Spin currents probed by magnetic circular dichroism in X-ray absorption spectroscopy (XMCD) - a theoretical study

• Applicant: Professor Dr. Hubert Ebert
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 327501276

The theory project aims to supply an ab-initio description of XMCD (X-ray magnetic circular dichroism in X-ray absorption) experiments used to monitor dynamical magnetic properties in the field of spintronics. The focus will be on experiments that monitor spin currents induced by an electric field (EFI-XMCD) or ferromagnetic resonance (FMR-XMCD) as well as the motivation of new experiments. All numerical work will be based on local spin density functional theory (LSDA) and its extensions with the electronic structure represented in terms of Green functions evaluated using the fully relativistic multiple scattering formalism.A first simplified description of the EFI-XMCD in FM/NM (FM and NM: ferromagnetic and normal metals, respectively) layered systems will be given by combining Kubo's linear response formalism with the theory on XMCD. Corresponding numerical work allow for a first comparison with experiments on systems in a steady state (SS). In the next step a more reliable description of the EFI-XMCD will be achieved by adopting the steady state non-equilibrium Green function (SS-NEGF) formalism. Corresponding calculations will clarify the applicability of linear-response theory and provide a detailed interpretation for on-going experiments meant to probe spin currents in the NM as well as FM part of FM/NM layered systems. This work will in particular reveal the mechanism giving rise to the observed XMCD together with its connection to the spin current. To describe recent FMR-XMCD experiments for which spin currents are stimulated by spin-pumping via FMR the adiabatic approximation will be used in a first step. This allows in particular to discuss the relation of the induced spin current and the XMCD spectra. Within further work the general Keldysh NEGF formalism will be used, making use of developments done by the applicant when dealing with pump-probe experiments in photo emission.Further work is meant to investigate the use of the EFI-XMCD to probe and verify new magneto-transport properties and to stimulate corresponding experiments. The Edelstein effect, i.e. the occurrence of a magnetization induced by an electric field in systems without inversion symmetry, will be investigated for NM bulk and NM/NM layered systems. The resulting spectra will be interpreted in terms of the magneto-electric susceptibility by adapting the XMCD sum rules. In addition calculations of EFI-XMCD spectra will be done to motivate experiments that verify the occurrence of induced longitudinal spin currents in low-dimensional solids predicted by the applicant. For this purpose NM/NM layered systems will be investigated that should show a pronounced effect that can be clearly distinguished from the Edelstein effect by symmetry considerations.

• DFG Programme: Research Grants