Origin and stability of highly spin polarized surface resonances

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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.