromagnetism for the narrow-band compounds at the beginning of the band filling. In particular, our results suggest that the strong ferromagnetism can be achieved in the thin films of VO2, whose crystal structure is controlled by the substrate. © 2016 IOP Publishing Ltd

ПЕРСПЕКТИВЫ РАЗВИТИЯ СЕЛЬСКОГО ХОЗЯЙСТВА РОССИИ НА СОВРЕМЕННОМ ЭТАПЕ. РЕЗУЛЬТАТЫ РАБОТЫ КОМИТЕТА ГОСУДАРСТВЕННОЙ ДУМЫ ПО ПРИРОДНЫМ РЕСУРСАМ, ПРИРОДОПОЛЬЗОВАНИЮ И ЭКОЛОГИИ

The perspectives of the development of agriculture in Russia in modern times and main results of work of the Committee of the State Duma on Natural Resources, Management and Ecology are presented in the article. For the purpose of food sovereignty, the import ration should not be exceeded 25% from overall volume of food production. The Russian Academy of Agricultural Science worked on the potential of agricultural sector, which has to be the duty-bearer of food supply security of Russia and the source of raw materials for the trade. Up to now, the Russian scientists have developed a lot of competitive varieties and hybrids, widespread commercial introduction of which is able to guarantee of high quality products.В статье, сформированной на основе доклада на Пленарном заседании общего собрания Отделения сельскохозяйственных наук РАН, состоявшемся 21 марта 2016 года, представлены состояние и перспективы развития сельского хозяйства РФ на современном этапе, показаны основные результаты работы Комитета Государственной Думы по природным ресурсам, природопользованию и экологии. Для достижения продовольственной независимости страны доля импорта не должна превышать 25% от общего объема продовольственной продукции. Российская академия сельскохозяйственных наук с момента ее основания предметно занималась созданием аграрного потенциала России, делая все, чтобы сельское хозяйство являлось гарантом продовольственной безопасности великой страны и источником сырья для промышленности. В настоящее время отечественными учеными-аграриями создано большое количество конкурентоспособных сортов и гибридов, широкое внедрение которых в сельскохозяйственное производство способно обеспечить свое население собственной качественной продукцией

Non-Trivial Electronic Configurations of Two-Dimensional Ferromagnetic Crystals with Standard Geometries

In this work we numerically determine non-trivial trends in electron structure of 2D materials, which favors the ferromagnetic ordering. For this purpose we use local force theorem and genetic algorithm of optimization. It was found that the most stable FM crystals possess a moderate metallicity.Работа выполнена при поддержке гранта Президента РФ № МК-2578.2021.1.2

ALGORITHM OF SEARCHING THE STABLE FERROMAGNETIC QUASI-TWO-DIMENSIONAL MATERIALS

Here we suggest a theoretical approach designed to seek an optimized electronic structure of quasi-two-dimensional material by means of ferromagnetic ordering stability. Correspond-ing tight-binding Hamiltonian is treated using Green's functions formalism in order to esti-mate Curie temperature value.Работа выполнена при поддержке гранта Президента РФ № МК-2578.2021.1.2

Influence of surface modification of nitinol with silicon using plasma-immersion ion implantation on the alloy corrosion resistance in artificial physiological solutions

Cyclic voltammetry and potentiostatic polarization have been applied to study electrochemical behavior and to determine corrosion resistance of nitinol, which surface was modified with silicon using plasma-immersion ion implantation, in 0.9% NaCl solution and in artificial blood plasma. It was found out that continuous, and also homogeneous in composition, thin Si-containing layers are resistant to corrosion damage at high positive potentials in artificial physiological solutions due to formation of stable passive films. Breakdown potential Eb of Si-modified NiTi depends on the character of silicon and Ni distribution at the alloy surface, Eb values amounted to 0.9–1.5 V (Ag/AgCl/KCl sat.) for the alloy samples with continuous Si-containing surface layers and with decreased Ni surface concentration