We revisit a problem of theoretical description of α-iron. By performing LDA+DMFT calculations in the paramagnetic phase we find that Coulomb interaction and, in particular Hund exchange, yields the formation of local moments in eg electron band, which can be traced from imaginary time dependence of the spin-spin correlation function. This behavior is accompanied by non-Fermi-liquid behavior of eg electrons and suggests using local moment variables in the effective model of iron. By investigating orbital-selective contributions to the Curie-Weiss law for Hund exchange I = 0.9 eV we obtain an effective value of local moment of eg electrons 2p = 1.04µ B . The effective bosonic model, which allows to describe magnetic properties of iron near the magnetic phase transition, is proposed.

Electronic correlations in the Fe-pnictide BaFe2As2 are explored within LDA+DMFT, the combination of density functional theory with dynamical mean-field theory. While the correlated band structure is substantially renormalized there is only little transfer of spectral weight. The computed k-integrated and k-resolved spectral functions are in good agreement with photoemission spectroscopy (PES) and angular resolved PES experiments. Making use of a general classification scheme for the strength of electronic correlations we conclude that BaFe2As2 is a moderately correlated system.

Applying the local density and dynamical mean field approximations to paramagnetic γ -iron we revisit the problem of the theoretical description of its magnetic properties in a wide temperature range. We show that contrary to α-iron, the frequency dependence of the electronic self-energy has a quasiparticle form for both t 2g and e g states. In the temperature range T = 1200-1500 K, where γ -iron exists in nature, this substance can be nevertheless characterized by temperature-dependent effective local moments, which yield relatively narrow peaks in the real part of the local magnetic susceptibility as a function of frequency. At the same time, at low temperatures γ -iron (which is realized in precipitates) is better described in terms of the itinerant picture. In particular, the nesting features of the Fermi surfaces yield the maximum of the static magnetic susceptibility at the incommensurate wave vector q max belonging in the direction q X − q W (q X ≡ (2π/a)(1,0,0),q W ≡ (2π/a)(1,1/2,0), a is a lattice parameter) in agreement with the experimental data. This state is found, however, to compete closely with the states characterized by magnetic wave vectors along the directions q X − q L − q K , where q L ≡ (2π/a)(1/2,1/2,1/2), q K ≡ (2π/a)(3/4,3/4,0). From the analysis of the uniform magnetic susceptibility we find that contrary to α-iron, the Curie-Weiss law is not fulfilled in a broad temperature range, although the inverse susceptibility is nearly linear in the moderate-temperature region (1200-1500 K). The nonlinearity of the inverse uniform magnetic susceptibility in a broader temperature range is due to the density of states peak located close to the Fermi level. The effective exchange integrals in the paramagnetic phase are estimated on the base of momentum-dependent susceptibility.

Applying the local density approximation (LDA) and dynamical mean field theory (DMFT) to paramagnetic α-iron, we revisit a problem of theoretical description of its magnetic properties. The analysis of local magnetic susceptibility shows that at sufficiently low temperatures T < 1500 K, both, eg and t2g states equally contribute to the formation of the effective magnetic moment with spin S = 1. The self-energy of t2g states shows sizable deviations from Fermi-liquid form, which accompanies earlier found non-quasiparticle form of eg states. By considering the non-uniform magnetic susceptibility we find that the non-quasiparticle form of eg states is crucial for obtaining ferromagnetic instability in α-iron. The main contribution to the exchange interaction, renormalized by the effects of electron interaction, comes from the hybridization between t2g and eg states. We furthermore suggest the effective spin-fermion model for α-iron, which allows us to estimate the exchange interaction from paramagnetic phase, which is in agreement with previous calculations in the ordered state within the LDA approaches.

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