Schedule Nov 02, 2009
Spin-Excitations from Density-Functional Theory and Beyond
Stefan Bluegel (FZ-Juelich)

Ersoy Sasioglu, Christoph Friedrich, Arno Schindlmayr, and Stefan Blügel

Institut für Festkörperforschung and Institute for Advanced Simulation, Forschungszentrum Jülich, D-52425 Jülich, Germany

Spin and magnon excitations, splin-flip life times, spin-diffusion lengths or Gilbert damping are quantities of great importance with respect to the design of materials and devices in the field of spintronics. In the recent past, various progresses has been made to determine these quantities from materials specific theories. For example in the last years, magnon dispersions have been calculated employing the density functional theory in the vector-spin density formulation combined with the generalized Bloch theorem [1] relying on the adiabatic approximation. In this contribution we go beyond the present framework. To study excitation spectra of magnetic materials from first principles we have developed a computational scheme based on many-body perturbation theory [2]. The main quantity of interest is the dynamical and q-dependent transverse spin susceptibility, from which magnetic excitations, including single-particle spin-flip Stoner excitations and collective spin-wave modes as well as their lifetimes, can be obtained. In order to describe spin waves we include appropriate vertex corrections in the form of a multiple-scattering T-matrix, which describes the coupling of electrons and holes with different spin. To reduce the numerical cost for the calculation of the four-point T-matrix we exploit a transformation to maximally localized Wannier functions [3] that takes advantage of the short spatial range of electronic correlation in the partially filled d or f orbitals of magnetic materials. Our implementation is based on the full-potential linearized augmented plane-wave (FLAPW) method as implemented in FLEUR and SPEX codes [4]. The latter is an efficient implementation of the GW approximation for the electronic self-energy within the all-electron FLAPW method [5]. The magnetic excitations in bulk Fe, Co, and Ni as well as other magnets are studied in detail using the local spin-density approximation (LSDA) as well as the LSDA+U approach. Employing the developed scheme we have calculated the matrix elements of the bare and screened Coulomb potential for a series of 3d transition metals in the Wannier basis.

[1] Ph. Kurz, F. Förster, L. Nordström, G. Bihlmayer, and S. Blügel, Phys. Rev. B 69, 024415 (2004).
[2] E. Sas?oglu , A. Schindlmayr, C. Friedrich, and Stefan Blügel, to be published.
[3] F. Freimuth, Y. Mokrousov, D. Wortmann, S. Heinze, S. Blügel, Phys. Rev. B. 78, 035120 (2008).
[5] C. Friedrich, A. Schindlmayr, and S. Blügel, Comput. Phys. Commun. 180, 347 (2009).

We acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) through the Priority Programs 1145, 1153, the ESF EUROCORES Programme SONS under contract N. ERAS-CT-2003-980409 and a grant from EU Network of Excellence Nanoquanta.

¶ New Address: Department Physik, Universität Paderborn, D-33095 Paderborn, Germany

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