Magnetic fluctuations and effective magnetic moments inγ-iron due to electronic structure peculiarities

Abstract

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.

Nonlocal correlations in the vicinity of the α−γ phase transition in iron within a DMFT plus spin-fermion model approach

“…However, since the corresponding Neel temperature is much lower, than T α C (see, e.g., Ref. 14), and pronounced corrections are obtained only in the vicinity of the magnetic transition temperature, we do not expect a substantial correction in that case.…”

“…Looking at the partial contributions from kinetic and potential energies, one can see that the increase of the energy of the α phase with decreasing T is due to the strong increase of the kinetic energy, while the potential energy expectedly decreases, reflecting the increase of instantaneous magnetic moment S 2 i,α (Ref. 14). Although the energy of the γ phase also increases with decreasing T , it saturates in the temperature range 1000 − 1500 K. Moreover, inspection of kinetic and potential energies shows the opposite tendencies to those in the α phase: the mentioned increase of total energy upon cooling is provided by a strong increase of the potential energy, and a weaker decrease of the kinetic energy.…”

“…To treat the effect of local correlations we apply in the present paper the combination of dynamical mean-field theory (DMFT) 10 with density functional theory (DFT) methods, usually called LDA+DMFT 11 . Previous studies by LDA+DMFT allowed one to obtain the correct values of magnetic moments in α and γ phases [12][13][14][15][16] , the linear behavior of the temperature dependence of the inverse local [13][14][15][16] (α,γ phases) and uniform magnetic 12,17,18 (α phase) susceptibilities, and revealed the non-monotonic temperature dependence of inverse the uniform magnetic susceptibility in the γ phase in a broad temperature range 14 .…”

“…(2). We fix its static component by the equality of the obtained static part of the on-site spin correlation function to that, obtained in DMFT µ 2 eff,α = 3T χ loc (0); the latter is found to be almost temperature independent (contrary to the instantaneous moment S 2 i ) in a broad temperature range 14 . The corresponding condition reads…”

Ab initiocalculation of the solution enthalpies of substitutional and interstitial impurities in paramagnetic fcc Fe

“…In principle, one could use the dynamical mean-field theory (DMFT) [2], combined with local or semilocal DFT band structure calculations. It has been used in papers on finite-temperature magnetism in body-centered cubic (bcc) [3,4] and face-centered cubic (fcc) Fe [5], as well as structural Fe [6] and electronic [7] phase transition in this metal. However, reducing symmetry of a system by introducing defects makes DMFT calculations quite time consuming.…”