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

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

To the center of cold spot with Planck

The structure of the cold spot, of a non-Gaussian anomaly in the cosmic microwave background (CMB) sky first detected by Vielva et al. is studied using the data by Planck satellite. The obtained map of the degree of stochasticity (K-map) of CMB for the cold spot, reveals, most clearly in 100 GHz band, a shell-type structure with a center coinciding with the minima of the temperature distribution. The shell structure is non-Gaussian at a 4\sigma confidence level. Such behavior of the K-map supports the void nature of the cold spot. The applied method can be used for tracing voids that have no signatures in redshift surveys.Comment: A & A (in press), 4 pages, 5 figures; to match the published versio

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

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

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

The Chemical Composition and Productivity of Vaccinium myrtillus L. under Influence of Industrial Pollution

Определены уровни накопления и особенности распределения химических элементов (S, Mn, Fe, Zn, Cu, Co, Cr, Ni, Pb, Cd), а также фенольных соединений в растениях черники обыкновенной (Vaccinium myrtillus L.) на фоновых и загрязненных аэропромвыбросами ОАО «Байкальский целлюлозно-бумажный комбинат» территориях Южного Прибайкалья. Анализ распределения элементов по органам черники показал, что в фоновых условиях корни черники отличаются наиболее высоким содержанием Fe, Ni, Cr, Pb, Cd, стебли – Mn, Zn, листья – S и Co, ягоды – Cu. Общее содержание фенольных соединений в растениях черники снижается в ряду листья > стебли > ягоды > корни, флавоноидов – листья > ягоды > стебли > корни. В фоновых условиях количество ягод на одном парциальном кусте варьирует от 0 до 17 шт., урожайность ягод – от 11 до 34 г/м2. Отмечено, что урожайность ягод в большей степени зависит от их количества и массы на одном парциальном кусте (r=0.55-0.68, р<0.05) и в меньшей – от количества плодоносящих кустов (r=0.34-0.45, р<0.05). Высота куста, параметры кроны существенного влияния на величину урожайности ягод не оказывают. В зоне атмосферных промышленных выбросов ОАО «БЦБК» в надземных частях черники при увеличении содержания S в 1.7-1.8 раза наблюдалось снижение концентрации Mn и Zn в 1.5-2.3 раза и повышение уровня флавоноидов в 1.3-1.5 раза. Коэффициенты накопления большинства рассмотренных элементов снижались, причем для биофильных элементов в 1.4- 2.0 раза. Обнаружено уменьшение средней высоты парциальных кустов и протяженности кроны в 1.4 раза, количества ягод и их массы на парциальном кусте в 1.5-1.8 раза по сравнению с фоновыми значениями, тогда как диаметр кроны и масса одной ягоды практически не изменялись. Снижение морфометрических параметров парциальных кустов приводит к уменьшению их надземной фитомассы в среднем в 1.5 раза, а урожайности – в 1.7 раза.The content and distribution of chemical elements (S, Mn, Fe, Zn, Cu, Co, Cr, Ni, Pb and Cd) and phenolic compounds were determined in the above-ground and underground parts of bilberry (Vaccinium myrtillus L.), grown on the background and technogenic polluted territories of the Southern Baikal region. Plants and soil were sampled at polluted site situated 0.5-1.0 km from Baikal Pulp and Paper Mill (East Siberia) and at a relatively clean site. It was shown that in background conditions Fe, Ni, Cr, Pb and Cd accumulated in roots of V. myrtillus, Mn and Zn accumulated in stems, S and Co in leaves and Cu in berries. It should be noted, that the absorption intensity of the elements of the V. myrtillus reproductive organs is lower than of the vegetative one. The content of phenolic compounds in bilberries plants decreases in the following order: leaves > stems > berries > roots, and the content of flavonoids in the order leaves > berries > stems > roots. Amount of berries on a shrub varies from 0 to 17 units in the background conditions. The berries productivity was from 11 to 34 g/m2. It was shown, that the berries productivity depends on the number and weight of the berries on the shrub (r = 0.55-0.68, p <0.05). In the site influenced by the industrial emissions of Baikal Pulp and Paper Mill bilberry above-ground parts contained significantly higher concentration of S and significantly lower concentrations of Mn and Zn. Coefficients of biophilic elements accumulation decreased 1.4- 2.0 times, that indicates a violation of their absorption by plant roots. Furthermore, leaves had significantly higher level of total phenolics and flavonoids as compared with bilberry leaves from non-polluted site. Plants of the polluted site had a lower height and amount of berries. The berries productivity was significantly lower (1.1±0.3 g per shrub in the polluted site compared with 2.0±0.5 g per shrub in the background one). At the same time the weight of one berry did not differ between the two sites. Reduction of the shrub morphometric parameters leads to a decrease in their aboveground biomass of an average of 1.5 times, berries productivity (g/m2) decreased 1.7 time