First-principles modelling of magnetic excitations in correlated materials

General ILL Seminar organised by College 4 et 7
Wednesday, 16 May, 2018, 14h00
Seminar room, 1st floor, ILL 4

« First-principles modelling of magnetic excitations in correlated materials »

Yaroslav Kvashnin
Department of Physics and Astronomy, Uppsala University, SE-75120, Uppsala, Sweden

In the past decades there has been an enormous progress in developing the realistic electronic structure theory for strongly correlated systems. Most of the modern first-principles studies are based on a combination of density functional theory and dynamical mean field theory (DFT+DMFT) [1]. Within this approach, the DFT is used as a starting point for more sophisticated DMFT calculations for a selected subset of “correlated” orbitals. The approach has proven to be extremely successful and is commonly employed to model the electronic structure of a wide range of d- and f-based correlated materials. In my talk I will show various applications of DFT+DMFT with an emphasis on magnetic properties.

Magnon excitations are non-local and are therefore not directly accessible within DMFT. Thus we have to employ a so-called two-step approach. We first map the system on a Heisenberg model and extract the effective exchange parameters Jij’s from DFT+DMFT, following Ref.[2]. Then the atomistic spin dynamics simulations are employed to simulate magnon spectra and predict the magnetic ordering temperatures [3]. I will demonstrate how such a combined approach can successfully predict properties of different types of materials from transition metal oxides to rare-earth metals.

The Jij’s in real materials have a complicated nature and depend on how the correlation effects are taken into account [4]. I will show how the decomposition of the exchange parameters in terms of their orbital contributions can provide a useful insight into the essence of magnetism in a material. In particular, we found an intrinsic competition between ferromagnetic and antiferromagnetic interactions between different orbitals in elemental Mn and Fe [5]. Moreover, I will demonstrate that iron is a unique example where one can even distinguish RKKY, double- and super-exchange contributions to the magnetic couplings [6].

References:
[1] G. Kotliar, S. Y. Savrasov, K. Haule, V. S. Oudovenko, O. Parcollet, and C. A. Marianetti, Rev. Mod. Phys., 78, 865 (2006)
[2] M.I. Katsnelson and A.I. Lichtenstein, Phys. Rev. B 61 8906 (2000)
[3] O. Eriksson et al, "Atomistic Spin Dynamics: Foundations and Applications" (Oxford University Press, 2017)
[4] YK et al, Phys. Rev. B 91, 125133 (2015)
[5] R. Cardias, et al, Sci. Rep. 7, 4058 (2017)
[6] YK et al, Phys. Rev. Lett. 116, 217202 (2016)

A. Piovano
(College VII Secretary)

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