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

该理论项目旨在提供用于监测自旋电子学领域动态磁性的XMCD(X射线磁圆二色谱)实验的从头算描述。重点将放在监测电场(EFI-XMCD)或铁磁共振(FMR-XMCD)产生的自旋电流的实验以及新实验的动机上。所有的数值工作将基于局域自旋密度泛函理论(LSDA)及其推广，电子结构用格林函数表示，用完全相对论多重散射公式计算。将Kubo的线性响应公式与XMCD理论相结合，首次给出FM/NM(FM和NM：铁磁和正常金属)层状系统中EFI-XMCD的简化描述。相应的数值计算工作允许首次与处于稳定状态(SS)的系统的实验进行比较。在下一步中，将采用稳态非平衡格林函数(SS-NEGF)形式对EFI-XMCD进行更可靠的描述。相应的计算将阐明线性响应理论的适用性，并为正在进行的实验提供详细的解释，这些实验旨在探测FM/NM层状系统中的自旋流以及FM/NM层状系统的FM部分。这项工作将特别揭示产生观测到的XMCD的机制及其与自旋流的联系。为了描述最近的FMR-XMCD实验，在这些实验中，通过FMR的自旋泵浦激发了自旋流，第一步将使用绝热近似。这特别允许讨论感应自旋电流与XMCD谱之间的关系。在进一步的工作中，将使用一般的Keldysh NEGF形式，利用申请人在处理光发射中的泵浦-探测实验时所做的发展。进一步的工作旨在调查使用EFI-XMCD来探测和验证新的磁输运性质并模拟相应的实验。对于NM块体和NM/NM层状系统，我们将研究Edelstein效应，即在没有反转对称性的系统中发生电场诱导的磁化。由此产生的光谱将通过采用XMCD求和规则来解释磁电极化率。此外，EFI-XMCD谱的计算将被用来激励实验，以验证申请人预测的低维固体中出现感应纵向自旋流。为此目的，将研究NM/NM分层系统，它应该表现出明显的效应，该效应可以通过对称性考虑与Edelstein效应明显地区分开来。

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.