Magnetic exchange inα-iron fromab initiocalculations in the paramagnetic phase

Abstract

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.

Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe

“…Moreover, one can see the crucial role of the E g − T 2g interactions on the ferromagnetism of iron: it provides the dominant FM component of the J 1 coupling. This observation is in line with a recent study by Igoshev et al 21 who analysed the paramagnetic susceptibility in bcc Fe and also emphasised an importance of the mixed E g −T 2g interactions for the formation of the local moment. The results shown in Fig.…”

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

“…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 .…”

“…15; χ loc (iω n ) is the dynamic local susceptibility and χ irr ≈ 2µ 2 B /eV is the static local two-particle irreducible susceptibility in the α phase. Decoupling the four-spin interaction in the soft constraint part in Eq.…”

“…is the exchange interaction, χ irr q is the static two-particle irreducible susceptibility, which can be calculated as a bubble, constructed from itinerant Green functions 15 . The determination of the function Λ(T, iω n ) is a rather complicated problem, since it requires knowledge of the S 2 2 interaction potential in Eq.…”

“…To calculate the nonlocal corrections to the Curie temperature and the energy of the α phase, we treat the effect of local moments on the energy within the spin-fermion model of Ref. 15, supplemented by a soft spin constraint,…”