Origin and stability of highly spin polarized surface resonances

铁磁材料在室温下的有效自旋极化大小是其在自旋电子学中应用的关键性质。然而，对于大多数应用来说，它不是本体的电子结构，而是材料的表面或界面，这是相关的。在研究Heusler化合物Co2MnSi时，我们最近与我们的实验合作伙伴（JGU Mainz的M. Jourdan博士）合作观察到高自旋极化（93%），这与大多数自旋带延伸到体深处的稳定表面共振有关。在这里，我们想研究这种高自旋极化表面共振的物理起源。根据这一中心目标，将从理论上研究具有不同表面取向和不同覆盖层界面的各种Heusler和相关化合物。特别是，半金属体带结构是否是出现100%自旋极化表面共振的必要前提的问题将得到回答。考虑到潜在的应用，如果自由表面被带有绝缘体或金属的界面所取代，由表面共振引起的高自旋极化是否守恒是非常重要的。我们的实验伙伴将原位自旋分辨光电子能谱（sr - up）应用于Heusler化合物和相关体系，对于表面和界面状态的研究已经非常成功。特别是使用自旋集成高能x射线光发射光谱（HAXPES）可以实现更大的探测深度。因此，我们的实验伙伴项目并行运行的结果将通过与考虑所有相对论，表面和高能效应的第一性原理能带结构和光发射计算的直接比较来支持。特别强调电子相关和各种形式的无序对自旋极化的影响。除了由于所研究的合金系统的非化学计量学和界面上的相互扩散引起的化学无序外，还将特别考虑热诱导的无序。这一重要方面包括晶格振动和热自旋涨落。为了解释光发射计算中的自旋波动，将采用申请人最近开发的一种基于CPA（相干势近似）合金理论的方法。

The magnitude of the effective spin polarization of ferromagnetic materials at room temperature is a key property for their application in spintronics. However, for most applications it is not the electronic structure of the bulk, but of the surface or interface of the material, which is relevant. Investigating the Heusler compound Co2MnSi we recently observed in collaboration with our experimentally working partner (Dr. M. Jourdan, JGU Mainz) a high spin polarization (93%), which is related to a stable surface resonance in the majority spin band extending deep into the bulk. Here we want to investigate the physical origin of such highly spin polarized surface resonances. According to this central aim various Heusler and related compounds with different orientations of their surface and interface to suitable cover layers, respectively, will be investigated theoretically. In particular, the question whether a half metallic bulk band structure is a necessary precondition for the appearance of a 100% spin-polarized surface resonance will be answered. Considering potential applications it is of major importance whether the high spin polarization due to a surface resonance is conserved, if the free surface is replaced by an interface with an insulator or metal. The in-situ spin resolved photo electron spectroscopy (SR-UPS) applied by our experimental partner to Heusler compounds and related systems proved already extremely successful for the investigation of the surface and interface states. In particular the use of spin integrated high energy x-ray photo emission spectroscopy (HAXPES) allows to achieve a larger probing depth. Accordingly, the interpretation of the results of our experimental partner project running in parallel will be supported by direct comparison with first-principles band structure and photo emission calculations which consider all relativistic, surface and high energy effects. Special emphasize will be put on the influence of electronic correlations as well as various forms of disorder on the spin polarization. Apart from chemical disorder due to non-stoichiometry of the investigated alloy systems and inter-diffusion at an interface, respectively, thermally induced disorder will be considered in particular. This important aspect includes lattice vibrations as well as thermal spin fluctuations. To account for spin fluctuations within the photo emission calculations a method will be applied that was developed recently by the applicant and that is based on the CPA (Coherent Potential Approximation) alloy theory.