Electronic Structure and Excitation Spectra of Magnetic Materials within First-Principles Many-Body Perturbation Theory

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We develop a computational method based on a many-body perturbation theory to enable first-principle calculations of the electronic structure and excitation spectra of magnetic materials. To this effect we extend the GW-approximation for the electronic self-energy to take the spin polarisation of the electron system into account. At this level the self-energy is still calculated separately for each spin channel. As a major goal, we then treat possible spin-flip processes by including appropriate vertex corrections in the form of a scattering matrix that couples the two spin channels. The implementation is based on an all-electron approach and avoids the use of pseudopotentials. Key quantities that can be obtained with this method are the quasi-particle band structure, including the exchange splitting between the two spin channels, as well as the spectral function, which in addition to the main quasi-particle peaks also shows additional collective excitations in the form of satellite resonances. The lifetime of the excitations can be obtained from the imaginary part of the complex quasi-particle energies. The theoretical approach is universal and can be applied to magnetic bulk materials, surfaces and nanostructures if spin polarisation plays an important role.