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Few-layer CrI3 is the most known example among two-dimensional (2D) ferromagnets, which have attracted growing interest in recent years. Despite considerable efforts and progress in understanding the properties of 2D magnets both from theory and experiment, the mechanism behind the formation of in-plane magnetic ordering in chromium halides is still under debate. Here, we propose a microscopic orbitally-resolved description of ferromagnetism in monolayer CrI3. Starting from first-principles calculations, we construct a low-energy model for the isotropic Heisenberg exchange interactions. We find that there are two competing contributions to the long-range magnetic ordering in CrI3: (i) Antiferromagnetic Anderson's superexchange between half-filled t 2g orbitals of Cr atoms; and (ii) Ferromagnetic exchange governed by the Kugel-Khomskii mechanism, involving the transitions between half-filled t 2g and empty e g orbitals. Using numerical calculations, we estimate the exchange interactions in momentum-space, which allows us to restore the spin-wave spectrum, as well as estimate the Curie temperature. Contrary to the nearest-neighbor effective models, our calculations suggest the presence of sharp resonances in the spin-wave spectrum at 5-7 meV, depending on the vertical bias voltage. Our estimation of the Curie temperature in monolayer CrI3 yields 55-65 K, which is in good agreement with experimental data. © 2020 IOP Publishing Ltd.

Contains fulltext : 219077.pdf (Publisher’s version ) (Open Access)

Contains fulltext : 219077.pdf (Publisher’s version ) (Open Access)

This article discusses the practical experience of planning, organization and conducting a charity event involving volunteers. The key aspect of such events is the preliminary training of volunteers. The quality of training that the volunteers receive to a great extent determines the event‘s future success or failure., Работа посвящена практическому изучению специфики планирования, организации и проведения мероприятий социально-благотворительной направленности с участием волонтерского движения. Важным аспектом успешного проведения данных мероприятий является предварительное обучение и консультирование волонтеров. От качества подготовки волонтеров во многом зависит социальный эффект мероприятия.

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.

We investigate behavior of the interatomic magnetic interactions in CrO2, starting from original and static variant of DMFT, based on the exact diagonalization of the effective Anderson impurity Hamiltonian, and Hartree-Fock approach. The contributions of double exchange, superexchange and higher order interactions, obtained from 1/A expansion (A being the intraatomic exchange splitting), were found to play an important role in stabilizing the ferromagnetic phase 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 ferr

We study the role of static and dynamical Coulomb correlation effects on the electronic and magnetic properties of individual Mn, Fe, and Co adatoms deposited on the CuN surface. For these purposes, we construct a realistic Anderson model, solve it by using the finite-temperature exact diagonalization method, and compare the calculated one-particle spectral functions with the LDA+U densities of states. In contrast to Mn/CuN and Fe/CuN, the cobalt system tends to form the electronic excitations at the Fermi level. Based on the calculated magnetic response functions, transverse relaxation times for the magnetic moments of impurity orbitals are estimated. To study the effect of the dynamical correlations on the exchange interaction in nanoclusters, we solve the two-impurity Anderson model for the Mn dimer on the CuN surface. It is found that the experimental exchange interaction can be well reproduced by employing U=3 eV, which is two times smaller than the value used in static mean-field LDA+U calculations. This suggests an important role of dynamical correlations in the interaction between adatoms on a surface. To estimate the correlated exchange interaction in the general case we derive a simple and transparent analytical expression demonstrating that the renormalization of the electronic spectrum due to dynamical correlations leads to a rescaling of the magnetic interactions compared to density functional results. © 2013 American Physical Society.