By means of first principles calculations we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the 3d orbitals of Eg and T2g symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly-interacting impurity levels. We demonstrate that, as a result of this, in Fe the T2g orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the Eg states the Heisenberg picture breaks down, since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbour coupling indicates that the interactions among Eg states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin.Iron is one of the most abundant elements in the universe. Its elemental phase has several polymorphs and among those the most stable crystal structure at ambient conditions is a body-centered cubic (bcc) one. Ferromagnetism is known to play a decisive role in defining the stability of this structure.1,2 The bcc phase is ferromagnetic (FM) up to critical temperature (T c ) of 1045 K. Quite importantly, above the Curie point, the bcc structure is preserved in a certain temperature range before it undergoes a transition to the fcc phase. This fact implies that the local magnetic moments exist in the paramagnetic (PM) phase, where strong short-ranged magnetic order was proposed.3 The magnetism of Fe is of mixed itinerant and localised nature and it is a matter of debate, which model describes it the best. Both the T c and the magnetic excitation spectra at low temperatures can be well described by means of the Heisenberg Hamiltonian (HH), parameterised by ab initio calculations.4-10 However, in several works 11,12 it was argued that in order to describe a large palette of magnetic states, higher-order (biquadratic) exchange interactions have to be taken into account. The results of the self-consistent spin spiral calculations also indicate that the magnitude of the magnetic moment in bcc Fe can differ by almost 30% in various states, 13,14 which in principle disagrees with the assumptions of the Heisenberg picture. Indeed, it was previously pointed out that the parameterisation of HH for bcc Fe depends on the magnetic configuration they are extracted from. 15-17The strong correlation effects are known to be important for bcc Fe at finite temperatures, as was shown in Ref. 18 by means of density functional theory plus dynamical mean field theory (DFT+DMFT) calculations.Earlier, it was suggested from a qualitative analysis of the electronic structure that the E g electrons in iron are much more correlated than the T 2 g ones.19 This statement was qua...

We have derived an expression of the Dzyaloshinskii–Moriya interaction (DMI), where all the three components of the DMI vector can be calculated independently, for a general, non-collinear magnetic configuration. The formalism is implemented in a real space—linear muffin-tin orbital—atomic sphere approximation (RS-LMTO-ASA) method. We have chosen the Cr triangular trimer on Au(111) and Mn triangular trimers on Ag(111) and Au(111) surfaces as numerical examples. The results show that the DMI (module and direction) is drastically different between collinear and non-collinear states. Based on the relation between the spin and charge currents flowing in the system and their coupling to the non-collinear magnetic configuration of the triangular trimer, we demonstrate that the DMI interaction can be significant, even in the absence of spin-orbit coupling. This is shown to emanate from the non-collinear magnetic structure, that can induce significant spin and charge currents even with spin-orbit coupling is ignored.